(x-handle-display): Add doc string.

[bpt/emacs.git] / lispref / display.texi

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999, 2000, 2001,
@c   2002, 2003, 2004, 2005, 2006 Free Software Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@setfilename ../info/display
@node Display, System Interface, Processes, Top
@chapter Emacs Display

  This chapter describes a number of features related to the display
that Emacs presents to the user.

@menu
* Refresh Screen::      Clearing the screen and redrawing everything on it.
* Forcing Redisplay::   Forcing redisplay.
* Truncation::          Folding or wrapping long text lines.
* The Echo Area::       Displaying messages at the bottom of the screen.
* Warnings::            Displaying warning messages for the user.
* Invisible Text::      Hiding part of the buffer text.
* Selective Display::   Hiding part of the buffer text (the old way).
* Temporary Displays::  Displays that go away automatically.
* Overlays::            Use overlays to highlight parts of the buffer.
* Width::               How wide a character or string is on the screen.
* Line Height::         Controlling the height of lines.
* Faces::               A face defines a graphics style for text characters:
                          font, colors, etc.
* Fringes::             Controlling window fringes.
* Scroll Bars::         Controlling vertical scroll bars.
* Display Property::    Enabling special display features.
* Images::              Displaying images in Emacs buffers.
* Buttons::             Adding clickable buttons to Emacs buffers.
* Abstract Display::    Emacs' Widget for Object Collections.
* Blinking::            How Emacs shows the matching open parenthesis.
* Usual Display::       The usual conventions for displaying nonprinting chars.
* Display Tables::      How to specify other conventions.
* Beeping::             Audible signal to the user.
* Window Systems::      Which window system is being used.
@end menu

@node Refresh Screen
@section Refreshing the Screen

  The function @code{redraw-frame} clears and redisplays the entire
contents of a given frame (@pxref{Frames}).  This is useful if the
screen is corrupted.

@c Emacs 19 feature
@defun redraw-frame frame
This function clears and redisplays frame @var{frame}.
@end defun

  Even more powerful is @code{redraw-display}:

@deffn Command redraw-display
This function clears and redisplays all visible frames.
@end deffn

  This function calls for redisplay of certain windows, the next time
redisplay is done, but does not clear them first.

@defun force-window-update &optional object
This function forces some or all windows to be updated on next redisplay.
If @var{object} is a window, it forces redisplay of that window.  If
@var{object} is a buffer or buffer name, it forces redisplay of all
windows displaying that buffer.  If @var{object} is @code{nil} (or
omitted), it forces redisplay of all windows.
@end defun

  Processing user input takes absolute priority over redisplay.  If you
call these functions when input is available, they do nothing
immediately, but a full redisplay does happen eventually---after all the
input has been processed.

  Normally, suspending and resuming Emacs also refreshes the screen.
Some terminal emulators record separate contents for display-oriented
programs such as Emacs and for ordinary sequential display.  If you are
using such a terminal, you might want to inhibit the redisplay on
resumption.

@defvar no-redraw-on-reenter
@cindex suspend (cf. @code{no-redraw-on-reenter})
@cindex resume (cf. @code{no-redraw-on-reenter})
This variable controls whether Emacs redraws the entire screen after it
has been suspended and resumed.  Non-@code{nil} means there is no need
to redraw, @code{nil} means redrawing is needed.  The default is @code{nil}.
@end defvar

@node Forcing Redisplay
@section Forcing Redisplay
@cindex forcing redisplay

  Emacs redisplay normally stops if input arrives, and does not happen
at all if input is available before it starts.  Most of the time, this
is exactly what you want.  However, you can prevent preemption by
binding @code{redisplay-dont-pause} to a non-@code{nil} value.

@defvar redisplay-preemption-period
This variable specifies how many seconds Emacs waits between checks
for new input during redisplay.  (The default is 0.1 seconds.)  If
input has arrived when Emacs checks, it pre-empts redisplay and
processes the available input before trying again to redisplay.

If this variable is @code{nil}, Emacs does not check for input during
redisplay, and redisplay cannot be preempted by input.

@emph{Note} that this variable is only available if Emacs is built
with support for sub-second timers.
@end defvar

@defvar redisplay-dont-pause
If this variable is non-@code{nil}, pending input does not
prevent or halt redisplay; redisplay occurs, and finishes,
regardless of whether input is available.
@end defvar

@findex sit-for
  You can request a display update, but only if no input is pending,
with @code{(sit-for 0)}.  To force a display update even when input is
pending, use @code{(sit-for -1)}.

@node Truncation
@section Truncation
@cindex line wrapping
@cindex continuation lines
@cindex @samp{$} in display
@cindex @samp{\} in display

  When a line of text extends beyond the right edge of a window, the
line can either be continued on the next screen line, or truncated to
one screen line.  The additional screen lines used to display a long
text line are called @dfn{continuation} lines.  Normally, tiny arrow images
are displayed in the window fringes to indicate truncated and continued
lines (*note Fringes).

   On a text terminal, a `$' in the rightmost column of the window
indicates truncation; a `\' on the rightmost column indicates a
continued line that "wraps" onto the next line.  (The display table can
specify alternative indicators; see *Note Display Tables.)

  On a graphical display, the @samp{$} and @samp{\} indicators are
replaced with arrow images displayed in the window fringes
(@pxref{Fringes}).

  Note that continuation is different from filling; continuation happens
on the screen only, not in the buffer contents, and it breaks a line
precisely at the right margin, not at a word boundary.  @xref{Filling}.

@defopt truncate-lines
This buffer-local variable controls how Emacs displays lines that extend
beyond the right edge of the window.  The default is @code{nil}, which
specifies continuation.  If the value is non-@code{nil}, then these
lines are truncated.

If the variable @code{truncate-partial-width-windows} is non-@code{nil},
then truncation is always used for side-by-side windows (within one
frame) regardless of the value of @code{truncate-lines}.
@end defopt

@defopt default-truncate-lines
This variable is the default value for @code{truncate-lines}, for
buffers that do not have buffer-local values for it.
@end defopt

@defopt truncate-partial-width-windows
This variable controls display of lines that extend beyond the right
edge of the window, in side-by-side windows (@pxref{Splitting Windows}).
If it is non-@code{nil}, these lines are truncated; otherwise,
@code{truncate-lines} says what to do with them.
@end defopt

  When horizontal scrolling (@pxref{Horizontal Scrolling}) is in use in
a window, that forces truncation.

  If your buffer contains @emph{very} long lines, and you use
continuation to display them, just thinking about them can make Emacs
redisplay slow.  The column computation and indentation functions also
become slow.  Then you might find it advisable to set
@code{cache-long-line-scans} to @code{t}.

@defvar cache-long-line-scans
If this variable is non-@code{nil}, various indentation and motion
functions, and Emacs redisplay, cache the results of scanning the
buffer, and consult the cache to avoid rescanning regions of the buffer
unless they are modified.

Turning on the cache slows down processing of short lines somewhat.

This variable is automatically buffer-local in every buffer.
@end defvar

@node The Echo Area
@section The Echo Area
@cindex error display
@cindex echo area

  The @dfn{echo area} is used for displaying error messages
(@pxref{Errors}), for messages made with the @code{message} primitive,
and for echoing keystrokes.  It is not the same as the minibuffer,
despite the fact that the minibuffer appears (when active) in the same
place on the screen as the echo area.  The @cite{GNU Emacs Manual}
specifies the rules for resolving conflicts between the echo area and
the minibuffer for use of that screen space (@pxref{Minibuffer,, The
Minibuffer, emacs, The GNU Emacs Manual}).

  You can write output in the echo area by using the Lisp printing
functions with @code{t} as the stream (@pxref{Output Functions}), or
explicitly.

@menu
* Displaying Messages:: Explicitly displaying text in the echo area.
* Progress::            Informing user about progress of a long operation.
* Logging Messages::    Echo area messages are logged for the user.
* Echo Area Customization:: Controlling the echo area.
@end menu

@node Displaying Messages
@subsection Displaying Messages in the Echo Area

  This section describes the functions for explicitly producing echo
area messages.  Many other Emacs features display messages there, too.

@defun message format-string &rest arguments
This function displays a message in the echo area.  The argument
@var{format-string} is similar to a C language @code{printf} format
string.  See @code{format} in @ref{Formatting Strings}, for the details
on the conversion specifications.  @code{message} returns the
constructed string.

In batch mode, @code{message} prints the message text on the standard
error stream, followed by a newline.

If @var{format-string}, or strings among the @var{arguments}, have
@code{face} text properties, these affect the way the message is displayed.

@c Emacs 19 feature
If @var{format-string} is @code{nil} or the empty string,
@code{message} clears the echo area; if the echo area has been
expanded automatically, this brings it back to its normal size.
If the minibuffer is active, this brings the minibuffer contents back
onto the screen immediately.

@example
@group
(message "Minibuffer depth is %d."
         (minibuffer-depth))
 @print{} Minibuffer depth is 0.
@result{} "Minibuffer depth is 0."
@end group

@group
---------- Echo Area ----------
Minibuffer depth is 0.
---------- Echo Area ----------
@end group
@end example

To automatically display a message in the echo area or in a pop-buffer,
depending on its size, use @code{display-message-or-buffer} (see below).
@end defun

@defmac with-temp-message message &rest body
This construct displays a message in the echo area temporarily, during
the execution of @var{body}.  It displays @var{message}, executes
@var{body}, then returns the value of the last body form while restoring
the previous echo area contents.
@end defmac

@defun message-or-box format-string &rest arguments
This function displays a message like @code{message}, but may display it
in a dialog box instead of the echo area.  If this function is called in
a command that was invoked using the mouse---more precisely, if
@code{last-nonmenu-event} (@pxref{Command Loop Info}) is either
@code{nil} or a list---then it uses a dialog box or pop-up menu to
display the message.  Otherwise, it uses the echo area.  (This is the
same criterion that @code{y-or-n-p} uses to make a similar decision; see
@ref{Yes-or-No Queries}.)

You can force use of the mouse or of the echo area by binding
@code{last-nonmenu-event} to a suitable value around the call.
@end defun

@defun message-box format-string &rest arguments
This function displays a message like @code{message}, but uses a dialog
box (or a pop-up menu) whenever that is possible.  If it is impossible
to use a dialog box or pop-up menu, because the terminal does not
support them, then @code{message-box} uses the echo area, like
@code{message}.
@end defun

@defun display-message-or-buffer message &optional buffer-name not-this-window frame
This function displays the message @var{message}, which may be either a
string or a buffer.  If it is shorter than the maximum height of the
echo area, as defined by @code{max-mini-window-height}, it is displayed
in the echo area, using @code{message}.  Otherwise,
@code{display-buffer} is used to show it in a pop-up buffer.

Returns either the string shown in the echo area, or when a pop-up
buffer is used, the window used to display it.

If @var{message} is a string, then the optional argument
@var{buffer-name} is the name of the buffer used to display it when a
pop-up buffer is used, defaulting to @samp{*Message*}.  In the case
where @var{message} is a string and displayed in the echo area, it is
not specified whether the contents are inserted into the buffer anyway.

The optional arguments @var{not-this-window} and @var{frame} are as for
@code{display-buffer}, and only used if a buffer is displayed.
@end defun

@defun current-message
This function returns the message currently being displayed in the
echo area, or @code{nil} if there is none.
@end defun

@node Progress
@subsection Reporting Operation Progress
@cindex progress reporting

  When an operation can take a while to finish, you should inform the
user about the progress it makes.  This way the user can estimate
remaining time and clearly see that Emacs is busy working, not hung.

  Functions listed in this section provide simple and efficient way of
reporting operation progress.  Here is a working example that does
nothing useful:

@smallexample
(let ((progress-reporter
       (make-progress-reporter "Collecting mana for Emacs..."
                               0  500)))
  (dotimes (k 500)
    (sit-for 0.01)
    (progress-reporter-update progress-reporter k))
  (progress-reporter-done progress-reporter))
@end smallexample

@defun make-progress-reporter message min-value max-value &optional current-value min-change min-time
This function creates and returns a @dfn{progress reporter}---an
object you will use as an argument for all other functions listed
here.  The idea is to precompute as much data as possible to make
progress reporting very fast.

When this progress reporter is subsequently used, it will display
@var{message} in the echo area, followed by progress percentage.
@var{message} is treated as a simple string.  If you need it to depend
on a filename, for instance, use @code{format} before calling this
function.

@var{min-value} and @var{max-value} arguments stand for starting and
final states of your operation.  For instance, if you scan a buffer,
they should be the results of @code{point-min} and @code{point-max}
correspondingly.  It is required that @var{max-value} is greater than
@var{min-value}.  If you create progress reporter when some part of
the operation has already been completed, then specify
@var{current-value} argument.  But normally you should omit it or set
it to @code{nil}---it will default to @var{min-value} then.

Remaining arguments control the rate of echo area updates.  Progress
reporter will wait for at least @var{min-change} more percents of the
operation to be completed before printing next message.
@var{min-time} specifies the minimum time in seconds to pass between
successive prints.  It can be fractional.  Depending on Emacs and
system capabilities, progress reporter may or may not respect this
last argument or do it with varying precision.  Default value for
@var{min-change} is 1 (one percent), for @var{min-time}---0.2
(seconds.)

This function calls @code{progress-reporter-update}, so the first
message is printed immediately.
@end defun

@defun progress-reporter-update reporter value
This function does the main work of reporting progress of your
operation.  It displays the message of @var{reporter}, followed by
progress percentage determined by @var{value}.  If percentage is zero,
or close enough according to the @var{min-change} and @var{min-time}
arguments, then it is omitted from the output.

@var{reporter} must be the result of a call to
@code{make-progress-reporter}.  @var{value} specifies the current
state of your operation and must be between @var{min-value} and
@var{max-value} (inclusive) as passed to
@code{make-progress-reporter}.  For instance, if you scan a buffer,
then @var{value} should be the result of a call to @code{point}.

This function respects @var{min-change} and @var{min-time} as passed
to @code{make-progress-reporter} and so does not output new messages
on every invocation.  It is thus very fast and normally you should not
try to reduce the number of calls to it: resulting overhead will most
likely negate your effort.
@end defun

@defun progress-reporter-force-update reporter value &optional new-message
This function is similar to @code{progress-reporter-update} except
that it prints a message in the echo area unconditionally.

The first two arguments have the same meaning as for
@code{progress-reporter-update}.  Optional @var{new-message} allows
you to change the message of the @var{reporter}.  Since this functions
always updates the echo area, such a change will be immediately
presented to the user.
@end defun

@defun progress-reporter-done reporter
This function should be called when the operation is finished.  It
prints the message of @var{reporter} followed by word ``done'' in the
echo area.

You should always call this function and not hope for
@code{progress-reporter-update} to print ``100%.''  Firstly, it may
never print it, there are many good reasons for this not to happen.
Secondly, ``done'' is more explicit.
@end defun

@defmac dotimes-with-progress-reporter (var count [result]) message body@dots{}
This is a convenience macro that works the same way as @code{dotimes}
does, but also reports loop progress using the functions described
above.  It allows you to save some typing.

You can rewrite the example in the beginning of this node using
this macro this way:

@example
(dotimes-with-progress-reporter
    (k 500)
    "Collecting some mana for Emacs..."
  (sit-for 0.01))
@end example
@end defmac

@node Logging Messages
@subsection Logging Messages in @samp{*Messages*}
@cindex logging echo-area messages

  Almost all the messages displayed in the echo area are also recorded
in the @samp{*Messages*} buffer so that the user can refer back to
them.  This includes all the messages that are output with
@code{message}.

@defopt message-log-max
This variable specifies how many lines to keep in the @samp{*Messages*}
buffer.  The value @code{t} means there is no limit on how many lines to
keep.  The value @code{nil} disables message logging entirely.  Here's
how to display a message and prevent it from being logged:

@example
(let (message-log-max)
  (message @dots{}))
@end example
@end defopt

  To make @samp{*Messages*} more convenient for the user, the logging
facility combines successive identical messages.  It also combines
successive related messages for the sake of two cases: question
followed by answer, and a series of progress messages.

  A ``question followed by an answer'' means two messages like the
ones produced by @code{y-or-n-p}: the first is @samp{@var{question}},
and the second is @samp{@var{question}...@var{answer}}.  The first
message conveys no additional information beyond what's in the second,
so logging the second message discards the first from the log.

  A ``series of progress messages'' means successive messages like
those produced by @code{make-progress-reporter}.  They have the form
@samp{@var{base}...@var{how-far}}, where @var{base} is the same each
time, while @var{how-far} varies.  Logging each message in the series
discards the previous one, provided they are consecutive.

  The functions @code{make-progress-reporter} and @code{y-or-n-p}
don't have to do anything special to activate the message log
combination feature.  It operates whenever two consecutive messages
are logged that share a common prefix ending in @samp{...}.

@node Echo Area Customization
@subsection Echo Area Customization

  These variables control details of how the echo area works.

@defvar cursor-in-echo-area
This variable controls where the cursor appears when a message is
displayed in the echo area.  If it is non-@code{nil}, then the cursor
appears at the end of the message.  Otherwise, the cursor appears at
point---not in the echo area at all.

The value is normally @code{nil}; Lisp programs bind it to @code{t}
for brief periods of time.
@end defvar

@defvar echo-area-clear-hook
This normal hook is run whenever the echo area is cleared---either by
@code{(message nil)} or for any other reason.
@end defvar

@defvar echo-keystrokes
This variable determines how much time should elapse before command
characters echo.  Its value must be an integer or floating point number,
which specifies the
number of seconds to wait before echoing.  If the user types a prefix
key (such as @kbd{C-x}) and then delays this many seconds before
continuing, the prefix key is echoed in the echo area.  (Once echoing
begins in a key sequence, all subsequent characters in the same key
sequence are echoed immediately.)

If the value is zero, then command input is not echoed.
@end defvar

@defvar message-truncate-lines
Normally, displaying a long message resizes the echo area to display
the entire message.  But if the variable @code{message-truncate-lines}
is non-@code{nil}, the echo area does not resize, and the message is
truncated to fit it, as in Emacs 20 and before.
@end defvar

  The variable @code{max-mini-window-height}, which specifies the
maximum height for resizing minibuffer windows, also applies to the
echo area (which is really a special use of the minibuffer window.
@xref{Minibuffer Misc}.

@node Warnings
@section Reporting Warnings
@cindex warnings

  @dfn{Warnings} are a facility for a program to inform the user of a
possible problem, but continue running.

@menu
* Warning Basics::      Warnings concepts and functions to report them.
* Warning Variables::   Variables programs bind to customize their warnings.
* Warning Options::     Variables users set to control display of warnings.
@end menu

@node Warning Basics
@subsection Warning Basics
@cindex severity level

  Every warning has a textual message, which explains the problem for
the user, and a @dfn{severity level} which is a symbol.  Here are the
possible severity levels, in order of decreasing severity, and their
meanings:

@table @code
@item :emergency
A problem that will seriously impair Emacs operation soon
if you do not attend to it promptly.
@item :error
A report of data or circumstances that are inherently wrong.
@item :warning
A report of data or circumstances that are not inherently wrong, but
raise suspicion of a possible problem.
@item :debug
A report of information that may be useful if you are debugging.
@end table

  When your program encounters invalid input data, it can either
signal a Lisp error by calling @code{error} or @code{signal} or report
a warning with severity @code{:error}.  Signaling a Lisp error is the
easiest thing to do, but it means the program cannot continue
processing.  If you want to take the trouble to implement a way to
continue processing despite the bad data, then reporting a warning of
severity @code{:error} is the right way to inform the user of the
problem.  For instance, the Emacs Lisp byte compiler can report an
error that way and continue compiling other functions.  (If the
program signals a Lisp error and then handles it with
@code{condition-case}, the user won't see the error message; it could
show the message to the user by reporting it as a warning.)

@cindex warning type
  Each warning has a @dfn{warning type} to classify it.  The type is a
list of symbols.  The first symbol should be the custom group that you
use for the program's user options.  For example, byte compiler
warnings use the warning type @code{(bytecomp)}.  You can also
subcategorize the warnings, if you wish, by using more symbols in the
list.

@defun display-warning type message &optional level buffer-name
This function reports a warning, using @var{message} as the message
and @var{type} as the warning type.  @var{level} should be the
severity level, with @code{:warning} being the default.

@var{buffer-name}, if non-@code{nil}, specifies the name of the buffer
for logging the warning.  By default, it is @samp{*Warnings*}.
@end defun

@defun lwarn type level message &rest args
This function reports a warning using the value of @code{(format
@var{message} @var{args}...)} as the message.  In other respects it is
equivalent to @code{display-warning}.
@end defun

@defun warn message &rest args
This function reports a warning using the value of @code{(format
@var{message} @var{args}...)} as the message, @code{(emacs)} as the
type, and @code{:warning} as the severity level.  It exists for
compatibility only; we recommend not using it, because you should
specify a specific warning type.
@end defun

@node Warning Variables
@subsection Warning Variables

  Programs can customize how their warnings appear by binding
the variables described in this section.

@defvar warning-levels
This list defines the meaning and severity order of the warning
severity levels.  Each element defines one severity level,
and they are arranged in order of decreasing severity.

Each element has the form @code{(@var{level} @var{string}
@var{function})}, where @var{level} is the severity level it defines.
@var{string} specifies the textual description of this level.
@var{string} should use @samp{%s} to specify where to put the warning
type information, or it can omit the @samp{%s} so as not to include
that information.

The optional @var{function}, if non-@code{nil}, is a function to call
with no arguments, to get the user's attention.

Normally you should not change the value of this variable.
@end defvar

@defvar warning-prefix-function
If non-@code{nil}, the value is a function to generate prefix text for
warnings.  Programs can bind the variable to a suitable function.
@code{display-warning} calls this function with the warnings buffer
current, and the function can insert text in it.  That text becomes
the beginning of the warning message.

The function is called with two arguments, the severity level and its
entry in @code{warning-levels}.  It should return a list to use as the
entry (this value need not be an actual member of
@code{warning-levels}).  By constructing this value, the function can
change the severity of the warning, or specify different handling for
a given severity level.

If the variable's value is @code{nil} then there is no function
to call.
@end defvar

@defvar warning-series
Programs can bind this variable to @code{t} to say that the next
warning should begin a series.  When several warnings form a series,
that means to leave point on the first warning of the series, rather
than keep moving it for each warning so that it appears on the last one.
The series ends when the local binding is unbound and
@code{warning-series} becomes @code{nil} again.

The value can also be a symbol with a function definition.  That is
equivalent to @code{t}, except that the next warning will also call
the function with no arguments with the warnings buffer current.  The
function can insert text which will serve as a header for the series
of warnings.

Once a series has begun, the value is a marker which points to the
buffer position in the warnings buffer of the start of the series.

The variable's normal value is @code{nil}, which means to handle
each warning separately.
@end defvar

@defvar warning-fill-prefix
When this variable is non-@code{nil}, it specifies a fill prefix to
use for filling each warning's text.
@end defvar

@defvar warning-type-format
This variable specifies the format for displaying the warning type
in the warning message.  The result of formatting the type this way
gets included in the message under the control of the string in the
entry in @code{warning-levels}.  The default value is @code{" (%s)"}.
If you bind it to @code{""} then the warning type won't appear at
all.
@end defvar

@node Warning Options
@subsection Warning Options

  These variables are used by users to control what happens
when a Lisp program reports a warning.

@defopt warning-minimum-level
This user option specifies the minimum severity level that should be
shown immediately to the user.  The default is @code{:warning}, which
means to immediately display all warnings except @code{:debug}
warnings.
@end defopt

@defopt warning-minimum-log-level
This user option specifies the minimum severity level that should be
logged in the warnings buffer.  The default is @code{:warning}, which
means to log all warnings except @code{:debug} warnings.
@end defopt

@defopt warning-suppress-types
This list specifies which warning types should not be displayed
immediately for the user.  Each element of the list should be a list
of symbols.  If its elements match the first elements in a warning
type, then that warning is not displayed immediately.
@end defopt

@defopt warning-suppress-log-types
This list specifies which warning types should not be logged in the
warnings buffer.  Each element of the list should be a list of
symbols.  If it matches the first few elements in a warning type, then
that warning is not logged.
@end defopt

@node Invisible Text
@section Invisible Text

@cindex invisible text
You can make characters @dfn{invisible}, so that they do not appear on
the screen, with the @code{invisible} property.  This can be either a
text property (@pxref{Text Properties}) or a property of an overlay
(@pxref{Overlays}).  Cursor motion also partly ignores these
characters; if the command loop finds point within them, it moves
point to the other side of them.

In the simplest case, any non-@code{nil} @code{invisible} property makes
a character invisible.  This is the default case---if you don't alter
the default value of @code{buffer-invisibility-spec}, this is how the
@code{invisible} property works.  You should normally use @code{t}
as the value of the @code{invisible} property if you don't plan
to set @code{buffer-invisibility-spec} yourself.

More generally, you can use the variable @code{buffer-invisibility-spec}
to control which values of the @code{invisible} property make text
invisible.  This permits you to classify the text into different subsets
in advance, by giving them different @code{invisible} values, and
subsequently make various subsets visible or invisible by changing the
value of @code{buffer-invisibility-spec}.

Controlling visibility with @code{buffer-invisibility-spec} is
especially useful in a program to display the list of entries in a
database.  It permits the implementation of convenient filtering
commands to view just a part of the entries in the database.  Setting
this variable is very fast, much faster than scanning all the text in
the buffer looking for properties to change.

@defvar buffer-invisibility-spec
This variable specifies which kinds of @code{invisible} properties
actually make a character invisible.  Setting this variable makes it
buffer-local.

@table @asis
@item @code{t}
A character is invisible if its @code{invisible} property is
non-@code{nil}.  This is the default.

@item a list
Each element of the list specifies a criterion for invisibility; if a
character's @code{invisible} property fits any one of these criteria,
the character is invisible.  The list can have two kinds of elements:

@table @code
@item @var{atom}
A character is invisible if its @code{invisible} property value
is @var{atom} or if it is a list with @var{atom} as a member.

@item (@var{atom} . t)
A character is invisible if its @code{invisible} property value
is @var{atom} or if it is a list with @var{atom} as a member.
Moreover, if this character is at the end of a line and is followed
by a visible newline, it displays an ellipsis.
@end table
@end table
@end defvar

  Two functions are specifically provided for adding elements to
@code{buffer-invisibility-spec} and removing elements from it.

@defun add-to-invisibility-spec element
This function adds the element @var{element} to
@code{buffer-invisibility-spec}.  If @code{buffer-invisibility-spec}
was @code{t}, it changes to a list, @code{(t)}, so that text whose
@code{invisible} property is @code{t} remains invisible.
@end defun

@defun remove-from-invisibility-spec element
This removes the element @var{element} from
@code{buffer-invisibility-spec}.  This does nothing if @var{element}
is not in the list.
@end defun

  A convention for use of @code{buffer-invisibility-spec} is that a
major mode should use the mode's own name as an element of
@code{buffer-invisibility-spec} and as the value of the
@code{invisible} property:

@example
;; @r{If you want to display an ellipsis:}
(add-to-invisibility-spec '(my-symbol . t))
;; @r{If you don't want ellipsis:}
(add-to-invisibility-spec 'my-symbol)

(overlay-put (make-overlay beginning end)
             'invisible 'my-symbol)

;; @r{When done with the overlays:}
(remove-from-invisibility-spec '(my-symbol . t))
;; @r{Or respectively:}
(remove-from-invisibility-spec 'my-symbol)
@end example

@vindex line-move-ignore-invisible
  Ordinarily, functions that operate on text or move point do not care
whether the text is invisible.  The user-level line motion commands
explicitly ignore invisible newlines if
@code{line-move-ignore-invisible} is non-@code{nil} (the default), but
only because they are explicitly programmed to do so.

  However, if a command ends with point inside or immediately before
invisible text, the main editing loop moves point further forward or
further backward (in the same direction that the command already moved
it) until that condition is no longer true.  Thus, if the command
moved point back into an invisible range, Emacs moves point back to
the beginning of that range, and then back one more character.  If the
command moved point forward into an invisible range, Emacs moves point
forward up to the first visible character that follows the invisible
text.

  Incremental search can make invisible overlays visible temporarily
and/or permanently when a match includes invisible text.  To enable
this, the overlay should have a non-@code{nil}
@code{isearch-open-invisible} property.  The property value should be a
function to be called with the overlay as an argument.  This function
should make the overlay visible permanently; it is used when the match
overlaps the overlay on exit from the search.

  During the search, such overlays are made temporarily visible by
temporarily modifying their invisible and intangible properties.  If you
want this to be done differently for a certain overlay, give it an
@code{isearch-open-invisible-temporary} property which is a function.
The function is called with two arguments: the first is the overlay, and
the second is @code{nil} to make the overlay visible, or @code{t} to
make it invisible again.

@node Selective Display
@section Selective Display
@cindex selective display

  @dfn{Selective display} refers to a pair of related features for
hiding certain lines on the screen.

  The first variant, explicit selective display, is designed for use
in a Lisp program: it controls which lines are hidden by altering the
text.  This kind of hiding in some ways resembles the effect of the
@code{invisible} property (@pxref{Invisible Text}), but the two
features are different and do not work the same way.

  In the second variant, the choice of lines to hide is made
automatically based on indentation.  This variant is designed to be a
user-level feature.

  The way you control explicit selective display is by replacing a
newline (control-j) with a carriage return (control-m).  The text that
was formerly a line following that newline is now hidden.  Strictly
speaking, it is temporarily no longer a line at all, since only
newlines can separate lines; it is now part of the previous line.

  Selective display does not directly affect editing commands.  For
example, @kbd{C-f} (@code{forward-char}) moves point unhesitatingly
into hidden text.  However, the replacement of newline characters with
carriage return characters affects some editing commands.  For
example, @code{next-line} skips hidden lines, since it searches only
for newlines.  Modes that use selective display can also define
commands that take account of the newlines, or that control which
parts of the text are hidden.

  When you write a selectively displayed buffer into a file, all the
control-m's are output as newlines.  This means that when you next read
in the file, it looks OK, with nothing hidden.  The selective display
effect is seen only within Emacs.

@defvar selective-display
This buffer-local variable enables selective display.  This means that
lines, or portions of lines, may be made hidden.

@itemize @bullet
@item
If the value of @code{selective-display} is @code{t}, then the character
control-m marks the start of hidden text; the control-m, and the rest
of the line following it, are not displayed.  This is explicit selective
display.

@item
If the value of @code{selective-display} is a positive integer, then
lines that start with more than that many columns of indentation are not
displayed.
@end itemize

When some portion of a buffer is hidden, the vertical movement
commands operate as if that portion did not exist, allowing a single
@code{next-line} command to skip any number of hidden lines.
However, character movement commands (such as @code{forward-char}) do
not skip the hidden portion, and it is possible (if tricky) to insert
or delete text in an hidden portion.

In the examples below, we show the @emph{display appearance} of the
buffer @code{foo}, which changes with the value of
@code{selective-display}.  The @emph{contents} of the buffer do not
change.

@example
@group
(setq selective-display nil)
     @result{} nil

---------- Buffer: foo ----------
1 on this column
 2on this column
  3n this column
  3n this column
 2on this column
1 on this column
---------- Buffer: foo ----------
@end group

@group
(setq selective-display 2)
     @result{} 2

---------- Buffer: foo ----------
1 on this column
 2on this column
 2on this column
1 on this column
---------- Buffer: foo ----------
@end group
@end example
@end defvar

@defvar selective-display-ellipses
If this buffer-local variable is non-@code{nil}, then Emacs displays
@samp{@dots{}} at the end of a line that is followed by hidden text.
This example is a continuation of the previous one.

@example
@group
(setq selective-display-ellipses t)
     @result{} t

---------- Buffer: foo ----------
1 on this column
 2on this column ...
 2on this column
1 on this column
---------- Buffer: foo ----------
@end group
@end example

You can use a display table to substitute other text for the ellipsis
(@samp{@dots{}}).  @xref{Display Tables}.
@end defvar

@node Temporary Displays
@section Temporary Displays

  Temporary displays are used by Lisp programs to put output into a
buffer and then present it to the user for perusal rather than for
editing.  Many help commands use this feature.

@defspec with-output-to-temp-buffer buffer-name forms@dots{}
This function executes @var{forms} while arranging to insert any output
they print into the buffer named @var{buffer-name}, which is first
created if necessary, and put into Help mode.  Finally, the buffer is
displayed in some window, but not selected.

If the @var{forms} do not change the major mode in the output buffer,
so that it is still Help mode at the end of their execution, then
@code{with-output-to-temp-buffer} makes this buffer read-only at the
end, and also scans it for function and variable names to make them
into clickable cross-references.  @xref{Docstring hyperlinks, , Tips
for Documentation Strings}, in particular the item on hyperlinks in
documentation strings, for more details.

The string @var{buffer-name} specifies the temporary buffer, which
need not already exist.  The argument must be a string, not a buffer.
The buffer is erased initially (with no questions asked), and it is
marked as unmodified after @code{with-output-to-temp-buffer} exits.

@code{with-output-to-temp-buffer} binds @code{standard-output} to the
temporary buffer, then it evaluates the forms in @var{forms}.  Output
using the Lisp output functions within @var{forms} goes by default to
that buffer (but screen display and messages in the echo area, although
they are ``output'' in the general sense of the word, are not affected).
@xref{Output Functions}.

Several hooks are available for customizing the behavior
of this construct; they are listed below.

The value of the last form in @var{forms} is returned.

@example
@group
---------- Buffer: foo ----------
 This is the contents of foo.
---------- Buffer: foo ----------
@end group

@group
(with-output-to-temp-buffer "foo"
    (print 20)
    (print standard-output))
@result{} #<buffer foo>

---------- Buffer: foo ----------
20

#<buffer foo>

---------- Buffer: foo ----------
@end group
@end example
@end defspec

@defvar temp-buffer-show-function
If this variable is non-@code{nil}, @code{with-output-to-temp-buffer}
calls it as a function to do the job of displaying a help buffer.  The
function gets one argument, which is the buffer it should display.

It is a good idea for this function to run @code{temp-buffer-show-hook}
just as @code{with-output-to-temp-buffer} normally would, inside of
@code{save-selected-window} and with the chosen window and buffer
selected.
@end defvar

@defvar temp-buffer-setup-hook
This normal hook is run by @code{with-output-to-temp-buffer} before
evaluating @var{body}.  When the hook runs, the temporary buffer is
current.  This hook is normally set up with a function to put the
buffer in Help mode.
@end defvar

@defvar temp-buffer-show-hook
This normal hook is run by @code{with-output-to-temp-buffer} after
displaying the temporary buffer.  When the hook runs, the temporary buffer
is current, and the window it was displayed in is selected.  This hook
is normally set up with a function to make the buffer read only, and
find function names and variable names in it, provided the major mode
is Help mode.
@end defvar

@defun momentary-string-display string position &optional char message
This function momentarily displays @var{string} in the current buffer at
@var{position}.  It has no effect on the undo list or on the buffer's
modification status.

The momentary display remains until the next input event.  If the next
input event is @var{char}, @code{momentary-string-display} ignores it
and returns.  Otherwise, that event remains buffered for subsequent use
as input.  Thus, typing @var{char} will simply remove the string from
the display, while typing (say) @kbd{C-f} will remove the string from
the display and later (presumably) move point forward.  The argument
@var{char} is a space by default.

The return value of @code{momentary-string-display} is not meaningful.

If the string @var{string} does not contain control characters, you can
do the same job in a more general way by creating (and then subsequently
deleting) an overlay with a @code{before-string} property.
@xref{Overlay Properties}.

If @var{message} is non-@code{nil}, it is displayed in the echo area
while @var{string} is displayed in the buffer.  If it is @code{nil}, a
default message says to type @var{char} to continue.

In this example, point is initially located at the beginning of the
second line:

@example
@group
---------- Buffer: foo ----------
This is the contents of foo.
@point{}Second line.
---------- Buffer: foo ----------
@end group

@group
(momentary-string-display
  "**** Important Message! ****"
  (point) ?\r
  "Type RET when done reading")
@result{} t
@end group

@group
---------- Buffer: foo ----------
This is the contents of foo.
**** Important Message! ****Second line.
---------- Buffer: foo ----------

---------- Echo Area ----------
Type RET when done reading
---------- Echo Area ----------
@end group
@end example
@end defun

@node Overlays
@section Overlays
@cindex overlays

You can use @dfn{overlays} to alter the appearance of a buffer's text on
the screen, for the sake of presentation features.  An overlay is an
object that belongs to a particular buffer, and has a specified
beginning and end.  It also has properties that you can examine and set;
these affect the display of the text within the overlay.

An overlay uses markers to record its beginning and end; thus,
editing the text of the buffer adjusts the beginning and end of each
overlay so that it stays with the text.  When you create the overlay,
you can specify whether text inserted at the beginning should be
inside the overlay or outside, and likewise for the end of the overlay.

@menu
* Managing Overlays::   Creating and moving overlays.
* Overlay Properties::  How to read and set properties.
			What properties do to the screen display.
* Finding Overlays::    Searching for overlays.
@end menu

@node Managing Overlays
@subsection Managing Overlays

  This section describes the functions to create, delete and move
overlays, and to examine their contents.  Overlay changes are not
recorded in the buffer's undo list, since the overlays are not
part of the buffer's contents.

@defun overlayp object
This function returns @code{t} if @var{object} is an overlay.
@end defun

@defun make-overlay start end &optional buffer front-advance rear-advance
This function creates and returns an overlay that belongs to
@var{buffer} and ranges from @var{start} to @var{end}.  Both @var{start}
and @var{end} must specify buffer positions; they may be integers or
markers.  If @var{buffer} is omitted, the overlay is created in the
current buffer.

The arguments @var{front-advance} and @var{rear-advance} specify the
marker insertion type for the start of the overlay and for the end of
the overlay, respectively.  @xref{Marker Insertion Types}.  If they
are both @code{nil}, the default, then the overlay extends to include
any text inserted at the beginning, but not text inserted at the end.
If @var{front-advance} is non-@code{nil}, text inserted at the
beginning of the overlay is excluded from the overlay.  If
@var{rear-advance} is non-@code{nil}, text inserted at the end of the
overlay is included in the overlay.
@end defun

@defun overlay-start overlay
This function returns the position at which @var{overlay} starts,
as an integer.
@end defun

@defun overlay-end overlay
This function returns the position at which @var{overlay} ends,
as an integer.
@end defun

@defun overlay-buffer overlay
This function returns the buffer that @var{overlay} belongs to.  It
returns @code{nil} if @var{overlay} has been deleted.
@end defun

@defun delete-overlay overlay
This function deletes @var{overlay}.  The overlay continues to exist as
a Lisp object, and its property list is unchanged, but it ceases to be
attached to the buffer it belonged to, and ceases to have any effect on
display.

A deleted overlay is not permanently disconnected.  You can give it a
position in a buffer again by calling @code{move-overlay}.
@end defun

@defun move-overlay overlay start end &optional buffer
This function moves @var{overlay} to @var{buffer}, and places its bounds
at @var{start} and @var{end}.  Both arguments @var{start} and @var{end}
must specify buffer positions; they may be integers or markers.

If @var{buffer} is omitted, @var{overlay} stays in the same buffer it
was already associated with; if @var{overlay} was deleted, it goes into
the current buffer.

The return value is @var{overlay}.

This is the only valid way to change the endpoints of an overlay.  Do
not try modifying the markers in the overlay by hand, as that fails to
update other vital data structures and can cause some overlays to be
``lost''.
@end defun

@defun remove-overlays &optional start end name value
This function removes all the overlays between @var{start} and
@var{end} whose property @var{name} has the value @var{value}.  It can
move the endpoints of the overlays in the region, or split them.

If @var{name} is omitted or @code{nil}, it means to delete all overlays in
the specified region.  If @var{start} and/or @var{end} are omitted or
@code{nil}, that means the beginning and end of the buffer respectively.
Therefore, @code{(remove-overlays)} removes all the overlays in the
current buffer.
@end defun

  Here are some examples:

@example
;; @r{Create an overlay.}
(setq foo (make-overlay 1 10))
     @result{} #<overlay from 1 to 10 in display.texi>
(overlay-start foo)
     @result{} 1
(overlay-end foo)
     @result{} 10
(overlay-buffer foo)
     @result{} #<buffer display.texi>
;; @r{Give it a property we can check later.}
(overlay-put foo 'happy t)
     @result{} t
;; @r{Verify the property is present.}
(overlay-get foo 'happy)
     @result{} t
;; @r{Move the overlay.}
(move-overlay foo 5 20)
     @result{} #<overlay from 5 to 20 in display.texi>
(overlay-start foo)
     @result{} 5
(overlay-end foo)
     @result{} 20
;; @r{Delete the overlay.}
(delete-overlay foo)
     @result{} nil
;; @r{Verify it is deleted.}
foo
     @result{} #<overlay in no buffer>
;; @r{A deleted overlay has no position.}
(overlay-start foo)
     @result{} nil
(overlay-end foo)
     @result{} nil
(overlay-buffer foo)
     @result{} nil
;; @r{Undelete the overlay.}
(move-overlay foo 1 20)
     @result{} #<overlay from 1 to 20 in display.texi>
;; @r{Verify the results.}
(overlay-start foo)
     @result{} 1
(overlay-end foo)
     @result{} 20
(overlay-buffer foo)
     @result{} #<buffer display.texi>
;; @r{Moving and deleting the overlay does not change its properties.}
(overlay-get foo 'happy)
     @result{} t
@end example

@node Overlay Properties
@subsection Overlay Properties

  Overlay properties are like text properties in that the properties that
alter how a character is displayed can come from either source.  But in
most respects they are different.  @xref{Text Properties}, for comparison.

  Text properties are considered a part of the text; overlays and
their properties are specifically considered not to be part of the
text.  Thus, copying text between various buffers and strings
preserves text properties, but does not try to preserve overlays.
Changing a buffer's text properties marks the buffer as modified,
while moving an overlay or changing its properties does not.  Unlike
text property changes, overlay property changes are not recorded in
the buffer's undo list.

  These functions read and set the properties of an overlay:

@defun overlay-get overlay prop
This function returns the value of property @var{prop} recorded in
@var{overlay}, if any.  If @var{overlay} does not record any value for
that property, but it does have a @code{category} property which is a
symbol, that symbol's @var{prop} property is used.  Otherwise, the value
is @code{nil}.
@end defun

@defun overlay-put overlay prop value
This function sets the value of property @var{prop} recorded in
@var{overlay} to @var{value}.  It returns @var{value}.
@end defun

@defun overlay-properties overlay
This returns a copy of the property list of @var{overlay}.
@end defun

  See also the function @code{get-char-property} which checks both
overlay properties and text properties for a given character.
@xref{Examining Properties}.

  Many overlay properties have special meanings; here is a table
of them:

@table @code
@item priority
@kindex priority @r{(overlay property)}
This property's value (which should be a nonnegative integer number)
determines the priority of the overlay.  The priority matters when two
or more overlays cover the same character and both specify the same
property; the one whose @code{priority} value is larger takes priority
over the other.  For the @code{face} property, the higher priority
value does not completely replace the other; instead, its face
attributes override the face attributes of the lower priority
@code{face} property.

Currently, all overlays take priority over text properties.  Please
avoid using negative priority values, as we have not yet decided just
what they should mean.

@item window
@kindex window @r{(overlay property)}
If the @code{window} property is non-@code{nil}, then the overlay
applies only on that window.

@item category
@kindex category @r{(overlay property)}
If an overlay has a @code{category} property, we call it the
@dfn{category} of the overlay.  It should be a symbol.  The properties
of the symbol serve as defaults for the properties of the overlay.

@item face
@kindex face @r{(overlay property)}
This property controls the way text is displayed---for example, which
font and which colors.  @xref{Faces}, for more information.

In the simplest case, the value is a face name.  It can also be a list;
then each element can be any of these possibilities:

@itemize @bullet
@item
A face name (a symbol or string).

@item
A property list of face attributes.  This has the form (@var{keyword}
@var{value} @dots{}), where each @var{keyword} is a face attribute
name and @var{value} is a meaningful value for that attribute.  With
this feature, you do not need to create a face each time you want to
specify a particular attribute for certain text.  @xref{Face
Attributes}.

@item
A cons cell of the form @code{(foreground-color . @var{color-name})} or
@code{(background-color . @var{color-name})}.  These elements specify
just the foreground color or just the background color.

@code{(foreground-color . @var{color-name})} has the same effect as
@code{(:foreground @var{color-name})}; likewise for the background.
@end itemize

@item mouse-face
@kindex mouse-face @r{(overlay property)}
This property is used instead of @code{face} when the mouse is within
the range of the overlay.

@item display
@kindex display @r{(overlay property)}
This property activates various features that change the
way text is displayed.  For example, it can make text appear taller
or shorter, higher or lower, wider or narrower, or replaced with an image.
@xref{Display Property}.

@item help-echo
@kindex help-echo @r{(overlay property)}
If an overlay has a @code{help-echo} property, then when you move the
mouse onto the text in the overlay, Emacs displays a help string in the
echo area, or in the tooltip window.  For details see @ref{Text
help-echo}.

@item modification-hooks
@kindex modification-hooks @r{(overlay property)}
This property's value is a list of functions to be called if any
character within the overlay is changed or if text is inserted strictly
within the overlay.

The hook functions are called both before and after each change.
If the functions save the information they receive, and compare notes
between calls, they can determine exactly what change has been made
in the buffer text.

When called before a change, each function receives four arguments: the
overlay, @code{nil}, and the beginning and end of the text range to be
modified.

When called after a change, each function receives five arguments: the
overlay, @code{t}, the beginning and end of the text range just
modified, and the length of the pre-change text replaced by that range.
(For an insertion, the pre-change length is zero; for a deletion, that
length is the number of characters deleted, and the post-change
beginning and end are equal.)

If these functions modify the buffer, they should bind
@code{inhibit-modification-hooks} to @code{t} around doing so, to
avoid confusing the internal mechanism that calls these hooks.

@item insert-in-front-hooks
@kindex insert-in-front-hooks @r{(overlay property)}
This property's value is a list of functions to be called before and
after inserting text right at the beginning of the overlay.  The calling
conventions are the same as for the @code{modification-hooks} functions.

@item insert-behind-hooks
@kindex insert-behind-hooks @r{(overlay property)}
This property's value is a list of functions to be called before and
after inserting text right at the end of the overlay.  The calling
conventions are the same as for the @code{modification-hooks} functions.

@item invisible
@kindex invisible @r{(overlay property)}
The @code{invisible} property can make the text in the overlay
invisible, which means that it does not appear on the screen.
@xref{Invisible Text}, for details.

@item intangible
@kindex intangible @r{(overlay property)}
The @code{intangible} property on an overlay works just like the
@code{intangible} text property.  @xref{Special Properties}, for details.

@item isearch-open-invisible
This property tells incremental search how to make an invisible overlay
visible, permanently, if the final match overlaps it.  @xref{Invisible
Text}.

@item isearch-open-invisible-temporary
This property tells incremental search how to make an invisible overlay
visible, temporarily, during the search.  @xref{Invisible Text}.

@item before-string
@kindex before-string @r{(overlay property)}
This property's value is a string to add to the display at the beginning
of the overlay.  The string does not appear in the buffer in any
sense---only on the screen.

@item after-string
@kindex after-string @r{(overlay property)}
This property's value is a string to add to the display at the end of
the overlay.  The string does not appear in the buffer in any
sense---only on the screen.

@item evaporate
@kindex evaporate @r{(overlay property)}
If this property is non-@code{nil}, the overlay is deleted automatically
if it becomes empty (i.e., if its length becomes zero).  If you give
an empty overlay a non-@code{nil} @code{evaporate} property, that deletes
it immediately.

@item local-map
@cindex keymap of character (and overlays)
@kindex local-map @r{(overlay property)}
If this property is non-@code{nil}, it specifies a keymap for a portion
of the text.  The property's value replaces the buffer's local map, when
the character after point is within the overlay.  @xref{Active Keymaps}.

@item keymap
@kindex keymap @r{(overlay property)}
The @code{keymap} property is similar to @code{local-map} but overrides the
buffer's local map (and the map specified by the @code{local-map}
property) rather than replacing it.
@end table

@node Finding Overlays
@subsection Searching for Overlays

@defun overlays-at pos
This function returns a list of all the overlays that cover the
character at position @var{pos} in the current buffer.  The list is in
no particular order.  An overlay contains position @var{pos} if it
begins at or before @var{pos}, and ends after @var{pos}.

To illustrate usage, here is a Lisp function that returns a list of the
overlays that specify property @var{prop} for the character at point:

@smallexample
(defun find-overlays-specifying (prop)
  (let ((overlays (overlays-at (point)))
        found)
    (while overlays
      (let ((overlay (car overlays)))
        (if (overlay-get overlay prop)
            (setq found (cons overlay found))))
      (setq overlays (cdr overlays)))
    found))
@end smallexample
@end defun

@defun overlays-in beg end
This function returns a list of the overlays that overlap the region
@var{beg} through @var{end}.  ``Overlap'' means that at least one
character is contained within the overlay and also contained within the
specified region; however, empty overlays are included in the result if
they are located at @var{beg}, or strictly between @var{beg} and @var{end}.
@end defun

@defun next-overlay-change pos
This function returns the buffer position of the next beginning or end
of an overlay, after @var{pos}.  If there is none, it returns
@code{(point-max)}.
@end defun

@defun previous-overlay-change pos
This function returns the buffer position of the previous beginning or
end of an overlay, before @var{pos}.  If there is none, it returns
@code{(point-min)}.
@end defun

  Here's a function which uses @code{next-overlay-change} to search
for the next character which gets a given property @code{prop} from
either its overlays or its text properties (@pxref{Property Search}):

@smallexample
(defun find-overlay-prop (prop)
  (save-excursion
    (while (and (not (eobp))
                (not (get-char-property (point) prop)))
      (goto-char (min (next-overlay-change (point))
                      (next-single-property-change (point) prop))))
    (point)))
@end smallexample

  Now you can search for a @code{happy} property like this:

@smallexample
(find-overlay-prop 'happy)
@end smallexample

@node Width
@section Width

Since not all characters have the same width, these functions let you
check the width of a character.  @xref{Primitive Indent}, and
@ref{Screen Lines}, for related functions.

@defun char-width char
This function returns the width in columns of the character @var{char},
if it were displayed in the current buffer and the selected window.
@end defun

@defun string-width string
This function returns the width in columns of the string @var{string},
if it were displayed in the current buffer and the selected window.
@end defun

@defun truncate-string-to-width string width &optional start-column padding ellipsis
This function returns the part of @var{string} that fits within
@var{width} columns, as a new string.

If @var{string} does not reach @var{width}, then the result ends where
@var{string} ends.  If one multi-column character in @var{string}
extends across the column @var{width}, that character is not included in
the result.  Thus, the result can fall short of @var{width} but cannot
go beyond it.

The optional argument @var{start-column} specifies the starting column.
If this is non-@code{nil}, then the first @var{start-column} columns of
the string are omitted from the value.  If one multi-column character in
@var{string} extends across the column @var{start-column}, that
character is not included.

The optional argument @var{padding}, if non-@code{nil}, is a padding
character added at the beginning and end of the result string, to extend
it to exactly @var{width} columns.  The padding character is used at the
end of the result if it falls short of @var{width}.  It is also used at
the beginning of the result if one multi-column character in
@var{string} extends across the column @var{start-column}.

If @var{ellipsis} is non-@code{nil}, it should be a string which will
replace the end of @var{str} (including any padding) if it extends
beyond @var{end-column}, unless the display width of @var{str} is
equal to or less than the display width of @var{ellipsis}.  If
@var{ellipsis} is non-@code{nil} and not a string, it stands for
@code{"..."}.

@example
(truncate-string-to-width "\tab\t" 12 4)
     @result{} "ab"
(truncate-string-to-width "\tab\t" 12 4 ?\s)
     @result{} "    ab  "
@end example
@end defun

@node Line Height
@section Line Height
@cindex line height

  The total height of each display line consists of the height of the
contents of the line, plus optional additional vertical line spacing
above or below the display line.

  The height of the line contents is the maximum height of any
character or image on that display line, including the final newline
if there is one.  (A display line that is continued doesn't include a
final newline.)  That is the default line height, if you do nothing to
specify a greater height.  (In the most common case, this equals the
height of the default frame font.)

  There are several ways to explicitly specify a larger line height,
either by specifying an absolute height for the display line, or by
specifying vertical space.  However, no matter what you specify, the
actual line height can never be less than the default.

@kindex line-height @r{(text property)}
  A newline can have a @code{line-height} text or overlay property
that controls the total height of the display line ending in that
newline.

  If the property value is @code{t}, the newline character has no
effect on the displayed height of the line---the visible contents
alone determine the height.  This is useful for tiling small images
(or image slices) without adding blank areas between the images.

  If the property value is a list of the form @code{(@var{height}
@var{total})}, that adds extra space @emph{below} the display line.
First Emacs uses @var{height} as a height spec to control extra space
@emph{above} the line; then it adds enough space @emph{below} the line
to bring the total line height up to @var{total}.  In this case, the
other ways to specify the line spacing are ignored.

  Any other kind of property value is a height spec, which translates
into a number---the specified line height.  There are several ways to
write a height spec; here's how each of them translates into a number:

@table @code
@item @var{integer}
If the height spec is a positive integer, the height value is that integer.
@item @var{float}
If the height spec is a float, @var{float}, the numeric height value
is @var{float} times the frame's default line height.
@item (@var{face} . @var{ratio})
If the height spec is a cons of the format shown, the numeric height
is @var{ratio} times the height of face @var{face}.  @var{ratio} can
be any type of number, or @code{nil} which means a ratio of 1.
If @var{face} is @code{t}, it refers to the current face.
@item (nil . @var{ratio})
If the height spec is a cons of the format shown, the numeric height
is @var{ratio} times the height of the contents of the line.
@end table

  Thus, any valid height spec determines the height in pixels, one way
or another.  If the line contents' height is less than that, Emacs
adds extra vertical space above the line to achieve the specified
total height.

  If you don't specify the @code{line-height} property, the line's
height consists of the contents' height plus the line spacing.
There are several ways to specify the line spacing for different
parts of Emacs text.

@vindex default-line-spacing
  You can specify the line spacing for all lines in a frame with the
@code{line-spacing} frame parameter (@pxref{Layout Parameters}).
However, if the variable @code{default-line-spacing} is
non-@code{nil}, it overrides the frame's @code{line-spacing}
parameter.  An integer value specifies the number of pixels put below
lines on graphical displays.  A floating point number specifies the
spacing relative to the frame's default line height.

@vindex line-spacing
  You can specify the line spacing for all lines in a buffer via the
buffer-local @code{line-spacing} variable.  An integer value specifies
the number of pixels put below lines on graphical displays.  A floating
point number specifies the spacing relative to the default frame line
height.  This overrides line spacings specified for the frame.

@kindex line-spacing @r{(text property)}
  Finally, a newline can have a @code{line-spacing} text or overlay
property that overrides the default frame line spacing and the buffer
local @code{line-spacing} variable, for the display line ending in
that newline.

  One way or another, these mechanisms specify a Lisp value for the
spacing of each line.  The value is a height spec, and it translates
into a Lisp value as described above.  However, in this case the
numeric height value specifies the line spacing, rather than the line
height.

@node Faces
@section Faces
@cindex faces

  A @dfn{face} is a named collection of graphical attributes: font
family, foreground color, background color, optional underlining, and
many others.  Faces are used in Emacs to control the style of display of
particular parts of the text or the frame.  @xref{Standard Faces,,,
emacs, The GNU Emacs Manual}, for the list of faces Emacs normally
comes with.

@cindex face id
Each face has its own @dfn{face number}, which distinguishes faces at
low levels within Emacs.  However, for most purposes, you refer to
faces in Lisp programs by the symbols that name them.

@defun facep object
This function returns @code{t} if @var{object} is a face name string
or symbol (or if it is a vector of the kind used internally to record
face data).  It returns @code{nil} otherwise.
@end defun

Each face name is meaningful for all frames, and by default it has the
same meaning in all frames.  But you can arrange to give a particular
face name a special meaning in one frame if you wish.

@menu
* Defining Faces::      How to define a face with @code{defface}.
* Face Attributes::     What is in a face?
* Attribute Functions::  Functions to examine and set face attributes.
* Displaying Faces::     How Emacs combines the faces specified for a character.
* Font Selection::      Finding the best available font for a face.
* Face Functions::      How to define and examine faces.
* Auto Faces::          Hook for automatic face assignment.
* Font Lookup::         Looking up the names of available fonts
                          and information about them.
* Fontsets::            A fontset is a collection of fonts
                          that handle a range of character sets.
@end menu

@node Defining Faces
@subsection Defining Faces

  The way to define a new face is with @code{defface}.  This creates a
kind of customization item (@pxref{Customization}) which the user can
customize using the Customization buffer (@pxref{Easy Customization,,,
emacs, The GNU Emacs Manual}).

@defmac defface face spec doc [keyword value]@dots{}
This declares @var{face} as a customizable face that defaults
according to @var{spec}.  You should not quote the symbol @var{face},
and it should not end in @samp{-face} (that would be redundant).  The
argument @var{doc} specifies the face documentation.  The keywords you
can use in @code{defface} are the same as in @code{defgroup} and
@code{defcustom} (@pxref{Common Keywords}).

When @code{defface} executes, it defines the face according to
@var{spec}, then uses any customizations that were read from the
init file (@pxref{Init File}) to override that specification.

The purpose of @var{spec} is to specify how the face should appear on
different kinds of terminals.  It should be an alist whose elements
have the form @code{(@var{display} @var{atts})}.  Each element's
@sc{car}, @var{display}, specifies a class of terminals.  (The first
element, if its @sc{car} is @code{default}, is special---it specifies
defaults for the remaining elements).  The element's @sc{cadr},
@var{atts}, is a list of face attributes and their values; it
specifies what the face should look like on that kind of terminal.
The possible attributes are defined in the value of
@code{custom-face-attributes}.

The @var{display} part of an element of @var{spec} determines which
frames the element matches.  If more than one element of @var{spec}
matches a given frame, the first element that matches is the one used
for that frame.  There are three possibilities for @var{display}:

@table @asis
@item @code{default}
This element of @var{spec} doesn't match any frames; instead, it
specifies defaults that apply to all frames.  This kind of element, if
used, must be the first element of @var{spec}.  Each of the following
elements can override any or all of these defaults.

@item @code{t}
This element of @var{spec} matches all frames.  Therefore, any
subsequent elements of @var{spec} are never used.  Normally
@code{t} is used in the last (or only) element of @var{spec}.

@item a list
If @var{display} is a list, each element should have the form
@code{(@var{characteristic} @var{value}@dots{})}.  Here
@var{characteristic} specifies a way of classifying frames, and the
@var{value}s are possible classifications which @var{display} should
apply to.  Here are the possible values of @var{characteristic}:

@table @code
@item type
The kind of window system the frame uses---either @code{graphic} (any
graphics-capable display), @code{x}, @code{pc} (for the MS-DOS console),
@code{w32} (for MS Windows 9X/NT/2K/XP), @code{mac} (for the Macintosh
display), or @code{tty} (a non-graphics-capable display).
@xref{Window Systems, window-system}.

@item class
What kinds of colors the frame supports---either @code{color},
@code{grayscale}, or @code{mono}.

@item background
The kind of background---either @code{light} or @code{dark}.

@item min-colors
An integer that represents the minimum number of colors the frame
should support.  This matches a frame if its
@code{display-color-cells} value is at least the specified integer.

@item supports
Whether or not the frame can display the face attributes given in
@var{value}@dots{} (@pxref{Face Attributes}).  See the documentation
for the function @code{display-supports-face-attributes-p} for more
information on exactly how this testing is done.  @xref{Display Face
Attribute Testing}.
@end table

If an element of @var{display} specifies more than one @var{value} for a
given @var{characteristic}, any of those values is acceptable.  If
@var{display} has more than one element, each element should specify a
different @var{characteristic}; then @emph{each} characteristic of the
frame must match one of the @var{value}s specified for it in
@var{display}.
@end table
@end defmac

  Here's how the standard face @code{region} is defined:

@example
@group
  '((((class color) (min-colors 88) (background dark))
     :background "blue3")
@end group
    (((class color) (min-colors 88) (background light))
     :background "lightgoldenrod2")
    (((class color) (min-colors 16) (background dark))
     :background "blue3")
    (((class color) (min-colors 16) (background light))
     :background "lightgoldenrod2")
    (((class color) (min-colors 8))
     :background "blue" :foreground "white")
    (((type tty) (class mono))
     :inverse-video t)
    (t :background "gray"))
@group
  "Basic face for highlighting the region."
  :group 'basic-faces)
@end group
@end example

  Internally, @code{defface} uses the symbol property
@code{face-defface-spec} to record the face attributes specified in
@code{defface}, @code{saved-face} for the attributes saved by the user
with the customization buffer, @code{customized-face} for the
attributes customized by the user for the current session, but not
saved, and @code{face-documentation} for the documentation string.

@defopt frame-background-mode
This option, if non-@code{nil}, specifies the background type to use for
interpreting face definitions.  If it is @code{dark}, then Emacs treats
all frames as if they had a dark background, regardless of their actual
background colors.  If it is @code{light}, then Emacs treats all frames
as if they had a light background.
@end defopt

@node Face Attributes
@subsection Face Attributes
@cindex face attributes

  The effect of using a face is determined by a fixed set of @dfn{face
attributes}.  This table lists all the face attributes, and what they
mean.  Note that in general, more than one face can be specified for a
given piece of text; when that happens, the attributes of all the faces
are merged to specify how to display the text.  @xref{Displaying Faces}.

  Any attribute in a face can have the value @code{unspecified}.  This
means the face doesn't specify that attribute.  In face merging, when
the first face fails to specify a particular attribute, that means the
next face gets a chance.  However, the @code{default} face must
specify all attributes.

  Some of these font attributes are meaningful only on certain kinds of
displays---if your display cannot handle a certain attribute, the
attribute is ignored.  (The attributes @code{:family}, @code{:width},
@code{:height}, @code{:weight}, and @code{:slant} correspond to parts of
an X Logical Font Descriptor.)

@table @code
@item :family
Font family name, or fontset name (@pxref{Fontsets}).  If you specify a
font family name, the wild-card characters @samp{*} and @samp{?} are
allowed.

@item :width
Relative proportionate width, also known as the character set width or
set width.  This should be one of the symbols @code{ultra-condensed},
@code{extra-condensed}, @code{condensed}, @code{semi-condensed},
@code{normal}, @code{semi-expanded}, @code{expanded},
@code{extra-expanded}, or @code{ultra-expanded}.

@item :height
Either the font height, an integer in units of 1/10 point, a floating
point number specifying the amount by which to scale the height of any
underlying face, or a function, which is called with the old height
(from the underlying face), and should return the new height.

@item :weight
Font weight---a symbol from this series (from most dense to most faint):
@code{ultra-bold}, @code{extra-bold}, @code{bold}, @code{semi-bold},
@code{normal}, @code{semi-light}, @code{light}, @code{extra-light},
or @code{ultra-light}.

On a text-only terminal, any weight greater than normal is displayed as
extra bright, and any weight less than normal is displayed as
half-bright (provided the terminal supports the feature).

@item :slant
Font slant---one of the symbols @code{italic}, @code{oblique}, @code{normal},
@code{reverse-italic}, or @code{reverse-oblique}.

On a text-only terminal, slanted text is displayed as half-bright, if
the terminal supports the feature.

@item :foreground
Foreground color, a string.  The value can be a system-defined color
name, or a hexadecimal color specification of the form
@samp{#@var{rr}@var{gg}@var{bb}}.  (@samp{#000000} is black,
@samp{#ff0000} is red, @samp{#00ff00} is green, @samp{#0000ff} is
blue, and @samp{#ffffff} is white.)

@item :background
Background color, a string, like the foreground color.

@item :inverse-video
Whether or not characters should be displayed in inverse video.  The
value should be @code{t} (yes) or @code{nil} (no).

@item :stipple
The background stipple, a bitmap.

The value can be a string; that should be the name of a file containing
external-format X bitmap data.  The file is found in the directories
listed in the variable @code{x-bitmap-file-path}.

Alternatively, the value can specify the bitmap directly, with a list
of the form @code{(@var{width} @var{height} @var{data})}.  Here,
@var{width} and @var{height} specify the size in pixels, and
@var{data} is a string containing the raw bits of the bitmap, row by
row.  Each row occupies @math{(@var{width} + 7) / 8} consecutive bytes
in the string (which should be a unibyte string for best results).
This means that each row always occupies at least one whole byte.

If the value is @code{nil}, that means use no stipple pattern.

Normally you do not need to set the stipple attribute, because it is
used automatically to handle certain shades of gray.

@item :underline
Whether or not characters should be underlined, and in what color.  If
the value is @code{t}, underlining uses the foreground color of the
face.  If the value is a string, underlining uses that color.  The
value @code{nil} means do not underline.

@item :overline
Whether or not characters should be overlined, and in what color.
The value is used like that of @code{:underline}.

@item :strike-through
Whether or not characters should be strike-through, and in what
color.  The value is used like that of @code{:underline}.

@item :inherit
The name of a face from which to inherit attributes, or a list of face
names.  Attributes from inherited faces are merged into the face like an
underlying face would be, with higher priority than underlying faces.
If a list of faces is used, attributes from faces earlier in the list
override those from later faces.

@item :box
Whether or not a box should be drawn around characters, its color, the
width of the box lines, and 3D appearance.
@end table

  Here are the possible values of the @code{:box} attribute, and what
they mean:

@table @asis
@item @code{nil}
Don't draw a box.

@item @code{t}
Draw a box with lines of width 1, in the foreground color.

@item @var{color}
Draw a box with lines of width 1, in color @var{color}.

@item @code{(:line-width @var{width} :color @var{color} :style @var{style})}
This way you can explicitly specify all aspects of the box.  The value
@var{width} specifies the width of the lines to draw; it defaults to 1.

The value @var{color} specifies the color to draw with.  The default is
the foreground color of the face for simple boxes, and the background
color of the face for 3D boxes.

The value @var{style} specifies whether to draw a 3D box.  If it is
@code{released-button}, the box looks like a 3D button that is not being
pressed.  If it is @code{pressed-button}, the box looks like a 3D button
that is being pressed.  If it is @code{nil} or omitted, a plain 2D box
is used.
@end table

  In older versions of Emacs, before @code{:family}, @code{:height},
@code{:width}, @code{:weight}, and @code{:slant} existed, these
attributes were used to specify the type face.  They are now
semi-obsolete, but they still work:

@table @code
@item :font
This attribute specifies the font name.

@item :bold
A non-@code{nil} value specifies a bold font.

@item :italic
A non-@code{nil} value specifies an italic font.
@end table

  For compatibility, you can still set these ``attributes'', even
though they are not real face attributes.  Here is what that does:

@table @code
@item :font
You can specify an X font name as the ``value'' of this ``attribute'';
that sets the @code{:family}, @code{:width}, @code{:height},
@code{:weight}, and @code{:slant} attributes according to the font name.

If the value is a pattern with wildcards, the first font that matches
the pattern is used to set these attributes.

@item :bold
A non-@code{nil} makes the face bold; @code{nil} makes it normal.
This actually works by setting the @code{:weight} attribute.

@item :italic
A non-@code{nil} makes the face italic; @code{nil} makes it normal.
This actually works by setting the @code{:slant} attribute.
@end table

@defvar x-bitmap-file-path
This variable specifies a list of directories for searching
for bitmap files, for the @code{:stipple} attribute.
@end defvar

@defun bitmap-spec-p object
This returns @code{t} if @var{object} is a valid bitmap specification,
suitable for use with @code{:stipple} (see above).  It returns
@code{nil} otherwise.
@end defun

@node Attribute Functions
@subsection Face Attribute Functions

  You can modify the attributes of an existing face with the following
functions.  If you specify @var{frame}, they affect just that frame;
otherwise, they affect all frames as well as the defaults that apply to
new frames.

@defun set-face-attribute face frame &rest arguments
This function sets one or more attributes of face @var{face}
for frame @var{frame}.  If @var{frame} is @code{nil}, it sets
the attribute for all frames, and the defaults for new frames.

The extra arguments @var{arguments} specify the attributes to set, and
the values for them.  They should consist of alternating attribute names
(such as @code{:family} or @code{:underline}) and corresponding values.
Thus,

@example
(set-face-attribute 'foo nil
                    :width 'extended
                    :weight 'bold
                    :underline "red")
@end example

@noindent
sets the attributes @code{:width}, @code{:weight} and @code{:underline}
to the corresponding values.
@end defun

@defun face-attribute face attribute &optional frame inherit
This returns the value of the @var{attribute} attribute of face
@var{face} on @var{frame}.  If @var{frame} is @code{nil},
that means the selected frame (@pxref{Input Focus}).

If @var{frame} is @code{t}, the value is the default for
@var{face} for new frames.

If @var{inherit} is @code{nil}, only attributes directly defined by
@var{face} are considered, so the return value may be
@code{unspecified}, or a relative value.  If @var{inherit} is
non-@code{nil}, @var{face}'s definition of @var{attribute} is merged
with the faces specified by its @code{:inherit} attribute; however the
return value may still be @code{unspecified} or relative.  If
@var{inherit} is a face or a list of faces, then the result is further
merged with that face (or faces), until it becomes specified and
absolute.

To ensure that the return value is always specified and absolute, use
a value of @code{default} for @var{inherit}; this will resolve any
unspecified or relative values by merging with the @code{default} face
(which is always completely specified).

For example,

@example
(face-attribute 'bold :weight)
     @result{} bold
@end example
@end defun

  The functions above did not exist before Emacs 21.  For compatibility
with older Emacs versions, you can use the following functions to set
and examine the face attributes which existed in those versions.

@defun face-attribute-relative-p attribute value
This function returns non-@code{nil} if @var{value}, when used as the
value of the face attribute @var{attribute}, is relative.  This means
it would modify, rather than completely override, any value that comes
from a subsequent face in the face list or that is inherited from
another face.

@code{unspecified} is a relative value for all attributes.
For @code{:height}, floating point values are also relative.

For example:

@example
(read-face-name "Describe face" "= `default' face" t)
@end example

prompts with @samp{Describe face (default = `default' face): }.
@end defun

@defun merge-face-attribute attribute value1 value2
If @var{value1} is a relative value for the face attribute
@var{attribute}, returns it merged with the underlying value
@var{value2}; otherwise, if @var{value1} is an absolute value for the
face attribute @var{attribute}, returns @var{value1} unchanged.
@end defun

@defun set-face-foreground face color &optional frame
@defunx set-face-background face color &optional frame
These functions set the foreground (or background, respectively) color
of face @var{face} to @var{color}.  The argument @var{color} should be a
string, the name of a color.

Certain shades of gray are implemented by stipple patterns on
black-and-white screens.
@end defun

@defun set-face-stipple face pattern &optional frame
This function sets the background stipple pattern of face @var{face}
to @var{pattern}.  The argument @var{pattern} should be the name of a
stipple pattern defined by the X server, or actual bitmap data
(@pxref{Face Attributes}), or @code{nil} meaning don't use stipple.

Normally there is no need to pay attention to stipple patterns, because
they are used automatically to handle certain shades of gray.
@end defun

@defun set-face-font face font &optional frame
This function sets the font of face @var{face}.  This actually sets
the attributes @code{:family}, @code{:width}, @code{:height},
@code{:weight}, and @code{:slant} according to the font name
@var{font}.
@end defun

@defun set-face-bold-p face bold-p &optional frame
This function specifies whether @var{face} should be bold.  If
@var{bold-p} is non-@code{nil}, that means yes; @code{nil} means no.
This actually sets the @code{:weight} attribute.
@end defun

@defun set-face-italic-p face italic-p &optional frame
This function specifies whether @var{face} should be italic.  If
@var{italic-p} is non-@code{nil}, that means yes; @code{nil} means no.
This actually sets the @code{:slant} attribute.
@end defun

@defun set-face-underline-p face underline-p &optional frame
This function sets the underline attribute of face @var{face}.
Non-@code{nil} means do underline; @code{nil} means don't.
@end defun

@defun set-face-inverse-video-p face inverse-video-p &optional frame
This function sets the @code{:inverse-video} attribute of face
@var{face}.
@end defun

@defun invert-face face &optional frame
This function swaps the foreground and background colors of face
@var{face}.
@end defun

  These functions examine the attributes of a face.  If you don't
specify @var{frame}, they refer to the default data for new frames.
They return the symbol @code{unspecified} if the face doesn't define any
value for that attribute.

@defun face-foreground face &optional frame inherit
@defunx face-background face &optional frame inherit
These functions return the foreground color (or background color,
respectively) of face @var{face}, as a string.

If @var{inherit} is @code{nil}, only a color directly defined by the face is
returned.  If @var{inherit} is non-@code{nil}, any faces specified by its
@code{:inherit} attribute are considered as well, and if @var{inherit}
is a face or a list of faces, then they are also considered, until a
specified color is found.  To ensure that the return value is always
specified, use a value of @code{default} for @var{inherit}.
@end defun

@defun face-stipple face &optional frame inherit
This function returns the name of the background stipple pattern of face
@var{face}, or @code{nil} if it doesn't have one.

If @var{inherit} is @code{nil}, only a stipple directly defined by the
face is returned.  If @var{inherit} is non-@code{nil}, any faces
specified by its @code{:inherit} attribute are considered as well, and
if @var{inherit} is a face or a list of faces, then they are also
considered, until a specified stipple is found.  To ensure that the
return value is always specified, use a value of @code{default} for
@var{inherit}.
@end defun

@defun face-font face &optional frame
This function returns the name of the font of face @var{face}.
@end defun

@defun face-bold-p face &optional frame
This function returns @code{t} if @var{face} is bold---that is, if it is
bolder than normal.  It returns @code{nil} otherwise.
@end defun

@defun face-italic-p face &optional frame
This function returns @code{t} if @var{face} is italic or oblique,
@code{nil} otherwise.
@end defun

@defun face-underline-p face &optional frame
This function returns the @code{:underline} attribute of face @var{face}.
@end defun

@defun face-inverse-video-p face &optional frame
This function returns the @code{:inverse-video} attribute of face @var{face}.
@end defun

@node Displaying Faces
@subsection Displaying Faces

  Here are the ways to specify which faces to use for display of text:

@itemize @bullet
@item
With defaults.  The @code{default} face is used as the ultimate
default for all text.  (In Emacs 19 and 20, the @code{default}
face is used only when no other face is specified.)

@item
For a mode line or header line, the face @code{mode-line} or
@code{mode-line-inactive}, or @code{header-line}, is merged in just
before @code{default}.

@item
With text properties.  A character can have a @code{face} property; if
so, the faces and face attributes specified there apply.  @xref{Special
Properties}.

If the character has a @code{mouse-face} property, that is used instead
of the @code{face} property when the mouse is ``near enough'' to the
character.

@item
With overlays.  An overlay can have @code{face} and @code{mouse-face}
properties too; they apply to all the text covered by the overlay.

@item
With a region that is active.  In Transient Mark mode, the region is
highlighted with the face @code{region} (@pxref{Standard Faces,,,
emacs, The GNU Emacs Manual}).

@item
With special glyphs.  Each glyph can specify a particular face
number.  @xref{Glyphs}.
@end itemize

  If these various sources together specify more than one face for a
particular character, Emacs merges the attributes of the various faces
specified.  For each attribute, Emacs tries first the face of any
special glyph; then the face for region highlighting, if appropriate;
then the faces specified by overlays, followed by those specified by
text properties, then the @code{mode-line} or
@code{mode-line-inactive} or @code{header-line} face (if in a mode
line or a header line), and last the @code{default} face.

  When multiple overlays cover one character, an overlay with higher
priority overrides those with lower priority.  @xref{Overlays}.

@node Font Selection
@subsection Font Selection

  @dfn{Selecting a font} means mapping the specified face attributes for
a character to a font that is available on a particular display.  The
face attributes, as determined by face merging, specify most of the
font choice, but not all.  Part of the choice depends on what character
it is.

  If the face specifies a fontset name, that fontset determines a
pattern for fonts of the given charset.  If the face specifies a font
family, a font pattern is constructed.

  Emacs tries to find an available font for the given face attributes
and character's registry and encoding.  If there is a font that matches
exactly, it is used, of course.  The hard case is when no available font
exactly fits the specification.  Then Emacs looks for one that is
``close''---one attribute at a time.  You can specify the order to
consider the attributes.  In the case where a specified font family is
not available, you can specify a set of mappings for alternatives to
try.

@defvar face-font-selection-order
This variable specifies the order of importance of the face attributes
@code{:width}, @code{:height}, @code{:weight}, and @code{:slant}.  The
value should be a list containing those four symbols, in order of
decreasing importance.

Font selection first finds the best available matches for the first
attribute listed; then, among the fonts which are best in that way, it
searches for the best matches in the second attribute, and so on.

The attributes @code{:weight} and @code{:width} have symbolic values in
a range centered around @code{normal}.  Matches that are more extreme
(farther from @code{normal}) are somewhat preferred to matches that are
less extreme (closer to @code{normal}); this is designed to ensure that
non-normal faces contrast with normal ones, whenever possible.

The default is @code{(:width :height :weight :slant)}, which means first
find the fonts closest to the specified @code{:width}, then---among the
fonts with that width---find a best match for the specified font height,
and so on.

One example of a case where this variable makes a difference is when the
default font has no italic equivalent.  With the default ordering, the
@code{italic} face will use a non-italic font that is similar to the
default one.  But if you put @code{:slant} before @code{:height}, the
@code{italic} face will use an italic font, even if its height is not
quite right.
@end defvar

@defvar face-font-family-alternatives
This variable lets you specify alternative font families to try, if a
given family is specified and doesn't exist.  Each element should have
this form:

@example
(@var{family} @var{alternate-families}@dots{})
@end example

If @var{family} is specified but not available, Emacs will try the other
families given in @var{alternate-families}, one by one, until it finds a
family that does exist.
@end defvar

@defvar face-font-registry-alternatives
This variable lets you specify alternative font registries to try, if a
given registry is specified and doesn't exist.  Each element should have
this form:

@example
(@var{registry} @var{alternate-registries}@dots{})
@end example

If @var{registry} is specified but not available, Emacs will try the
other registries given in @var{alternate-registries}, one by one,
until it finds a registry that does exist.
@end defvar

  Emacs can make use of scalable fonts, but by default it does not use
them, since the use of too many or too big scalable fonts can crash
XFree86 servers.

@defvar scalable-fonts-allowed
This variable controls which scalable fonts to use.  A value of
@code{nil}, the default, means do not use scalable fonts.  @code{t}
means to use any scalable font that seems appropriate for the text.

Otherwise, the value must be a list of regular expressions.  Then a
scalable font is enabled for use if its name matches any regular
expression in the list.  For example,

@example
(setq scalable-fonts-allowed '("muleindian-2$"))
@end example

@noindent
allows the use of scalable fonts with registry @code{muleindian-2}.
@end defvar

@defvar face-font-rescale-alist
This variable specifies scaling for certain faces.  Its value should
be a list of elements of the form

@example
(@var{fontname-regexp} . @var{scale-factor})
@end example

If @var{fontname-regexp} matches the font name that is about to be
used, this says to choose a larger similar font according to the
factor @var{scale-factor}.  You would use this feature to normalize
the font size if certain fonts are bigger or smaller than their
nominal heights and widths would suggest.
@end defvar

@node Face Functions
@subsection Functions for Working with Faces

  Here are additional functions for creating and working with faces.

@defun make-face name
This function defines a new face named @var{name}, initially with all
attributes @code{nil}.  It does nothing if there is already a face named
@var{name}.
@end defun

@defun face-list
This function returns a list of all defined face names.
@end defun

@defun copy-face old-face new-name &optional frame new-frame
This function defines a face named @var{new-name} as a copy of the existing
face named @var{old-face}.  It creates the face @var{new-name} if that
doesn't already exist.

If the optional argument @var{frame} is given, this function applies
only to that frame.  Otherwise it applies to each frame individually,
copying attributes from @var{old-face} in each frame to @var{new-face}
in the same frame.

If the optional argument @var{new-frame} is given, then @code{copy-face}
copies the attributes of @var{old-face} in @var{frame} to @var{new-name}
in @var{new-frame}.
@end defun

@defun face-id face
This function returns the face number of face @var{face}.
@end defun

@defun face-documentation face
This function returns the documentation string of face @var{face}, or
@code{nil} if none was specified for it.
@end defun

@defun face-equal face1 face2 &optional frame
This returns @code{t} if the faces @var{face1} and @var{face2} have the
same attributes for display.
@end defun

@defun face-differs-from-default-p face &optional frame
This returns non-@code{nil} if the face @var{face} displays
differently from the default face.
@end defun

@cindex face alias
A @dfn{face alias} provides an equivalent name for a face.  You can
define a face alias by giving the alias symbol the @code{face-alias}
property, with a value of the target face name.  The following example
makes @code{modeline} an alias for the @code{mode-line} face.

@example
(put 'modeline 'face-alias 'mode-line)
@end example


@node Auto Faces
@subsection Automatic Face Assignment
@cindex automatic face assignment
@cindex faces, automatic choice

@cindex Font-Lock mode
  This hook is used for automatically assigning faces to text in the
buffer.  It is part of the implementation of Font-Lock mode.

@defvar fontification-functions
This variable holds a list of functions that are called by Emacs
redisplay as needed to assign faces automatically to text in the buffer.

The functions are called in the order listed, with one argument, a
buffer position @var{pos}.  Each function should attempt to assign faces
to the text in the current buffer starting at @var{pos}.

Each function should record the faces they assign by setting the
@code{face} property.  It should also add a non-@code{nil}
@code{fontified} property for all the text it has assigned faces to.
That property tells redisplay that faces have been assigned to that text
already.

It is probably a good idea for each function to do nothing if the
character after @var{pos} already has a non-@code{nil} @code{fontified}
property, but this is not required.  If one function overrides the
assignments made by a previous one, the properties as they are
after the last function finishes are the ones that really matter.

For efficiency, we recommend writing these functions so that they
usually assign faces to around 400 to 600 characters at each call.
@end defvar

@node Font Lookup
@subsection Looking Up Fonts

@defun x-list-fonts pattern &optional face frame maximum
This function returns a list of available font names that match
@var{pattern}.  If the optional arguments @var{face} and @var{frame} are
specified, then the list is limited to fonts that are the same size as
@var{face} currently is on @var{frame}.

The argument @var{pattern} should be a string, perhaps with wildcard
characters: the @samp{*} character matches any substring, and the
@samp{?} character matches any single character.  Pattern matching
of font names ignores case.

If you specify @var{face} and @var{frame}, @var{face} should be a face name
(a symbol) and @var{frame} should be a frame.

The optional argument @var{maximum} sets a limit on how many fonts to
return.  If this is non-@code{nil}, then the return value is truncated
after the first @var{maximum} matching fonts.  Specifying a small value
for @var{maximum} can make this function much faster, in cases where
many fonts match the pattern.
@end defun

@defun x-family-fonts &optional family frame
This function returns a list describing the available fonts for family
@var{family} on @var{frame}.  If @var{family} is omitted or @code{nil},
this list applies to all families, and therefore, it contains all
available fonts.  Otherwise, @var{family} must be a string; it may
contain the wildcards @samp{?} and @samp{*}.

The list describes the display that @var{frame} is on; if @var{frame} is
omitted or @code{nil}, it applies to the selected frame's display
(@pxref{Input Focus}).

The list contains a vector of the following form for each font:

@example
[@var{family} @var{width} @var{point-size} @var{weight} @var{slant}
 @var{fixed-p} @var{full} @var{registry-and-encoding}]
@end example

The first five elements correspond to face attributes; if you
specify these attributes for a face, it will use this font.

The last three elements give additional information about the font.
@var{fixed-p} is non-@code{nil} if the font is fixed-pitch.
@var{full} is the full name of the font, and
@var{registry-and-encoding} is a string giving the registry and
encoding of the font.

The result list is sorted according to the current face font sort order.
@end defun

@defun x-font-family-list &optional frame
This function returns a list of the font families available for
@var{frame}'s display.  If @var{frame} is omitted or @code{nil}, it
describes the selected frame's display (@pxref{Input Focus}).

The value is a list of elements of this form:

@example
(@var{family} . @var{fixed-p})
@end example

@noindent
Here @var{family} is a font family, and @var{fixed-p} is
non-@code{nil} if fonts of that family are fixed-pitch.
@end defun

@defvar font-list-limit
This variable specifies maximum number of fonts to consider in font
matching.  The function @code{x-family-fonts} will not return more than
that many fonts, and font selection will consider only that many fonts
when searching a matching font for face attributes.  The default is
currently 100.
@end defvar

@node Fontsets
@subsection Fontsets

  A @dfn{fontset} is a list of fonts, each assigned to a range of
character codes.  An individual font cannot display the whole range of
characters that Emacs supports, but a fontset can.  Fontsets have names,
just as fonts do, and you can use a fontset name in place of a font name
when you specify the ``font'' for a frame or a face.  Here is
information about defining a fontset under Lisp program control.

@defun create-fontset-from-fontset-spec fontset-spec &optional style-variant-p noerror
This function defines a new fontset according to the specification
string @var{fontset-spec}.  The string should have this format:

@smallexample
@var{fontpattern}, @r{[}@var{charsetname}:@var{fontname}@r{]@dots{}}
@end smallexample

@noindent
Whitespace characters before and after the commas are ignored.

The first part of the string, @var{fontpattern}, should have the form of
a standard X font name, except that the last two fields should be
@samp{fontset-@var{alias}}.

The new fontset has two names, one long and one short.  The long name is
@var{fontpattern} in its entirety.  The short name is
@samp{fontset-@var{alias}}.  You can refer to the fontset by either
name.  If a fontset with the same name already exists, an error is
signaled, unless @var{noerror} is non-@code{nil}, in which case this
function does nothing.

If optional argument @var{style-variant-p} is non-@code{nil}, that says
to create bold, italic and bold-italic variants of the fontset as well.
These variant fontsets do not have a short name, only a long one, which
is made by altering @var{fontpattern} to indicate the bold or italic
status.

The specification string also says which fonts to use in the fontset.
See below for the details.
@end defun

  The construct @samp{@var{charset}:@var{font}} specifies which font to
use (in this fontset) for one particular character set.  Here,
@var{charset} is the name of a character set, and @var{font} is the font
to use for that character set.  You can use this construct any number of
times in the specification string.

  For the remaining character sets, those that you don't specify
explicitly, Emacs chooses a font based on @var{fontpattern}: it replaces
@samp{fontset-@var{alias}} with a value that names one character set.
For the @acronym{ASCII} character set, @samp{fontset-@var{alias}} is replaced
with @samp{ISO8859-1}.

  In addition, when several consecutive fields are wildcards, Emacs
collapses them into a single wildcard.  This is to prevent use of
auto-scaled fonts.  Fonts made by scaling larger fonts are not usable
for editing, and scaling a smaller font is not useful because it is
better to use the smaller font in its own size, which Emacs does.

  Thus if @var{fontpattern} is this,

@example
-*-fixed-medium-r-normal-*-24-*-*-*-*-*-fontset-24
@end example

@noindent
the font specification for @acronym{ASCII} characters would be this:

@example
-*-fixed-medium-r-normal-*-24-*-ISO8859-1
@end example

@noindent
and the font specification for Chinese GB2312 characters would be this:

@example
-*-fixed-medium-r-normal-*-24-*-gb2312*-*
@end example

  You may not have any Chinese font matching the above font
specification.  Most X distributions include only Chinese fonts that
have @samp{song ti} or @samp{fangsong ti} in the @var{family} field.  In
such a case, @samp{Fontset-@var{n}} can be specified as below:

@smallexample
Emacs.Fontset-0: -*-fixed-medium-r-normal-*-24-*-*-*-*-*-fontset-24,\
        chinese-gb2312:-*-*-medium-r-normal-*-24-*-gb2312*-*
@end smallexample

@noindent
Then, the font specifications for all but Chinese GB2312 characters have
@samp{fixed} in the @var{family} field, and the font specification for
Chinese GB2312 characters has a wild card @samp{*} in the @var{family}
field.

@defun set-fontset-font name character fontname &optional frame
This function modifies the existing fontset @var{name} to
use the font name @var{fontname} for the character @var{character}.

If @var{name} is @code{nil}, this function modifies the default
fontset, whose short name is @samp{fontset-default}.

@var{character} may be a cons; @code{(@var{from} . @var{to})}, where
@var{from} and @var{to} are non-generic characters.  In that case, use
@var{fontname} for all characters in the range @var{from} and @var{to}
(inclusive).

@var{character} may be a charset.  In that case, use
@var{fontname} for all character in the charsets.

@var{fontname} may be a cons; @code{(@var{family} . @var{registry})},
where @var{family} is a family name of a font (possibly including a
foundry name at the head), @var{registry} is a registry name of a font
(possibly including an encoding name at the tail).

For instance, this changes the default fontset to use a font of which
registry name is @samp{JISX0208.1983} for all characters belonging to
the charset @code{japanese-jisx0208}.

@smallexample
(set-fontset-font nil 'japanese-jisx0208 '(nil . "JISX0208.1983"))
@end smallexample
@end defun

@defun char-displayable-p char
This function returns @code{t} if Emacs ought to be able to display
@var{char}.  More precisely, if the selected frame's fontset has a
font to display the character set that @var{char} belongs to.

Fontsets can specify a font on a per-character basis; when the fontset
does that, this function's value may not be accurate.
@end defun

@node Fringes
@section Fringes
@cindex Fringes

  The @dfn{fringes} of a window are thin vertical strips down the
sides that are used for displaying bitmaps that indicate truncation,
continuation, horizontal scrolling, and the overlay arrow.

@menu
* Fringe Size/Pos::     Specifying where to put the window fringes.
* Fringe Indicators::   Displaying indicator icons in the window fringes.
* Fringe Cursors::      Displaying cursors in the right fringe.
* Fringe Bitmaps::      Specifying bitmaps for fringe indicators.
* Customizing Bitmaps:: Specifying your own bitmaps to use in the fringes.
* Overlay Arrow::       Display of an arrow to indicate position.
@end menu

@node Fringe Size/Pos
@subsection Fringe Size and Position

  The following buffer-local variables control the position and width
of the window fringes.

@defvar fringes-outside-margins
The fringes normally appear between the display margins and the window
text.  If the value is non-@code{nil}, they appear outside the display
margins.  @xref{Display Margins}.
@end defvar

@defvar left-fringe-width
This variable, if non-@code{nil}, specifies the width of the left
fringe in pixels.  A value of @code{nil} means to use the left fringe
width from the window's frame.
@end defvar

@defvar right-fringe-width
This variable, if non-@code{nil}, specifies the width of the right
fringe in pixels.  A value of @code{nil} means to use the right fringe
width from the window's frame.
@end defvar

  The values of these variables take effect when you display the
buffer in a window.  If you change them while the buffer is visible,
you can call @code{set-window-buffer} to display it once again in the
same window, to make the changes take effect.

@defun set-window-fringes window left &optional right outside-margins
This function sets the fringe widths of window @var{window}.
If @var{window} is @code{nil}, the selected window is used.

The argument @var{left} specifies the width in pixels of the left
fringe, and likewise @var{right} for the right fringe.  A value of
@code{nil} for either one stands for the default width.  If
@var{outside-margins} is non-@code{nil}, that specifies that fringes
should appear outside of the display margins.
@end defun

@defun window-fringes &optional window
This function returns information about the fringes of a window
@var{window}.  If @var{window} is omitted or @code{nil}, the selected
window is used.  The value has the form @code{(@var{left-width}
@var{right-width} @var{outside-margins})}.
@end defun


@node Fringe Indicators
@subsection Fringe Indicators
@cindex fringe indicators
@cindex indicators, fringe

  The @dfn{fringe indicators} are tiny icons Emacs displays in the
window fringe (on a graphic display) to indicate truncated or
continued lines, buffer boundaries, overlay arrow, etc.

@defopt indicate-empty-lines
@cindex fringes, and empty line indication
When this is non-@code{nil}, Emacs displays a special glyph in the
fringe of each empty line at the end of the buffer, on graphical
displays.  @xref{Fringes}.  This variable is automatically
buffer-local in every buffer.
@end defopt

@defvar indicate-buffer-boundaries
This buffer-local variable controls how the buffer boundaries and
window scrolling are indicated in the window fringes.

Emacs can indicate the buffer boundaries---that is, the first and last
line in the buffer---with angle icons when they appear on the screen.
In addition, Emacs can display an up-arrow in the fringe to show
that there is text above the screen, and a down-arrow to show
there is text below the screen.

There are four kinds of basic values:

@table @asis
@item @code{nil}
Don't display the icons.
@item @code{left}
Display them in the left fringe.
@item @code{right}
Display them in the right fringe.
@item @var{anything-else}
Display the icon at the top of the window top in the left fringe, and other
in the right fringe.
@end table

If value is a cons @code{(@var{angles} . @var{arrows})}, @var{angles}
controls the angle icons, and @var{arrows} controls the arrows.  Both
@var{angles} and @var{arrows} work according to the table above.
Thus, @code{(t .  right)} places the top angle icon in the left
fringe, the bottom angle icon in the right fringe, and both arrows in
the right fringe.
@end defvar

@defvar default-indicate-buffer-boundaries
The value of this variable is the default value for
@code{indicate-buffer-boundaries} in buffers that do not override it.
@end defvar

@defvar fringe-indicator-alist
This buffer-local variable specifies the mapping from logical fringe
indicators to the actual bitmaps displayed in the window fringes.

These symbols identify the logical fringe indicators:

@table @asis
@item Truncation and continuation line indicators:
@code{truncation}, @code{continuation}.

@item Buffer position indicators:
@code{up}, @code{down},
@code{top}, @code{bottom},
@code{top-bottom}.

@item Empty line indicator:
@code{empty-line}.

@item Overlay arrow indicator:
@code{overlay-arrow}.

@item Unknown bitmap indicator:
@code{unknown}.
@end table

  The value is an alist where each element @code{(@var{indicator} . @var{bitmaps})}
specifies the fringe bitmaps used to display a specific logical
fringe indicator.

Here, @var{indicator} specifies the logical indicator type, and
@var{bitmaps} is list of symbols @code{(@var{left} @var{right}
[@var{left1} @var{right1}])} which specifies the actual bitmap shown
in the left or right fringe for the logical indicator.

The @var{left} and @var{right} symbols specify the bitmaps shown in
the left and/or right fringe for the specific indicator.  The
@var{left1} or @var{right1} bitmaps are used only for the `bottom' and
`top-bottom indicators when the last (only) line in has no final
newline.  Alternatively, @var{bitmaps} may be a single symbol which is
used in both left and right fringes.

When @code{fringe-indicator-alist} has a buffer-local value, and there
is no bitmap defined for a logical indicator, or the bitmap is
@code{t}, the corresponding value from the (non-local)
@code{default-fringe-indicator-alist} is used.

To completely hide a specific indicator, set the bitmap to @code{nil}.
@end defvar

@defvar default-fringe-indicator-alist
The value of this variable is the default value for
@code{fringe-indicator-alist} in buffers that do not override it.
@end defvar

Standard fringe bitmaps for indicators:
@example
left-arrow right-arrow up-arrow down-arrow
left-curly-arrow right-curly-arrow
left-triangle right-triangle
top-left-angle top-right-angle
bottom-left-angle bottom-right-angle
left-bracket right-bracket
filled-rectangle hollow-rectangle
filled-square hollow-square
vertical-bar horizontal-bar
empty-line question-mark
@end example

@node Fringe Cursors
@subsection Fringe Cursors
@cindex fringe cursors
@cindex cursor, fringe

  When a line is exactly as wide as the window, Emacs displays the
cursor in the right fringe instead of using two lines.  Different
bitmaps are used to represent the cursor in the fringe depending on
the current buffer's cursor type.

@table @asis
@item Logical cursor types:
@code{box} , @code{hollow}, @code{bar},
@code{hbar}, @code{hollow-small}.
@end table

The @code{hollow-small} type is used instead of @code{hollow} when the
normal @code{hollow-rectangle} bitmap is too tall to fit on a specific
display line.

@defvar overflow-newline-into-fringe
If this is non-@code{nil}, lines exactly as wide as the window (not
counting the final newline character) are not continued.  Instead,
when point is at the end of the line, the cursor appears in the right
fringe.
@end defvar

@defvar fringe-cursor-alist
This variable specifies the mapping from logical cursor type to the
actual fringe bitmaps displayed in the right fringe.  The value is an
alist where each element @code{(@var{cursor} . @var{bitmap})} specifies
the fringe bitmaps used to display a specific logical cursor type in
the fringe.  Here, @var{cursor} specifies the logical cursor type and
@var{bitmap} is a symbol specifying the fringe bitmap to be displayed
for that logical cursor type.

When @code{fringe-cursor-alist} has a buffer-local value, and there is
no bitmap defined for a cursor type, the corresponding value from the
(non-local) @code{default-fringes-indicator-alist} is used.
@end defvar

@defvar default-fringes-cursor-alist
The value of this variable is the default value for
@code{fringe-cursor-alist} in buffers that do not override it.
@end defvar

Standard bitmaps for displaying the cursor in right fringe:
@example
filled-rectangle hollow-rectangle filled-square hollow-square
vertical-bar horizontal-bar
@end example


@node Fringe Bitmaps
@subsection Fringe Bitmaps
@cindex fringe bitmaps
@cindex bitmaps, fringe

  The @dfn{fringe bitmaps} are the actual bitmaps which represent the
logical fringe indicators for truncated or continued lines, buffer
boundaries, overlay arrow, etc.  Fringe bitmap symbols have their own
name space.  The fringe bitmaps are shared by all frames and windows.
You can redefine the built-in fringe bitmaps, and you can define new
fringe bitmaps.

  The way to display a bitmap in the left or right fringes for a given
line in a window is by specifying the @code{display} property for one
of the characters that appears in it.  Use a display specification of
the form @code{(left-fringe @var{bitmap} [@var{face}])} or
@code{(right-fringe @var{bitmap} [@var{face}])} (@pxref{Display
Property}).  Here, @var{bitmap} is a symbol identifying the bitmap you
want, and @var{face} (which is optional) is the name of the face whose
colors should be used for displaying the bitmap, instead of the
default @code{fringe} face.  @var{face} is automatically merged with
the @code{fringe} face, so normally @var{face} need only specify the
foreground color for the bitmap.

@defun fringe-bitmaps-at-pos &optional pos window
This function returns the fringe bitmaps of the display line
containing position @var{pos} in window @var{window}.  The return
value has the form @code{(@var{left} @var{right} @var{ov})}, where @var{left}
is the symbol for the fringe bitmap in the left fringe (or @code{nil}
if no bitmap), @var{right} is similar for the right fringe, and @var{ov}
is non-@code{nil} if there is an overlay arrow in the left fringe.

The value is @code{nil} if @var{pos} is not visible in @var{window}.
If @var{window} is @code{nil}, that stands for the selected window.
If @var{pos} is @code{nil}, that stands for the value of point in
@var{window}.
@end defun

@node Customizing Bitmaps
@subsection Customizing Fringe Bitmaps

@defun define-fringe-bitmap bitmap bits &optional height width align
This function defines the symbol @var{bitmap} as a new fringe bitmap,
or replaces an existing bitmap with that name.

The argument @var{bits} specifies the image to use.  It should be
either a string or a vector of integers, where each element (an
integer) corresponds to one row of the bitmap.  Each bit of an integer
corresponds to one pixel of the bitmap, where the low bit corresponds
to the rightmost pixel of the bitmap.

The height is normally the length of @var{bits}.  However, you
can specify a different height with non-@code{nil} @var{height}.  The width
is normally 8, but you can specify a different width with non-@code{nil}
@var{width}.  The width must be an integer between 1 and 16.

The argument @var{align} specifies the positioning of the bitmap
relative to the range of rows where it is used; the default is to
center the bitmap.  The allowed values are @code{top}, @code{center},
or @code{bottom}.

The @var{align} argument may also be a list @code{(@var{align}
@var{periodic})} where @var{align} is interpreted as described above.
If @var{periodic} is non-@code{nil}, it specifies that the rows in
@code{bits} should be repeated enough times to reach the specified
height.
@end defun

@defun destroy-fringe-bitmap bitmap
This function destroy the fringe bitmap identified by @var{bitmap}.
If @var{bitmap} identifies a standard fringe bitmap, it actually
restores the standard definition of that bitmap, instead of
eliminating it entirely.
@end defun

@defun set-fringe-bitmap-face bitmap &optional face
This sets the face for the fringe bitmap @var{bitmap} to @var{face}.
If @var{face} is @code{nil}, it selects the @code{fringe} face.  The
bitmap's face controls the color to draw it in.

@var{face} is merged with the @code{fringe} face, so normally
@var{face} should specify only the foreground color.
@end defun

@node Overlay Arrow
@subsection The Overlay Arrow
@cindex overlay arrow

  The @dfn{overlay arrow} is useful for directing the user's attention
to a particular line in a buffer.  For example, in the modes used for
interface to debuggers, the overlay arrow indicates the line of code
about to be executed.  This feature has nothing to do with
@dfn{overlays} (@pxref{Overlays}).

@defvar overlay-arrow-string
This variable holds the string to display to call attention to a
particular line, or @code{nil} if the arrow feature is not in use.
On a graphical display the contents of the string are ignored; instead a
glyph is displayed in the fringe area to the left of the display area.
@end defvar

@defvar overlay-arrow-position
This variable holds a marker that indicates where to display the overlay
arrow.  It should point at the beginning of a line.  On a non-graphical
display the arrow text
appears at the beginning of that line, overlaying any text that would
otherwise appear.  Since the arrow is usually short, and the line
usually begins with indentation, normally nothing significant is
overwritten.

The overlay-arrow string is displayed in any given buffer if the value
of @code{overlay-arrow-position} in that buffer points into that
buffer.  Thus, it works to can display multiple overlay arrow strings
by creating buffer-local bindings of @code{overlay-arrow-position}.
However, it is usually cleaner to use
@code{overlay-arrow-variable-list} to achieve this result.
@c !!! overlay-arrow-position: but the overlay string may remain in the display
@c of some other buffer until an update is required.  This should be fixed
@c now.  Is it?
@end defvar

  You can do a similar job by creating an overlay with a
@code{before-string} property.  @xref{Overlay Properties}.

  You can define multiple overlay arrows via the variable
@code{overlay-arrow-variable-list}.

@defvar overlay-arrow-variable-list
This variable's value is a list of variables, each of which specifies
the position of an overlay arrow.  The variable
@code{overlay-arrow-position} has its normal meaning because it is on
this list.
@end defvar

Each variable on this list can have properties
@code{overlay-arrow-string} and @code{overlay-arrow-bitmap} that
specify an overlay arrow string (for text-only terminals) or fringe
bitmap (for graphical terminals) to display at the corresponding
overlay arrow position.  If either property is not set, the default
@code{overlay-arrow-string} or @code{overlay-arrow} fringe indicator
is used.

@node Scroll Bars
@section Scroll Bars

Normally the frame parameter @code{vertical-scroll-bars} controls
whether the windows in the frame have vertical scroll bars, and
whether they are on the left or right.  The frame parameter
@code{scroll-bar-width} specifies how wide they are (@code{nil}
meaning the default).  @xref{Layout Parameters}.

@defun frame-current-scroll-bars &optional frame
This function reports the scroll bar type settings for frame
@var{frame}.  The value is a cons cell
@code{(@var{vertical-type} .@: @var{horizontal-type})}, where
@var{vertical-type} is either @code{left}, @code{right}, or @code{nil}
(which means no scroll bar.)  @var{horizontal-type} is meant to
specify the horizontal scroll bar type, but since they are not
implemented, it is always @code{nil}.
@end defun

@vindex vertical-scroll-bar
  You can enable or disable scroll bars for a particular buffer,
by setting the variable @code{vertical-scroll-bar}.  This variable
automatically becomes buffer-local when set.  The possible values are
@code{left}, @code{right}, @code{t}, which means to use the
frame's default, and @code{nil} for no scroll bar.

  You can also control this for individual windows.  Call the function
@code{set-window-scroll-bars} to specify what to do for a specific window:

@defun set-window-scroll-bars window width &optional vertical-type horizontal-type
This function sets the width and type of scroll bars for window
@var{window}.

@var{width} specifies the scroll bar width in pixels (@code{nil} means
use the width specified for the frame).  @var{vertical-type} specifies
whether to have a vertical scroll bar and, if so, where.  The possible
values are @code{left}, @code{right} and @code{nil}, just like the
values of the @code{vertical-scroll-bars} frame parameter.

The argument @var{horizontal-type} is meant to specify whether and
where to have horizontal scroll bars, but since they are not
implemented, it has no effect.  If @var{window} is @code{nil}, the
selected window is used.
@end defun

@defun window-scroll-bars &optional window
Report the width and type of scroll bars specified for @var{window}.
If @var{window} is omitted or @code{nil}, the selected window is used.
The value is a list of the form @code{(@var{width}
@var{cols} @var{vertical-type} @var{horizontal-type})}.  The value
@var{width} is the value that was specified for the width (which may
be @code{nil}); @var{cols} is the number of columns that the scroll
bar actually occupies.

@var{horizontal-type} is not actually meaningful.
@end defun

If you don't specify these values for a window with
@code{set-window-scroll-bars}, the buffer-local variables
@code{scroll-bar-mode} and @code{scroll-bar-width} in the buffer being
displayed control the window's vertical scroll bars.  The function
@code{set-window-buffer} examines these variables.  If you change them
in a buffer that is already visible in a window, you can make the
window take note of the new values by calling @code{set-window-buffer}
specifying the same buffer that is already displayed.

@defvar scroll-bar-mode
This variable, always local in all buffers, controls whether and where
to put scroll bars in windows displaying the buffer.  The possible values
are @code{nil} for no scroll bar, @code{left} to put a scroll bar on
the left, and @code{right} to put a scroll bar on the right.
@end defvar

@defun window-current-scroll-bars &optional window
This function reports the scroll bar type for window @var{window}.
If @var{window} is omitted or @code{nil}, the selected window is used.
The value is a cons cell
@code{(@var{vertical-type} .@: @var{horizontal-type})}.  Unlike
@code{window-scroll-bars}, this reports the scroll bar type actually
used, once frame defaults and @code{scroll-bar-mode} are taken into
account.
@end defun

@defvar scroll-bar-width
This variable, always local in all buffers, specifies the width of the
buffer's scroll bars, measured in pixels.  A value of @code{nil} means
to use the value specified by the frame.
@end defvar

@node Display Property
@section The @code{display} Property
@cindex display specification
@kindex display @r{(text property)}

  The @code{display} text property (or overlay property) is used to
insert images into text, and also control other aspects of how text
displays.  The value of the @code{display} property should be a
display specification, or a list or vector containing several display
specifications.

  Some kinds of @code{display} properties specify something to display
instead of the text that has the property.  In this case, ``the text''
means all the consecutive characters that have the same Lisp object as
their @code{display} property; these characters are replaced as a
single unit.  By contrast, characters that have similar but distinct
Lisp objects as their @code{display} properties are handled
separately.  Here's a function that illustrates this point:

@smallexample
(defun foo ()
  (goto-char (point-min))
  (dotimes (i 5)
    (let ((string (concat "A")))
      (put-text-property (point) (1+ (point)) 'display string)
      (forward-char 1)
      (put-text-property (point) (1+ (point)) 'display string)
      (forward-char 1))))
@end smallexample

@noindent
It gives each of the first ten characters in the buffer string
@code{"A"} as the @code{display} property, but they don't all get the
same string.  The first two characters get the same string, so they
together are replaced with one @samp{A}.  The next two characters get
a second string, so they together are replaced with one @samp{A}.
Likewise for each following pair of characters.  Thus, the ten
characters appear as five A's.  This function would have the same
results:

@smallexample
(defun foo ()
  (goto-char (point-min))
  (dotimes (i 5)
    (let ((string (concat "A")))
      (put-text-property (point) (2+ (point)) 'display string)
      (put-text-property (point) (1+ (point)) 'display string)
      (forward-char 2))))
@end smallexample

@noindent
This illustrates that what matters is the property value for
each character.  If two consecutive characters have the same
object as the @code{display} property value, it's irrelevant
whether they got this property from a single call to
@code{put-text-property} or from two different calls.

  The rest of this section describes several kinds of
display specifications and what they mean.

@menu
* Specified Space::      Displaying one space with a specified width.
* Pixel Specification::  Specifying space width or height in pixels.
* Other Display Specs::  Displaying an image; magnifying text; moving it
                          up or down on the page; adjusting the width
                          of spaces within text.
* Display Margins::     Displaying text or images to the side of the main text.
@end menu

@node Specified Space
@subsection Specified Spaces
@cindex spaces, specified height or width
@cindex specified spaces
@cindex variable-width spaces

  To display a space of specified width and/or height, use a display
specification of the form @code{(space . @var{props})}, where
@var{props} is a property list (a list of alternating properties and
values).  You can put this property on one or more consecutive
characters; a space of the specified height and width is displayed in
place of @emph{all} of those characters.  These are the properties you
can use in @var{props} to specify the weight of the space:

@table @code
@item :width @var{width}
If @var{width} is an integer or floating point number, it specifies
that the space width should be @var{width} times the normal character
width.  @var{width} can also be a @dfn{pixel width} specification
(@pxref{Pixel Specification}).

@item :relative-width @var{factor}
Specifies that the width of the stretch should be computed from the
first character in the group of consecutive characters that have the
same @code{display} property.  The space width is the width of that
character, multiplied by @var{factor}.

@item :align-to @var{hpos}
Specifies that the space should be wide enough to reach @var{hpos}.
If @var{hpos} is a number, it is measured in units of the normal
character width.  @var{hpos} can also be a @dfn{pixel width}
specification (@pxref{Pixel Specification}).
@end table

  You should use one and only one of the above properties.  You can
also specify the height of the space, with these properties:

@table @code
@item :height @var{height}
Specifies the height of the space.
If @var{height} is an integer or floating point number, it specifies
that the space height should be @var{height} times the normal character
height.  The @var{height} may also be a @dfn{pixel height} specification
(@pxref{Pixel Specification}).

@item :relative-height @var{factor}
Specifies the height of the space, multiplying the ordinary height
of the text having this display specification by @var{factor}.

@item :ascent @var{ascent}
If the value of @var{ascent} is a non-negative number no greater than
100, it specifies that @var{ascent} percent of the height of the space
should be considered as the ascent of the space---that is, the part
above the baseline.  The ascent may also be specified in pixel units
with a @dfn{pixel ascent} specification (@pxref{Pixel Specification}).

@end table

  Don't use both @code{:height} and @code{:relative-height} together.

  The @code{:width} and @code{:align-to} properties are supported on
non-graphic terminals, but the other space properties in this section
are not.

@node Pixel Specification
@subsection Pixel Specification for Spaces
@cindex spaces, pixel specification

  The value of the @code{:width}, @code{:align-to}, @code{:height},
and @code{:ascent} properties can be a special kind of expression that
is evaluated during redisplay.  The result of the evaluation is used
as an absolute number of pixels.

  The following expressions are supported:

@smallexample
@group
  @var{expr} ::= @var{num} | (@var{num}) | @var{unit} | @var{elem} | @var{pos} | @var{image} | @var{form}
  @var{num}  ::= @var{integer} | @var{float} | @var{symbol}
  @var{unit} ::= in | mm | cm | width | height
@end group
@group
  @var{elem} ::= left-fringe | right-fringe | left-margin | right-margin
        |  scroll-bar | text
  @var{pos}  ::= left | center | right
  @var{form} ::= (@var{num} . @var{expr}) | (@var{op} @var{expr} ...)
  @var{op}   ::= + | -
@end group
@end smallexample

  The form @var{num} specifies a fraction of the default frame font
height or width.  The form @code{(@var{num})} specifies an absolute
number of pixels.  If @var{num} is a symbol, @var{symbol}, its
buffer-local variable binding is used.

  The @code{in}, @code{mm}, and @code{cm} units specify the number of
pixels per inch, millimeter, and centimeter, respectively.  The
@code{width} and @code{height} units correspond to the default width
and height of the current face.  An image specification @code{image}
corresponds to the width or height of the image.

  The @code{left-fringe}, @code{right-fringe}, @code{left-margin},
@code{right-margin}, @code{scroll-bar}, and @code{text} elements
specify to the width of the corresponding area of the window.

  The @code{left}, @code{center}, and @code{right} positions can be
used with @code{:align-to} to specify a position relative to the left
edge, center, or right edge of the text area.

  Any of the above window elements (except @code{text}) can also be
used with @code{:align-to} to specify that the position is relative to
the left edge of the given area.  Once the base offset for a relative
position has been set (by the first occurrence of one of these
symbols), further occurrences of these symbols are interpreted as the
width of the specified area.  For example, to align to the center of
the left-margin, use

@example
:align-to (+ left-margin (0.5 . left-margin))
@end example

  If no specific base offset is set for alignment, it is always relative
to the left edge of the text area.  For example, @samp{:align-to 0} in a
header-line aligns with the first text column in the text area.

  A value of the form @code{(@var{num} . @var{expr})} stands for the
product of the values of @var{num} and @var{expr}.  For example,
@code{(2 . in)} specifies a width of 2 inches, while @code{(0.5 .
@var{image})} specifies half the width (or height) of the specified
image.

  The form @code{(+ @var{expr} ...)} adds up the value of the
expressions.  The form @code{(- @var{expr} ...)} negates or subtracts
the value of the expressions.

@node Other Display Specs
@subsection Other Display Specifications

  Here are the other sorts of display specifications that you can use
in the @code{display} text property.

@table @code
@item @var{string}
Display @var{string} instead of the text that has this property.

Recursive display specifications are not supported---@var{string}'s
@code{display} properties, if any, are not used.

@item (image . @var{image-props})
This kind of display specification is an image descriptor (@pxref{Images}).
When used as a display specification, it means to display the image
instead of the text that has the display specification.

@item (slice @var{x} @var{y} @var{width} @var{height})
This specification together with @code{image} specifies a @dfn{slice}
(a partial area) of the image to display.  The elements @var{y} and
@var{x} specify the top left corner of the slice, within the image;
@var{width} and @var{height} specify the width and height of the
slice.  Integer values are numbers of pixels.  A floating point number
in the range 0.0--1.0 stands for that fraction of the width or height
of the entire image.

@item ((margin nil) @var{string})
A display specification of this form means to display @var{string}
instead of the text that has the display specification, at the same
position as that text.  It is equivalent to using just @var{string},
but it is done as a special case of marginal display (@pxref{Display
Margins}).

@item (space-width @var{factor})
This display specification affects all the space characters within the
text that has the specification.  It displays all of these spaces
@var{factor} times as wide as normal.  The element @var{factor} should
be an integer or float.  Characters other than spaces are not affected
at all; in particular, this has no effect on tab characters.

@item (height @var{height})
This display specification makes the text taller or shorter.
Here are the possibilities for @var{height}:

@table @asis
@item @code{(+ @var{n})}
This means to use a font that is @var{n} steps larger.  A ``step'' is
defined by the set of available fonts---specifically, those that match
what was otherwise specified for this text, in all attributes except
height.  Each size for which a suitable font is available counts as
another step.  @var{n} should be an integer.

@item @code{(- @var{n})}
This means to use a font that is @var{n} steps smaller.

@item a number, @var{factor}
A number, @var{factor}, means to use a font that is @var{factor} times
as tall as the default font.

@item a symbol, @var{function}
A symbol is a function to compute the height.  It is called with the
current height as argument, and should return the new height to use.

@item anything else, @var{form}
If the @var{height} value doesn't fit the previous possibilities, it is
a form.  Emacs evaluates it to get the new height, with the symbol
@code{height} bound to the current specified font height.
@end table

@item (raise @var{factor})
This kind of display specification raises or lowers the text
it applies to, relative to the baseline of the line.

@var{factor} must be a number, which is interpreted as a multiple of the
height of the affected text.  If it is positive, that means to display
the characters raised.  If it is negative, that means to display them
lower down.

If the text also has a @code{height} display specification, that does
not affect the amount of raising or lowering, which is based on the
faces used for the text.
@end table

@c We put all the `@code{(when ...)}' on one line to encourage
@c makeinfo's end-of-sentence heuristics to DTRT.  Previously, the dot
@c was at eol; the info file ended up w/ two spaces rendered after it.
  You can make any display specification conditional.  To do that,
package it in another list of the form
@code{(when @var{condition} . @var{spec})}.
Then the specification @var{spec} applies only when
@var{condition} evaluates to a non-@code{nil} value.  During the
evaluation, @code{object} is bound to the string or buffer having the
conditional @code{display} property.  @code{position} and
@code{buffer-position} are bound to the position within @code{object}
and the buffer position where the @code{display} property was found,
respectively.  Both positions can be different when @code{object} is a
string.

@node Display Margins
@subsection Displaying in the Margins
@cindex display margins
@cindex margins, display

  A buffer can have blank areas called @dfn{display margins} on the left
and on the right.  Ordinary text never appears in these areas, but you
can put things into the display margins using the @code{display}
property.

  To put text in the left or right display margin of the window, use a
display specification of the form @code{(margin right-margin)} or
@code{(margin left-margin)} on it.  To put an image in a display margin,
use that display specification along with the display specification for
the image.  Unfortunately, there is currently no way to make
text or images in the margin mouse-sensitive.

  If you put such a display specification directly on text in the
buffer, the specified margin display appears @emph{instead of} that
buffer text itself.  To put something in the margin @emph{in
association with} certain buffer text without preventing or altering
the display of that text, put a @code{before-string} property on the
text and put the display specification on the contents of the
before-string.

  Before the display margins can display anything, you must give
them a nonzero width.  The usual way to do that is to set these
variables:

@defvar left-margin-width
This variable specifies the width of the left margin.
It is buffer-local in all buffers.
@end defvar

@defvar right-margin-width
This variable specifies the width of the right margin.
It is buffer-local in all buffers.
@end defvar

  Setting these variables does not immediately affect the window.  These
variables are checked when a new buffer is displayed in the window.
Thus, you can make changes take effect by calling
@code{set-window-buffer}.

  You can also set the margin widths immediately.

@defun set-window-margins window left &optional right
This function specifies the margin widths for window @var{window}.
The argument @var{left} controls the left margin and
@var{right} controls the right margin (default @code{0}).
@end defun

@defun window-margins &optional window
This function returns the left and right margins of @var{window}
as a cons cell of the form @code{(@var{left} . @var{right})}.
If @var{window} is @code{nil}, the selected window is used.
@end defun

@node Images
@section Images
@cindex images in buffers

  To display an image in an Emacs buffer, you must first create an image
descriptor, then use it as a display specifier in the @code{display}
property of text that is displayed (@pxref{Display Property}).

  Emacs can display a number of different image formats; some of them
are supported only if particular support libraries are installed on
your machine.  In some environments, Emacs can load image
libraries on demand; if so, the variable @code{image-library-alist}
can be used to modify the set of known names for these dynamic
libraries (though it is not possible to add new image formats).

  The supported image formats include XBM, XPM (this requires the
libraries @code{libXpm} version 3.4k and @code{libz}), GIF (requiring
@code{libungif} 4.1.0), Postscript, PBM, JPEG (requiring the
@code{libjpeg} library version v6a), TIFF (requiring @code{libtiff}
v3.4), and PNG (requiring @code{libpng} 1.0.2).

  You specify one of these formats with an image type symbol.  The image
type symbols are @code{xbm}, @code{xpm}, @code{gif}, @code{postscript},
@code{pbm}, @code{jpeg}, @code{tiff}, and @code{png}.

@defvar image-types
This variable contains a list of those image type symbols that are
potentially supported in the current configuration.
@emph{Potentially} here means that Emacs knows about the image types,
not necessarily that they can be loaded (they could depend on
unavailable dynamic libraries, for example).

To know which image types are really available, use
@code{image-type-available-p}.
@end defvar

@defvar image-library-alist
This in an alist of image types vs external libraries needed to
display them.

Each element is a list @code{(@var{image-type} @var{library}...)},
where the car is a supported image format from @code{image-types}, and
the rest are strings giving alternate filenames for the corresponding
external libraries to load.

Emacs tries to load the libraries in the order they appear on the
list; if none is loaded, the running session of Emacs won't support
the image type.  @code{pbm} and @code{xbm} don't need to be listed;
they're always supported.

This variable is ignored if the image libraries are statically linked
into Emacs.
@end defvar

@defun  image-type-available-p type
@findex image-type-available-p

This function returns non-@code{nil} if image type @var{type} is
available, i.e., if images of this type can be loaded and displayed in
Emacs.  @var{type} should be one of the types contained in
@code{image-types}.

For image types whose support libraries are statically linked, this
function always returns @code{t}; for other image types, it returns
@code{t} if the dynamic library could be loaded, @code{nil} otherwise.
@end defun

@menu
* Image Descriptors::   How to specify an image for use in @code{:display}.
* XBM Images::          Special features for XBM format.
* XPM Images::          Special features for XPM format.
* GIF Images::          Special features for GIF format.
* Postscript Images::   Special features for Postscript format.
* Other Image Types::   Various other formats are supported.
* Defining Images::     Convenient ways to define an image for later use.
* Showing Images::      Convenient ways to display an image once it is defined.
* Image Cache::         Internal mechanisms of image display.
@end menu

@node Image Descriptors
@subsection Image Descriptors
@cindex image descriptor

  An image description is a list of the form @code{(image . @var{props})},
where @var{props} is a property list containing alternating keyword
symbols (symbols whose names start with a colon) and their values.
You can use any Lisp object as a property, but the only properties
that have any special meaning are certain symbols, all of them keywords.

  Every image descriptor must contain the property @code{:type
@var{type}} to specify the format of the image.  The value of @var{type}
should be an image type symbol; for example, @code{xpm} for an image in
XPM format.

  Here is a list of other properties that are meaningful for all image
types:

@table @code
@item :file @var{file}
The @code{:file} property says to load the image from file
@var{file}.  If @var{file} is not an absolute file name, it is expanded
in @code{data-directory}.

@item :data @var{data}
The @code{:data} property says the actual contents of the image.
Each image must use either @code{:data} or @code{:file}, but not both.
For most image types, the value of the @code{:data} property should be a
string containing the image data; we recommend using a unibyte string.

Before using @code{:data}, look for further information in the section
below describing the specific image format.  For some image types,
@code{:data} may not be supported; for some, it allows other data types;
for some, @code{:data} alone is not enough, so you need to use other
image properties along with @code{:data}.

@item :margin @var{margin}
The @code{:margin} property specifies how many pixels to add as an
extra margin around the image.  The value, @var{margin}, must be a
non-negative number, or a pair @code{(@var{x} . @var{y})} of such
numbers.  If it is a pair, @var{x} specifies how many pixels to add
horizontally, and @var{y} specifies how many pixels to add vertically.
If @code{:margin} is not specified, the default is zero.

@item :ascent @var{ascent}
The @code{:ascent} property specifies the amount of the image's
height to use for its ascent---that is, the part above the baseline.
The value, @var{ascent}, must be a number in the range 0 to 100, or
the symbol @code{center}.

If @var{ascent} is a number, that percentage of the image's height is
used for its ascent.

If @var{ascent} is @code{center}, the image is vertically centered
around a centerline which would be the vertical centerline of text drawn
at the position of the image, in the manner specified by the text
properties and overlays that apply to the image.

If this property is omitted, it defaults to 50.

@item :relief @var{relief}
The @code{:relief} property, if non-@code{nil}, adds a shadow rectangle
around the image.  The value, @var{relief}, specifies the width of the
shadow lines, in pixels.  If @var{relief} is negative, shadows are drawn
so that the image appears as a pressed button; otherwise, it appears as
an unpressed button.

@item :conversion @var{algorithm}
The @code{:conversion} property, if non-@code{nil}, specifies a
conversion algorithm that should be applied to the image before it is
displayed; the value, @var{algorithm}, specifies which algorithm.

@table @code
@item laplace
@itemx emboss
Specifies the Laplace edge detection algorithm, which blurs out small
differences in color while highlighting larger differences.  People
sometimes consider this useful for displaying the image for a
``disabled'' button.

@item (edge-detection :matrix @var{matrix} :color-adjust @var{adjust})
Specifies a general edge-detection algorithm.  @var{matrix} must be
either a nine-element list or a nine-element vector of numbers.  A pixel
at position @math{x/y} in the transformed image is computed from
original pixels around that position.  @var{matrix} specifies, for each
pixel in the neighborhood of @math{x/y}, a factor with which that pixel
will influence the transformed pixel; element @math{0} specifies the
factor for the pixel at @math{x-1/y-1}, element @math{1} the factor for
the pixel at @math{x/y-1} etc., as shown below:
@iftex
@tex
$$\pmatrix{x-1/y-1 & x/y-1  & x+1/y-1 \cr
   x-1/y  &   x/y &    x+1/y \cr
   x-1/y+1&   x/y+1 &  x+1/y+1 \cr}$$
@end tex
@end iftex
@ifnottex
@display
  (x-1/y-1  x/y-1  x+1/y-1
   x-1/y    x/y    x+1/y
   x-1/y+1  x/y+1  x+1/y+1)
@end display
@end ifnottex

The resulting pixel is computed from the color intensity of the color
resulting from summing up the RGB values of surrounding pixels,
multiplied by the specified factors, and dividing that sum by the sum
of the factors' absolute values.

Laplace edge-detection currently uses a matrix of
@iftex
@tex
$$\pmatrix{1 & 0 & 0 \cr
   0&  0 &  0 \cr
   9 & 9 & -1 \cr}$$
@end tex
@end iftex
@ifnottex
@display
  (1  0  0
   0  0  0
   9  9 -1)
@end display
@end ifnottex

Emboss edge-detection uses a matrix of
@iftex
@tex
$$\pmatrix{ 2 & -1 &  0 \cr
   -1 &  0 &  1 \cr
    0  & 1 & -2 \cr}$$
@end tex
@end iftex
@ifnottex
@display
  ( 2 -1  0
   -1  0  1
    0  1 -2)
@end display
@end ifnottex

@item disabled
Specifies transforming the image so that it looks ``disabled''.
@end table

@item :mask @var{mask}
If @var{mask} is @code{heuristic} or @code{(heuristic @var{bg})}, build
a clipping mask for the image, so that the background of a frame is
visible behind the image.  If @var{bg} is not specified, or if @var{bg}
is @code{t}, determine the background color of the image by looking at
the four corners of the image, assuming the most frequently occurring
color from the corners is the background color of the image.  Otherwise,
@var{bg} must be a list @code{(@var{red} @var{green} @var{blue})}
specifying the color to assume for the background of the image.

If @var{mask} is @code{nil}, remove a mask from the image, if it has
one.  Images in some formats include a mask which can be removed by
specifying @code{:mask nil}.

@item :pointer @var{shape}
This specifies the pointer shape when the mouse pointer is over this
image.  @xref{Pointer Shape}, for available pointer shapes.

@item :map @var{map}
This associates an image map of @dfn{hot spots} with this image.

An image map is an alist where each element has the format
@code{(@var{area} @var{id} @var{plist})}.  An @var{area} is specified
as either a rectangle, a circle, or a polygon.

A rectangle is a cons
@code{(rect . ((@var{x0} . @var{y0}) . (@var{x1} . @var{y1})))}
which specifies the pixel coordinates of the upper left and bottom right
corners of the rectangle area.

A circle is a cons
@code{(circle . ((@var{x0} . @var{y0}) . @var{r}))}
which specifies the center and the radius of the circle; @var{r} may
be a float or integer.

A polygon is a cons
@code{(poly . [@var{x0} @var{y0} @var{x1} @var{y1} ...])}
where each pair in the vector describes one corner in the polygon.

When the mouse pointer lies on a hot-spot area of an image, the
@var{plist} of that hot-spot is consulted; if it contains a @code{help-echo}
property, that defines a tool-tip for the hot-spot, and if it contains
a @code{pointer} property, that defines the shape of the mouse cursor when
it is on the hot-spot.
@xref{Pointer Shape}, for available pointer shapes.

When you click the mouse when the mouse pointer is over a hot-spot, an
event is composed by combining the @var{id} of the hot-spot with the
mouse event; for instance, @code{[area4 mouse-1]} if the hot-spot's
@var{id} is @code{area4}.
@end table

@defun image-mask-p spec &optional frame
This function returns @code{t} if image @var{spec} has a mask bitmap.
@var{frame} is the frame on which the image will be displayed.
@var{frame} @code{nil} or omitted means to use the selected frame
(@pxref{Input Focus}).
@end defun

@node XBM Images
@subsection XBM Images
@cindex XBM

  To use XBM format, specify @code{xbm} as the image type.  This image
format doesn't require an external library, so images of this type are
always supported.

  Additional image properties supported for the @code{xbm} image type are:

@table @code
@item :foreground @var{foreground}
The value, @var{foreground}, should be a string specifying the image
foreground color, or @code{nil} for the default color.  This color is
used for each pixel in the XBM that is 1.  The default is the frame's
foreground color.

@item :background @var{background}
The value, @var{background}, should be a string specifying the image
background color, or @code{nil} for the default color.  This color is
used for each pixel in the XBM that is 0.  The default is the frame's
background color.
@end table

  If you specify an XBM image using data within Emacs instead of an
external file, use the following three properties:

@table @code
@item :data @var{data}
The value, @var{data}, specifies the contents of the image.
There are three formats you can use for @var{data}:

@itemize @bullet
@item
A vector of strings or bool-vectors, each specifying one line of the
image.  Do specify @code{:height} and @code{:width}.

@item
A string containing the same byte sequence as an XBM file would contain.
You must not specify @code{:height} and @code{:width} in this case,
because omitting them is what indicates the data has the format of an
XBM file.  The file contents specify the height and width of the image.

@item
A string or a bool-vector containing the bits of the image (plus perhaps
some extra bits at the end that will not be used).  It should contain at
least @var{width} * @code{height} bits.  In this case, you must specify
@code{:height} and @code{:width}, both to indicate that the string
contains just the bits rather than a whole XBM file, and to specify the
size of the image.
@end itemize

@item :width @var{width}
The value, @var{width}, specifies the width of the image, in pixels.

@item :height @var{height}
The value, @var{height}, specifies the height of the image, in pixels.
@end table

@node XPM Images
@subsection XPM Images
@cindex XPM

  To use XPM format, specify @code{xpm} as the image type.  The
additional image property @code{:color-symbols} is also meaningful with
the @code{xpm} image type:

@table @code
@item :color-symbols @var{symbols}
The value, @var{symbols}, should be an alist whose elements have the
form @code{(@var{name} . @var{color})}.  In each element, @var{name} is
the name of a color as it appears in the image file, and @var{color}
specifies the actual color to use for displaying that name.
@end table

@node GIF Images
@subsection GIF Images
@cindex GIF

  For GIF images, specify image type @code{gif}.

@table @code
@item :index @var{index}
You can use @code{:index} to specify one image from a GIF file that
contains more than one image.  This property specifies use of image
number @var{index} from the file.  If the GIF file doesn't contain an
image with index @var{index}, the image displays as a hollow box.
@end table

@ignore
This could be used to implement limited support for animated GIFs.
For example, the following function displays a multi-image GIF file
at point-min in the current buffer, switching between sub-images
every 0.1 seconds.

(defun show-anim (file max)
  "Display multi-image GIF file FILE which contains MAX subimages."
  (display-anim (current-buffer) file 0 max t))

(defun display-anim (buffer file idx max first-time)
  (when (= idx max)
    (setq idx 0))
  (let ((img (create-image file nil :image idx)))
    (save-excursion
      (set-buffer buffer)
      (goto-char (point-min))
      (unless first-time (delete-char 1))
      (insert-image img))
    (run-with-timer 0.1 nil 'display-anim buffer file (1+ idx) max nil)))
@end ignore

@node Postscript Images
@subsection Postscript Images
@cindex Postscript images

  To use Postscript for an image, specify image type @code{postscript}.
This works only if you have Ghostscript installed.  You must always use
these three properties:

@table @code
@item :pt-width @var{width}
The value, @var{width}, specifies the width of the image measured in
points (1/72 inch).  @var{width} must be an integer.

@item :pt-height @var{height}
The value, @var{height}, specifies the height of the image in points
(1/72 inch).  @var{height} must be an integer.

@item :bounding-box @var{box}
The value, @var{box}, must be a list or vector of four integers, which
specifying the bounding box of the Postscript image, analogous to the
@samp{BoundingBox} comment found in Postscript files.

@example
%%BoundingBox: 22 171 567 738
@end example
@end table

  Displaying Postscript images from Lisp data is not currently
implemented, but it may be implemented by the time you read this.
See the @file{etc/NEWS} file to make sure.

@node Other Image Types
@subsection Other Image Types
@cindex PBM

  For PBM images, specify image type @code{pbm}.  Color, gray-scale and
monochromatic images are supported.   For mono PBM images, two additional
image properties are supported.

@table @code
@item :foreground @var{foreground}
The value, @var{foreground}, should be a string specifying the image
foreground color, or @code{nil} for the default color.  This color is
used for each pixel in the XBM that is 1.  The default is the frame's
foreground color.

@item :background @var{background}
The value, @var{background}, should be a string specifying the image
background color, or @code{nil} for the default color.  This color is
used for each pixel in the XBM that is 0.  The default is the frame's
background color.
@end table

  For JPEG images, specify image type @code{jpeg}.

  For TIFF images, specify image type @code{tiff}.

  For PNG images, specify image type @code{png}.

@node Defining Images
@subsection Defining Images

  The functions @code{create-image}, @code{defimage} and
@code{find-image} provide convenient ways to create image descriptors.

@defun create-image file-or-data &optional type data-p &rest props
This function creates and returns an image descriptor which uses the
data in @var{file-or-data}.  @var{file-or-data} can be a file name or
a string containing the image data; @var{data-p} should be @code{nil}
for the former case, non-@code{nil} for the latter case.

The optional argument @var{type} is a symbol specifying the image type.
If @var{type} is omitted or @code{nil}, @code{create-image} tries to
determine the image type from the file's first few bytes, or else
from the file's name.

The remaining arguments, @var{props}, specify additional image
properties---for example,

@example
(create-image "foo.xpm" 'xpm nil :heuristic-mask t)
@end example

The function returns @code{nil} if images of this type are not
supported.  Otherwise it returns an image descriptor.
@end defun

@defmac defimage symbol specs &optional doc
This macro defines @var{symbol} as an image name.  The arguments
@var{specs} is a list which specifies how to display the image.
The third argument, @var{doc}, is an optional documentation string.

Each argument in @var{specs} has the form of a property list, and each
one should specify at least the @code{:type} property and either the
@code{:file} or the @code{:data} property.  The value of @code{:type}
should be a symbol specifying the image type, the value of
@code{:file} is the file to load the image from, and the value of
@code{:data} is a string containing the actual image data.  Here is an
example:

@example
(defimage test-image
  ((:type xpm :file "~/test1.xpm")
   (:type xbm :file "~/test1.xbm")))
@end example

@code{defimage} tests each argument, one by one, to see if it is
usable---that is, if the type is supported and the file exists.  The
first usable argument is used to make an image descriptor which is
stored in @var{symbol}.

If none of the alternatives will work, then @var{symbol} is defined
as @code{nil}.
@end defmac

@defun find-image specs
This function provides a convenient way to find an image satisfying one
of a list of image specifications @var{specs}.

Each specification in @var{specs} is a property list with contents
depending on image type.  All specifications must at least contain the
properties @code{:type @var{type}} and either @w{@code{:file @var{file}}}
or @w{@code{:data @var{DATA}}}, where @var{type} is a symbol specifying
the image type, e.g.@: @code{xbm}, @var{file} is the file to load the
image from, and @var{data} is a string containing the actual image data.
The first specification in the list whose @var{type} is supported, and
@var{file} exists, is used to construct the image specification to be
returned.  If no specification is satisfied, @code{nil} is returned.

The image is looked for in @code{image-load-path}.
@end defun

@defvar image-load-path
This variable's value is a list of locations in which to search for
image files.  If an element is a string or a variable symbol whose
value is a string, the string is taken to be the name of a directory
to search.  If an element is a variable symbol whose value is a list,
that is taken to be a list of directory names to search.

The default is to search in the @file{images} subdirectory of the
directory specified by @code{data-directory}, then the directory
specified by @code{data-directory}, and finally in the directories in
@code{load-path}.  Subdirectories are not automatically included in
the search, so if you put an image file in a subdirectory, you have to
supply the subdirectory name explicitly.  For example, to find the
image @file{images/foo/bar.xpm} within @code{data-directory}, you
should specify the image as follows:

@example
(defimage foo-image '((:type xpm :file "foo/bar.xpm")))
@end example
@end defvar

@defun image-load-path-for-library library image &optional path no-error
This function returns a suitable search path for images used by the
Lisp package @var{library}.

The function searches for @var{image} first in @code{image-load-path}
(excluding @file{@code{data-directory}/images}) and then in
@code{load-path}, followed by a path suitable for @var{library}, which
includes @file{../../etc/images} and @file{../etc/images} relative to
the library file itself, and finally in
@file{@code{data-directory}/images}.

Then this function returns a list of directories which contains first
the directory in which @var{image} was found, followed by the value of
@code{load-path}.  If @var{path} is given, it is used instead of
@code{load-path}.

If @var{no-error} is non-@code{nil} and a suitable path can't be
found, don't signal an error.  Instead, return a list of directories as
before, except that @code{nil} appears in place of the image directory.

Here is an example that uses a common idiom to provide compatibility
with versions of Emacs that lack the variable @code{image-load-path}:

@example
(defvar image-load-path) ; shush compiler
(let* ((load-path (image-load-path-for-library
                        "mh-e" "mh-logo.xpm"))
       (image-load-path (cons (car load-path)
                              (when (boundp 'image-load-path)
                                image-load-path))))
  (mh-tool-bar-folder-buttons-init))
@end example
@end defun

@node Showing Images
@subsection Showing Images

  You can use an image descriptor by setting up the @code{display}
property yourself, but it is easier to use the functions in this
section.

@defun insert-image image &optional string area slice
This function inserts @var{image} in the current buffer at point.  The
value @var{image} should be an image descriptor; it could be a value
returned by @code{create-image}, or the value of a symbol defined with
@code{defimage}.  The argument @var{string} specifies the text to put
in the buffer to hold the image.  If it is omitted or @code{nil},
@code{insert-image} uses @code{" "} by default.

The argument @var{area} specifies whether to put the image in a margin.
If it is @code{left-margin}, the image appears in the left margin;
@code{right-margin} specifies the right margin.  If @var{area} is
@code{nil} or omitted, the image is displayed at point within the
buffer's text.

The argument @var{slice} specifies a slice of the image to insert.  If
@var{slice} is @code{nil} or omitted the whole image is inserted.
Otherwise, @var{slice} is a list @code{(@var{x} @var{y} @var{width}
@var{height})} which specifies the @var{x} and @var{y} positions and
@var{width} and @var{height} of the image area to insert.  Integer
values are in units of pixels.  A floating point number in the range
0.0--1.0 stands for that fraction of the width or height of the entire
image.

Internally, this function inserts @var{string} in the buffer, and gives
it a @code{display} property which specifies @var{image}.  @xref{Display
Property}.
@end defun

@defun insert-sliced-image image &optional string area rows cols
This function inserts @var{image} in the current buffer at point, like
@code{insert-image}, but splits the image into @var{rows}x@var{cols}
equally sized slices.
@end defun

@defun put-image image pos &optional string area
This function puts image @var{image} in front of @var{pos} in the
current buffer.  The argument @var{pos} should be an integer or a
marker.  It specifies the buffer position where the image should appear.
The argument @var{string} specifies the text that should hold the image
as an alternative to the default.

The argument @var{image} must be an image descriptor, perhaps returned
by @code{create-image} or stored by @code{defimage}.

The argument @var{area} specifies whether to put the image in a margin.
If it is @code{left-margin}, the image appears in the left margin;
@code{right-margin} specifies the right margin.  If @var{area} is
@code{nil} or omitted, the image is displayed at point within the
buffer's text.

Internally, this function creates an overlay, and gives it a
@code{before-string} property containing text that has a @code{display}
property whose value is the image.  (Whew!)
@end defun

@defun remove-images start end &optional buffer
This function removes images in @var{buffer} between positions
@var{start} and @var{end}.  If @var{buffer} is omitted or @code{nil},
images are removed from the current buffer.

This removes only images that were put into @var{buffer} the way
@code{put-image} does it, not images that were inserted with
@code{insert-image} or in other ways.
@end defun

@defun image-size spec &optional pixels frame
This function returns the size of an image as a pair
@w{@code{(@var{width} . @var{height})}}.  @var{spec} is an image
specification.  @var{pixels} non-@code{nil} means return sizes
measured in pixels, otherwise return sizes measured in canonical
character units (fractions of the width/height of the frame's default
font).  @var{frame} is the frame on which the image will be displayed.
@var{frame} null or omitted means use the selected frame (@pxref{Input
Focus}).
@end defun

@defvar max-image-size
This variable is used to define the maximum size of image that Emacs
will load.  Emacs will refuse to load (and display) any image that is
larger than this limit.

If the value is an integer, it directly specifies the maximum
image height and width, measured in pixels.  If it is a floating
point number, it specifies the maximum image height and width
as a ratio to the frame height and width.  If the value is
non-numeric, there is no explicit limit on the size of images.

The purpose of this variable is to prevent unreasonably large images
from accidentally being loaded into Emacs.  It only takes effect the
first time an image is loaded.  Once an image is placed in the image
cache, it can always be displayed, even if the value of
@var{max-image-size} is subsequently changed (@pxref{Image Cache}).
@end defvar

@node Image Cache
@subsection Image Cache

  Emacs stores images in an image cache when it displays them, so it can
display them again more efficiently.  It removes an image from the cache
when it hasn't been displayed for a specified period of time.

When an image is looked up in the cache, its specification is compared
with cached image specifications using @code{equal}.  This means that
all images with equal specifications share the same image in the cache.

@defvar image-cache-eviction-delay
This variable specifies the number of seconds an image can remain in the
cache without being displayed.  When an image is not displayed for this
length of time, Emacs removes it from the image cache.

If the value is @code{nil}, Emacs does not remove images from the cache
except when you explicitly clear it.  This mode can be useful for
debugging.
@end defvar

@defun clear-image-cache &optional frame
This function clears the image cache.  If @var{frame} is non-@code{nil},
only the cache for that frame is cleared.  Otherwise all frames' caches
are cleared.
@end defun

@node Buttons
@section Buttons
@cindex buttons
@cindex buttons in buffers
@cindex clickable buttons in buffers

  The @emph{button} package defines functions for inserting and
manipulating clickable (with the mouse, or via keyboard commands)
buttons in Emacs buffers, such as might be used for help hyper-links,
etc.  Emacs uses buttons for the hyper-links in help text and the like.

  A button is essentially a set of properties attached (via text
properties or overlays) to a region of text in an Emacs buffer.  These
properties are called @dfn{button properties}.

  One of the these properties (@code{action}) is a function, which will
be called when the user invokes it using the keyboard or the mouse.
The invoked function may then examine the button and use its other
properties as desired.

  In some ways the Emacs button package duplicates functionality offered
by the widget package (@pxref{Top, , Introduction, widget, The Emacs
Widget Library}), but the button package has the advantage that it is
much faster, much smaller, and much simpler to use (for elisp
programmers---for users, the result is about the same).  The extra
speed and space savings are useful mainly if you need to create many
buttons in a buffer (for instance an @code{*Apropos*} buffer uses
buttons to make entries clickable, and may contain many thousands of
entries).

@menu
* Button Properties::      Button properties with special meanings.
* Button Types::           Defining common properties for classes of buttons.
* Making Buttons::         Adding buttons to Emacs buffers.
* Manipulating Buttons::   Getting and setting properties of buttons.
* Button Buffer Commands:: Buffer-wide commands and bindings for buttons.
@end menu

@node Button Properties
@subsection Button Properties
@cindex button properties

  Buttons have an associated list of properties defining their
appearance and behavior, and other arbitrary properties may be used
for application specific purposes.  Some properties that have special
meaning to the button package include:

@table @code
@item action
@kindex action @r{(button property)}
The function to call when the user invokes the button, which is passed
the single argument @var{button}.  By default this is @code{ignore},
which does nothing.

@item mouse-action
@kindex mouse-action @r{(button property)}
This is similar to @code{action}, and when present, will be used
instead of @code{action} for button invocations resulting from
mouse-clicks (instead of the user hitting @key{RET}).  If not
present, mouse-clicks use @code{action} instead.

@item face
@kindex face @r{(button property)}
This is an Emacs face controlling how buttons of this type are
displayed; by default this is the @code{button} face.

@item mouse-face
@kindex mouse-face @r{(button property)}
This is an additional face which controls appearance during
mouse-overs (merged with the usual button face); by default this is
the usual Emacs @code{highlight} face.

@item keymap
@kindex keymap @r{(button property)}
The button's keymap, defining bindings active within the button
region.  By default this is the usual button region keymap, stored
in the variable @code{button-map}, which defines @key{RET} and
@key{mouse-2} to invoke the button.

@item type
@kindex type @r{(button property)}
The button-type of the button.  When creating a button, this is
usually specified using the @code{:type} keyword argument.
@xref{Button Types}.

@item help-echo
@kindex help-index @r{(button property)}
A string displayed by the Emacs tool-tip help system; by default,
@code{"mouse-2, RET: Push this button"}.

@item follow-link
@kindex follow-link @r{(button property)}
The follow-link property, defining how a @key{Mouse-1} click behaves
on this button, @xref{Links and Mouse-1}.

@item button
@kindex button @r{(button property)}
All buttons have a non-@code{nil} @code{button} property, which may be useful
in finding regions of text that comprise buttons (which is what the
standard button functions do).
@end table

  There are other properties defined for the regions of text in a
button, but these are not generally interesting for typical uses.

@node Button Types
@subsection Button Types
@cindex button types

  Every button has a button @emph{type}, which defines default values
for the button's properties.  Button types are arranged in a
hierarchy, with specialized types inheriting from more general types,
so that it's easy to define special-purpose types of buttons for
specific tasks.

@defun define-button-type name &rest properties
Define a `button type' called @var{name}.  The remaining arguments
form a sequence of @var{property value} pairs, specifying default
property values for buttons with this type (a button's type may be set
by giving it a @code{type} property when creating the button, using
the @code{:type} keyword argument).

In addition, the keyword argument @code{:supertype} may be used to
specify a button-type from which @var{name} inherits its default
property values.  Note that this inheritance happens only when
@var{name} is defined; subsequent changes to a supertype are not
reflected in its subtypes.
@end defun

  Using @code{define-button-type} to define default properties for
buttons is not necessary---buttons without any specified type use the
built-in button-type @code{button}---but it is encouraged, since
doing so usually makes the resulting code clearer and more efficient.

@node Making Buttons
@subsection Making Buttons
@cindex making buttons

  Buttons are associated with a region of text, using an overlay or
text properties to hold button-specific information, all of which are
initialized from the button's type (which defaults to the built-in
button type @code{button}).  Like all Emacs text, the appearance of
the button is governed by the @code{face} property; by default (via
the @code{face} property inherited from the @code{button} button-type)
this is a simple underline, like a typical web-page link.

  For convenience, there are two sorts of button-creation functions,
those that add button properties to an existing region of a buffer,
called @code{make-...button}, and those that also insert the button
text, called @code{insert-...button}.

  The button-creation functions all take the @code{&rest} argument
@var{properties}, which should be a sequence of @var{property value}
pairs, specifying properties to add to the button; see @ref{Button
Properties}.  In addition, the keyword argument @code{:type} may be
used to specify a button-type from which to inherit other properties;
see @ref{Button Types}.  Any properties not explicitly specified
during creation will be inherited from the button's type (if the type
defines such a property).

  The following functions add a button using an overlay
(@pxref{Overlays}) to hold the button properties:

@defun make-button beg end &rest properties
This makes a button from @var{beg} to @var{end} in the
current buffer, and returns it.
@end defun

@defun insert-button label &rest properties
This insert a button with the label @var{label} at point,
and returns it.
@end defun

  The following functions are similar, but use Emacs text properties
(@pxref{Text Properties}) to hold the button properties, making the
button actually part of the text instead of being a property of the
buffer.  Buttons using text properties do not create markers into the
buffer, which is important for speed when you use extremely large
numbers of buttons.  Both functions return the position of the start
of the new button:

@defun make-text-button beg end &rest properties
This makes a button from @var{beg} to @var{end} in the current buffer, using
text properties.
@end defun

@defun insert-text-button label &rest properties
This inserts a button with the label @var{label} at point, using text
properties.
@end defun

@node Manipulating Buttons
@subsection Manipulating Buttons
@cindex manipulating buttons

These are functions for getting and setting properties of buttons.
Often these are used by a button's invocation function to determine
what to do.

Where a @var{button} parameter is specified, it means an object
referring to a specific button, either an overlay (for overlay
buttons), or a buffer-position or marker (for text property buttons).
Such an object is passed as the first argument to a button's
invocation function when it is invoked.

@defun button-start button
Return the position at which @var{button} starts.
@end defun

@defun button-end button
Return the position at which @var{button} ends.
@end defun

@defun button-get button prop
Get the property of button @var{button} named @var{prop}.
@end defun

@defun button-put button prop val
Set @var{button}'s @var{prop} property to @var{val}.
@end defun

@defun button-activate button &optional use-mouse-action
Call @var{button}'s @code{action} property (i.e., invoke it).  If
@var{use-mouse-action} is non-@code{nil}, try to invoke the button's
@code{mouse-action} property instead of @code{action}; if the button
has no @code{mouse-action} property, use @code{action} as normal.
@end defun

@defun button-label button
Return @var{button}'s text label.
@end defun

@defun button-type button
Return @var{button}'s button-type.
@end defun

@defun button-has-type-p button type
Return @code{t} if @var{button} has button-type @var{type}, or one of
@var{type}'s subtypes.
@end defun

@defun button-at pos
Return the button at position @var{pos} in the current buffer, or @code{nil}.
@end defun

@defun button-type-put type prop val
Set the button-type @var{type}'s @var{prop} property to @var{val}.
@end defun

@defun button-type-get type prop
Get the property of button-type @var{type} named @var{prop}.
@end defun

@defun button-type-subtype-p type supertype
Return @code{t} if button-type @var{type} is a subtype of @var{supertype}.
@end defun

@node Button Buffer Commands
@subsection Button Buffer Commands
@cindex button buffer commands

These are commands and functions for locating and operating on
buttons in an Emacs buffer.

@code{push-button} is the command that a user uses to actually `push'
a button, and is bound by default in the button itself to @key{RET}
and to @key{mouse-2} using a region-specific keymap.  Commands
that are useful outside the buttons itself, such as
@code{forward-button} and @code{backward-button} are additionally
available in the keymap stored in @code{button-buffer-map}; a mode
which uses buttons may want to use @code{button-buffer-map} as a
parent keymap for its keymap.

If the button has a non-@code{nil} @code{follow-link} property, and
@var{mouse-1-click-follows-link} is set, a quick @key{Mouse-1} click
will also activate the @code{push-button} command.
@xref{Links and Mouse-1}.

@deffn Command push-button &optional pos use-mouse-action
Perform the action specified by a button at location @var{pos}.
@var{pos} may be either a buffer position or a mouse-event.  If
@var{use-mouse-action} is non-@code{nil}, or @var{pos} is a
mouse-event (@pxref{Mouse Events}), try to invoke the button's
@code{mouse-action} property instead of @code{action}; if the button
has no @code{mouse-action} property, use @code{action} as normal.
@var{pos} defaults to point, except when @code{push-button} is invoked
interactively as the result of a mouse-event, in which case, the mouse
event's position is used.  If there's no button at @var{pos}, do
nothing and return @code{nil}, otherwise return @code{t}.
@end deffn

@deffn Command forward-button n &optional wrap display-message
Move to the @var{n}th next button, or @var{n}th previous button if
@var{n} is negative.  If @var{n} is zero, move to the start of any
button at point.  If @var{wrap} is non-@code{nil}, moving past either
end of the buffer continues from the other end.  If
@var{display-message} is non-@code{nil}, the button's help-echo string
is displayed.  Any button with a non-@code{nil} @code{skip} property
is skipped over.  Returns the button found.
@end deffn

@deffn Command backward-button n &optional wrap display-message
Move to the @var{n}th previous button, or @var{n}th next button if
@var{n} is negative.  If @var{n} is zero, move to the start of any
button at point.  If @var{wrap} is non-@code{nil}, moving past either
end of the buffer continues from the other end.  If
@var{display-message} is non-@code{nil}, the button's help-echo string
is displayed.  Any button with a non-@code{nil} @code{skip} property
is skipped over.  Returns the button found.
@end deffn

@defun next-button pos &optional count-current
Return the next button after position @var{pos} in the current buffer.
If @var{count-current} is non-@code{nil}, count any button at
@var{pos} in the search, instead of starting at the next button.
@end defun

@defun previous-button pos &optional count-current
Return the @var{n}th button before position @var{pos} in the current
buffer.  If @var{count-current} is non-@code{nil}, count any button at
@var{pos} in the search, instead of starting at the next button.
@end defun

@node Abstract Display
@section Abstract Display
@cindex ewoc
@cindex display, abstract
@cindex display, arbitrary objects
@cindex model/view/controller
@cindex view part, model/view/controller

  The Ewoc package constructs buffer text that represents a structure
of Lisp objects, and updates the text to follow changes in that
structure.  This is like the ``view'' component in the
``model/view/controller'' design paradigm.

  An @dfn{ewoc} is a structure that organizes information required to
construct buffer text that represents certain Lisp data.  The buffer
text of the ewoc has three parts, in order: first, fixed @dfn{header}
text; next, textual descriptions of a series of data elements (Lisp
objects that you specify); and last, fixed @dfn{footer} text.
Specifically, an ewoc contains information on:

@itemize @bullet
@item
The buffer which its text is generated in.

@item
The text's start position in the buffer.

@item
The header and footer strings.

@item
A doubly-linked chain of @dfn{nodes}, each of which contains:

@itemize
@item
A @dfn{data element}, a single Lisp object.

@item
Links to the preceding and following nodes in the chain.
@end itemize

@item
A @dfn{pretty-printer} function which is responsible for
inserting the textual representation of a data
element value into the current buffer.
@end itemize

  Typically, you define an ewoc with @code{ewoc-create}, and then pass
the resulting ewoc structure to other functions in the Ewoc package to
build nodes within it, and display it in the buffer.  Once it is
displayed in the buffer, other functions determine the correspondance
between buffer positions and nodes, move point from one node's textual
representation to another, and so forth.  @xref{Abstract Display
Functions}.

  A node @dfn{encapsulates} a data element much the way a variable
holds a value.  Normally, encapsulation occurs as a part of adding a
node to the ewoc.  You can retrieve the data element value and place a
new value in its place, like so:

@lisp
(ewoc-data @var{node})
@result{} value

(ewoc-set-data @var{node} @var{new-value})
@result{} @var{new-value}
@end lisp

@noindent
You can also use, as the data element value, a Lisp object (list or
vector) that is a container for the ``real'' value, or an index into
some other structure.  The example (@pxref{Abstract Display Example})
uses the latter approach.

  When the data changes, you will want to update the text in the
buffer.  You can update all nodes by calling @code{ewoc-refresh}, or
just specific nodes using @code{ewoc-invalidate}, or all nodes
satisfying a predicate using @code{ewoc-map}.  Alternatively, you can
delete invalid nodes using @code{ewoc-delete} or @code{ewoc-filter},
and add new nodes in their place.  Deleting a node from an ewoc deletes
its associated textual description from buffer, as well.

@menu
* Abstract Display Functions::
* Abstract Display Example::
@end menu

@node Abstract Display Functions
@subsection Abstract Display Functions

  In this subsection, @var{ewoc} and @var{node} stand for the
structures described above (@pxref{Abstract Display}), while
@var{data} stands for an arbitrary Lisp object used as a data element.

@defun ewoc-create pretty-printer &optional header footer nosep
This constructs and returns a new ewoc, with no nodes (and thus no data
elements).  @var{pretty-printer} should be a function that takes one
argument, a data element of the sort you plan to use in this ewoc, and
inserts its textual description at point using @code{insert} (and never
@code{insert-before-markers}, because that would interfere with the
Ewoc package's internal mechanisms).

Normally, a newline is automatically inserted after the header,
the footer and every node's textual description.  If @var{nosep}
is non-@code{nil}, no newline is inserted.  This may be useful for
displaying an entire ewoc on a single line, for example, or for
making nodes ``invisible'' by arranging for @var{pretty-printer}
to do nothing for those nodes.

An ewoc maintains its text in the buffer that is current when
you create it, so switch to the intended buffer before calling
@code{ewoc-create}.
@end defun

@defun ewoc-buffer ewoc
This returns the buffer where @var{ewoc} maintains its text.
@end defun

@defun ewoc-get-hf ewoc
This returns a cons cell @code{(@var{header} . @var{footer})}
made from @var{ewoc}'s header and footer.
@end defun

@defun ewoc-set-hf ewoc header footer
This sets the header and footer of @var{ewoc} to the strings
@var{header} and @var{footer}, respectively.
@end defun

@defun ewoc-enter-first ewoc data
@defunx ewoc-enter-last ewoc data
These add a new node encapsulating @var{data}, putting it, respectively,
at the beginning or end of @var{ewoc}'s chain of nodes.
@end defun

@defun ewoc-enter-before ewoc node data
@defunx ewoc-enter-after ewoc node data
These add a new node encapsulating @var{data}, adding it to
@var{ewoc} before or after @var{node}, respectively.
@end defun

@defun ewoc-prev ewoc node
@defunx ewoc-next ewoc node
These return, respectively, the previous node and the next node of @var{node}
in @var{ewoc}.
@end defun

@defun ewoc-nth ewoc n
This returns the node in @var{ewoc} found at zero-based index @var{n}.
A negative @var{n} means count from the end.  @code{ewoc-nth} returns
@code{nil} if @var{n} is out of range.
@end defun

@defun ewoc-data node
This extracts the data encapsulated by @var{node} and returns it.
@end defun

@defun ewoc-set-data node data
This sets the data encapsulated by @var{node} to @var{data}.
@end defun

@defun ewoc-locate ewoc &optional pos guess
This determines the node in @var{ewoc} which contains point (or
@var{pos} if specified), and returns that node.  If @var{ewoc} has no
nodes, it returns @code{nil}.  If @var{pos} is before the first node,
it returns the first node; if @var{pos} is after the last node, it returns
the last node.  The optional third arg @var{guess}
should be a node that is likely to be near @var{pos}; this doesn't
alter the result, but makes the function run faster.
@end defun

@defun ewoc-location node
This returns the start position of @var{node}.
@end defun

@defun ewoc-goto-prev ewoc arg
@defunx ewoc-goto-next ewoc arg
These move point to the previous or next, respectively, @var{arg}th node
in @var{ewoc}.  @code{ewoc-goto-prev} does not move if it is already at
the first node or if @var{ewoc} is empty, whereas @code{ewoc-goto-next}
moves past the last node, returning @code{nil}.  Excepting this special
case, these functions return the node moved to.
@end defun

@defun ewoc-goto-node ewoc node
This moves point to the start of @var{node} in @var{ewoc}.
@end defun

@defun ewoc-refresh ewoc
This function regenerates the text of @var{ewoc}.  It works by
deleting the text between the header and the footer, i.e., all the
data elements' representations, and then calling the pretty-printer
function for each node, one by one, in order.
@end defun

@defun ewoc-invalidate ewoc &rest nodes
This is similar to @code{ewoc-refresh}, except that only @var{nodes} in
@var{ewoc} are updated instead of the entire set.
@end defun

@defun ewoc-delete ewoc &rest nodes
This deletes each node in @var{nodes} from @var{ewoc}.
@end defun

@defun ewoc-filter ewoc predicate &rest args
This calls @var{predicate} for each data element in @var{ewoc} and
deletes those nodes for which @var{predicate} returns @code{nil}.
Any @var{args} are passed to @var{predicate}.
@end defun

@defun ewoc-collect ewoc predicate &rest args
This calls @var{predicate} for each data element in @var{ewoc}
and returns a list of those elements for which @var{predicate}
returns non-@code{nil}.  The elements in the list are ordered
as in the buffer.  Any @var{args} are passed to @var{predicate}.
@end defun

@defun ewoc-map map-function ewoc &rest args
This calls @var{map-function} for each data element in @var{ewoc} and
updates those nodes for which @var{map-function} returns non-@code{nil}.
Any @var{args} are passed to @var{map-function}.
@end defun

@node Abstract Display Example
@subsection Abstract Display Example

  Here is a simple example using functions of the ewoc package to
implement a ``color components display'', an area in a buffer that
represents a vector of three integers (itself representing a 24-bit RGB
value) in various ways.

@example
(setq colorcomp-ewoc nil
      colorcomp-data nil
      colorcomp-mode-map nil
      colorcomp-labels ["Red" "Green" "Blue"])

(defun colorcomp-pp (data)
  (if data
      (let ((comp (aref colorcomp-data data)))
        (insert (aref colorcomp-labels data) "\t: #x"
                (format "%02X" comp) " "
                (make-string (ash comp -2) ?#) "\n"))
    (let ((cstr (format "#%02X%02X%02X"
                        (aref colorcomp-data 0)
                        (aref colorcomp-data 1)
                        (aref colorcomp-data 2)))
          (samp " (sample text) "))
      (insert "Color\t: "
              (propertize samp 'face `(foreground-color . ,cstr))
              (propertize samp 'face `(background-color . ,cstr))
              "\n"))))

(defun colorcomp (color)
  "Allow fiddling with COLOR in a new buffer.
The buffer is in Color Components mode."
  (interactive "sColor (name or #RGB or #RRGGBB): ")
  (when (string= "" color)
    (setq color "green"))
  (unless (color-values color)
    (error "No such color: %S" color))
  (switch-to-buffer
   (generate-new-buffer (format "originally: %s" color)))
  (kill-all-local-variables)
  (setq major-mode 'colorcomp-mode
        mode-name "Color Components")
  (use-local-map colorcomp-mode-map)
  (erase-buffer)
  (buffer-disable-undo)
  (let ((data (apply 'vector (mapcar (lambda (n) (ash n -8))
                                     (color-values color))))
        (ewoc (ewoc-create 'colorcomp-pp
                           "\nColor Components\n\n"
                           (substitute-command-keys
                            "\n\\@{colorcomp-mode-map@}"))))
    (set (make-local-variable 'colorcomp-data) data)
    (set (make-local-variable 'colorcomp-ewoc) ewoc)
    (ewoc-enter-last ewoc 0)
    (ewoc-enter-last ewoc 1)
    (ewoc-enter-last ewoc 2)
    (ewoc-enter-last ewoc nil)))
@end example

@cindex controller part, model/view/controller
  This example can be extended to be a ``color selection widget'' (in
other words, the controller part of the ``model/view/controller''
design paradigm) by defining commands to modify @code{colorcomp-data}
and to ``finish'' the selection process, and a keymap to tie it all
together conveniently.

@smallexample
(defun colorcomp-mod (index limit delta)
  (let ((cur (aref colorcomp-data index)))
    (unless (= limit cur)
      (aset colorcomp-data index (+ cur delta)))
    (ewoc-invalidate
     colorcomp-ewoc
     (ewoc-nth colorcomp-ewoc index)
     (ewoc-nth colorcomp-ewoc -1))))

(defun colorcomp-R-more () (interactive) (colorcomp-mod 0 255 1))
(defun colorcomp-G-more () (interactive) (colorcomp-mod 1 255 1))
(defun colorcomp-B-more () (interactive) (colorcomp-mod 2 255 1))
(defun colorcomp-R-less () (interactive) (colorcomp-mod 0 0 -1))
(defun colorcomp-G-less () (interactive) (colorcomp-mod 1 0 -1))
(defun colorcomp-B-less () (interactive) (colorcomp-mod 2 0 -1))

(defun colorcomp-copy-as-kill-and-exit ()
  "Copy the color components into the kill ring and kill the buffer.
The string is formatted #RRGGBB (hash followed by six hex digits)."
  (interactive)
  (kill-new (format "#%02X%02X%02X"
                    (aref colorcomp-data 0)
                    (aref colorcomp-data 1)
                    (aref colorcomp-data 2)))
  (kill-buffer nil))

(setq colorcomp-mode-map
      (let ((m (make-sparse-keymap)))
        (suppress-keymap m)
        (define-key m "i" 'colorcomp-R-less)
        (define-key m "o" 'colorcomp-R-more)
        (define-key m "k" 'colorcomp-G-less)
        (define-key m "l" 'colorcomp-G-more)
        (define-key m "," 'colorcomp-B-less)
        (define-key m "." 'colorcomp-B-more)
        (define-key m " " 'colorcomp-copy-as-kill-and-exit)
        m))
@end smallexample

Note that we never modify the data in each node, which is fixed when the
ewoc is created to be either @code{nil} or an index into the vector
@code{colorcomp-data}, the actual color components.

@node Blinking
@section Blinking Parentheses
@cindex parenthesis matching
@cindex blinking
@cindex balancing parentheses
@cindex close parenthesis

  This section describes the mechanism by which Emacs shows a matching
open parenthesis when the user inserts a close parenthesis.

@defvar blink-paren-function
The value of this variable should be a function (of no arguments) to
be called whenever a character with close parenthesis syntax is inserted.
The value of @code{blink-paren-function} may be @code{nil}, in which
case nothing is done.
@end defvar

@defopt blink-matching-paren
If this variable is @code{nil}, then @code{blink-matching-open} does
nothing.
@end defopt

@defopt blink-matching-paren-distance
This variable specifies the maximum distance to scan for a matching
parenthesis before giving up.
@end defopt

@defopt blink-matching-delay
This variable specifies the number of seconds for the cursor to remain
at the matching parenthesis.  A fraction of a second often gives
good results, but the default is 1, which works on all systems.
@end defopt

@deffn Command blink-matching-open
This function is the default value of @code{blink-paren-function}.  It
assumes that point follows a character with close parenthesis syntax and
moves the cursor momentarily to the matching opening character.  If that
character is not already on the screen, it displays the character's
context in the echo area.  To avoid long delays, this function does not
search farther than @code{blink-matching-paren-distance} characters.

Here is an example of calling this function explicitly.

@smallexample
@group
(defun interactive-blink-matching-open ()
@c Do not break this line! -- rms.
@c The first line of a doc string
@c must stand alone.
  "Indicate momentarily the start of sexp before point."
  (interactive)
@end group
@group
  (let ((blink-matching-paren-distance
         (buffer-size))
        (blink-matching-paren t))
    (blink-matching-open)))
@end group
@end smallexample
@end deffn

@node Usual Display
@section Usual Display Conventions

  The usual display conventions define how to display each character
code.  You can override these conventions by setting up a display table
(@pxref{Display Tables}).  Here are the usual display conventions:

@itemize @bullet
@item
Character codes 32 through 126 map to glyph codes 32 through 126.
Normally this means they display as themselves.

@item
Character code 9 is a horizontal tab.  It displays as whitespace
up to a position determined by @code{tab-width}.

@item
Character code 10 is a newline.

@item
All other codes in the range 0 through 31, and code 127, display in one
of two ways according to the value of @code{ctl-arrow}.  If it is
non-@code{nil}, these codes map to sequences of two glyphs, where the
first glyph is the @acronym{ASCII} code for @samp{^}.  (A display table can
specify a glyph to use instead of @samp{^}.)  Otherwise, these codes map
just like the codes in the range 128 to 255.

On MS-DOS terminals, Emacs arranges by default for the character code
127 to be mapped to the glyph code 127, which normally displays as an
empty polygon.  This glyph is used to display non-@acronym{ASCII} characters
that the MS-DOS terminal doesn't support.  @xref{MS-DOS and MULE,,,
emacs, The GNU Emacs Manual}.

@item
Character codes 128 through 255 map to sequences of four glyphs, where
the first glyph is the @acronym{ASCII} code for @samp{\}, and the others are
digit characters representing the character code in octal.  (A display
table can specify a glyph to use instead of @samp{\}.)

@item
Multibyte character codes above 256 are displayed as themselves, or as a
question mark or empty box if the terminal cannot display that
character.
@end itemize

  The usual display conventions apply even when there is a display
table, for any character whose entry in the active display table is
@code{nil}.  Thus, when you set up a display table, you need only
specify the characters for which you want special behavior.

  These display rules apply to carriage return (character code 13), when
it appears in the buffer.  But that character may not appear in the
buffer where you expect it, if it was eliminated as part of end-of-line
conversion (@pxref{Coding System Basics}).

  These variables affect the way certain characters are displayed on the
screen.  Since they change the number of columns the characters occupy,
they also affect the indentation functions.  These variables also affect
how the mode line is displayed; if you want to force redisplay of the
mode line using the new values, call the function
@code{force-mode-line-update} (@pxref{Mode Line Format}).

@defopt ctl-arrow
@cindex control characters in display
This buffer-local variable controls how control characters are
displayed.  If it is non-@code{nil}, they are displayed as a caret
followed by the character: @samp{^A}.  If it is @code{nil}, they are
displayed as a backslash followed by three octal digits: @samp{\001}.
@end defopt

@c Following may have overfull hbox.
@defvar default-ctl-arrow
The value of this variable is the default value for @code{ctl-arrow} in
buffers that do not override it.  @xref{Default Value}.
@end defvar

@defopt tab-width
The value of this buffer-local variable is the spacing between tab
stops used for displaying tab characters in Emacs buffers.  The value
is in units of columns, and the default is 8.  Note that this feature
is completely independent of the user-settable tab stops used by the
command @code{tab-to-tab-stop}.  @xref{Indent Tabs}.
@end defopt

@node Display Tables
@section Display Tables

@cindex display table
You can use the @dfn{display table} feature to control how all possible
character codes display on the screen.  This is useful for displaying
European languages that have letters not in the @acronym{ASCII} character
set.

The display table maps each character code into a sequence of
@dfn{glyphs}, each glyph being a graphic that takes up one character
position on the screen.  You can also define how to display each glyph
on your terminal, using the @dfn{glyph table}.

Display tables affect how the mode line is displayed; if you want to
force redisplay of the mode line using a new display table, call
@code{force-mode-line-update} (@pxref{Mode Line Format}).

@menu
* Display Table Format::  What a display table consists of.
* Active Display Table::  How Emacs selects a display table to use.
* Glyphs::              How to define a glyph, and what glyphs mean.
@end menu

@node Display Table Format
@subsection Display Table Format

  A display table is actually a char-table (@pxref{Char-Tables}) with
@code{display-table} as its subtype.

@defun make-display-table
This creates and returns a display table.  The table initially has
@code{nil} in all elements.
@end defun

  The ordinary elements of the display table are indexed by character
codes; the element at index @var{c} says how to display the character
code @var{c}.  The value should be @code{nil} or a vector of glyph
values (@pxref{Glyphs}).  If an element is @code{nil}, it says to
display that character according to the usual display conventions
(@pxref{Usual Display}).

  If you use the display table to change the display of newline
characters, the whole buffer will be displayed as one long ``line.''

  The display table also has six ``extra slots'' which serve special
purposes.  Here is a table of their meanings; @code{nil} in any slot
means to use the default for that slot, as stated below.

@table @asis
@item 0
The glyph for the end of a truncated screen line (the default for this
is @samp{$}).  @xref{Glyphs}.  On graphical terminals, Emacs uses
arrows in the fringes to indicate truncation, so the display table has
no effect.

@item 1
The glyph for the end of a continued line (the default is @samp{\}).
On graphical terminals, Emacs uses curved arrows in the fringes to
indicate continuation, so the display table has no effect.

@item 2
The glyph for indicating a character displayed as an octal character
code (the default is @samp{\}).

@item 3
The glyph for indicating a control character (the default is @samp{^}).

@item 4
A vector of glyphs for indicating the presence of invisible lines (the
default is @samp{...}).  @xref{Selective Display}.

@item 5
The glyph used to draw the border between side-by-side windows (the
default is @samp{|}).  @xref{Splitting Windows}.  This takes effect only
when there are no scroll bars; if scroll bars are supported and in use,
a scroll bar separates the two windows.
@end table

  For example, here is how to construct a display table that mimics the
effect of setting @code{ctl-arrow} to a non-@code{nil} value:

@example
(setq disptab (make-display-table))
(let ((i 0))
  (while (< i 32)
    (or (= i ?\t) (= i ?\n)
        (aset disptab i (vector ?^ (+ i 64))))
    (setq i (1+ i)))
  (aset disptab 127 (vector ?^ ??)))
@end example

@defun display-table-slot display-table slot
This function returns the value of the extra slot @var{slot} of
@var{display-table}.  The argument @var{slot} may be a number from 0 to
5 inclusive, or a slot name (symbol).  Valid symbols are
@code{truncation}, @code{wrap}, @code{escape}, @code{control},
@code{selective-display}, and @code{vertical-border}.
@end defun

@defun set-display-table-slot display-table slot value
This function stores @var{value} in the extra slot @var{slot} of
@var{display-table}.  The argument @var{slot} may be a number from 0 to
5 inclusive, or a slot name (symbol).  Valid symbols are
@code{truncation}, @code{wrap}, @code{escape}, @code{control},
@code{selective-display}, and @code{vertical-border}.
@end defun

@defun describe-display-table display-table
This function displays a description of the display table
@var{display-table} in a help buffer.
@end defun

@deffn Command describe-current-display-table
This command displays a description of the current display table in a
help buffer.
@end deffn

@node Active Display Table
@subsection Active Display Table
@cindex active display table

  Each window can specify a display table, and so can each buffer.  When
a buffer @var{b} is displayed in window @var{w}, display uses the
display table for window @var{w} if it has one; otherwise, the display
table for buffer @var{b} if it has one; otherwise, the standard display
table if any.  The display table chosen is called the @dfn{active}
display table.

@defun window-display-table &optional window
This function returns @var{window}'s display table, or @code{nil}
if @var{window} does not have an assigned display table.  The default
for @var{window} is the selected window.
@end defun

@defun set-window-display-table window table
This function sets the display table of @var{window} to @var{table}.
The argument @var{table} should be either a display table or
@code{nil}.
@end defun

@defvar buffer-display-table
This variable is automatically buffer-local in all buffers; its value in
a particular buffer specifies the display table for that buffer.  If it
is @code{nil}, that means the buffer does not have an assigned display
table.
@end defvar

@defvar standard-display-table
This variable's value is the default display table, used whenever a
window has no display table and neither does the buffer displayed in
that window.  This variable is @code{nil} by default.
@end defvar

  If there is no display table to use for a particular window---that is,
if the window specifies none, its buffer specifies none, and
@code{standard-display-table} is @code{nil}---then Emacs uses the usual
display conventions for all character codes in that window.  @xref{Usual
Display}.

A number of functions for changing the standard display table
are defined in the library @file{disp-table}.

@node Glyphs
@subsection Glyphs

@cindex glyph
  A @dfn{glyph} is a generalization of a character; it stands for an
image that takes up a single character position on the screen.  Glyphs
are represented in Lisp as integers, just as characters are.  Normally
Emacs finds glyphs in the display table (@pxref{Display Tables}).

  A glyph can be @dfn{simple} or it can be defined by the @dfn{glyph
table}.  A simple glyph is just a way of specifying a character and a
face to output it in.  The glyph code for a simple glyph, mod 524288,
is the character to output, and the glyph code divided by 524288
specifies the face number (@pxref{Face Functions}) to use while
outputting it.  (524288 is
@ifnottex
2**19.)
@end ifnottex
@tex
$2^{19}$.)
@end tex
@xref{Faces}.

  On character terminals, you can set up a @dfn{glyph table} to define
the meaning of glyph codes.  The glyph codes is the value of the
variable @code{glyph-table}.

@defvar glyph-table
The value of this variable is the current glyph table.  It should be a
vector; the @var{g}th element defines glyph code @var{g}.

If a glyph code is greater than or equal to the length of the glyph
table, that code is automatically simple.  If the value of
@code{glyph-table} is @code{nil} instead of a vector, then all glyphs
are simple.  The glyph table is not used on graphical displays, only
on character terminals.  On graphical displays, all glyphs are simple.
@end defvar

  Here are the possible types of elements in the glyph table:

@table @asis
@item @var{string}
Send the characters in @var{string} to the terminal to output
this glyph.  This alternative is available on character terminals,
but not on graphical displays.

@item @var{integer}
Define this glyph code as an alias for glyph code @var{integer}.  You
can use an alias to specify a face code for the glyph and use a small
number as its code.

@item @code{nil}
This glyph is simple.
@end table

@defun create-glyph string
This function returns a newly-allocated glyph code which is set up to
display by sending @var{string} to the terminal.
@end defun

@node Beeping
@section Beeping
@cindex beeping
@cindex bell

  This section describes how to make Emacs ring the bell (or blink the
screen) to attract the user's attention.  Be conservative about how
often you do this; frequent bells can become irritating.  Also be
careful not to use just beeping when signaling an error is more
appropriate.  (@xref{Errors}.)

@defun ding &optional do-not-terminate
@cindex keyboard macro termination
This function beeps, or flashes the screen (see @code{visible-bell} below).
It also terminates any keyboard macro currently executing unless
@var{do-not-terminate} is non-@code{nil}.
@end defun

@defun beep &optional do-not-terminate
This is a synonym for @code{ding}.
@end defun

@defopt visible-bell
This variable determines whether Emacs should flash the screen to
represent a bell.  Non-@code{nil} means yes, @code{nil} means no.  This
is effective on graphical displays, and on text-only terminals
provided the terminal's Termcap entry defines the visible bell
capability (@samp{vb}).
@end defopt

@defvar ring-bell-function
If this is non-@code{nil}, it specifies how Emacs should ``ring the
bell.''  Its value should be a function of no arguments.  If this is
non-@code{nil}, it takes precedence over the @code{visible-bell}
variable.
@end defvar

@node Window Systems
@section Window Systems

  Emacs works with several window systems, most notably the X Window
System.  Both Emacs and X use the term ``window'', but use it
differently.  An Emacs frame is a single window as far as X is
concerned; the individual Emacs windows are not known to X at all.

@defvar window-system
This variable tells Lisp programs what window system Emacs is running
under.  The possible values are

@table @code
@item x
@cindex X Window System
Emacs is displaying using X.
@item pc
Emacs is displaying using MS-DOS.
@item w32
Emacs is displaying using Windows.
@item mac
Emacs is displaying using a Macintosh.
@item nil
Emacs is using a character-based terminal.
@end table
@end defvar

@defvar window-setup-hook
This variable is a normal hook which Emacs runs after handling the
initialization files.  Emacs runs this hook after it has completed
loading your init file, the default initialization file (if
any), and the terminal-specific Lisp code, and running the hook
@code{term-setup-hook}.

This hook is used for internal purposes: setting up communication with
the window system, and creating the initial window.  Users should not
interfere with it.
@end defvar

@ignore
   arch-tag: ffdf5714-7ecf-415b-9023-fbc6b409c2c6
@end ignore