#f is a null pointer for glut callbacks

[clinton/guile-figl.git] / doc / gl.texi

@node GL
@chapter GL


@menu
* About OpenGL::                Know the past to understand the present.
* GL Contexts::                 Finding a square of pixels.
* Rendering::                   How to paint.
* GL API::                      The OpenGL interface, organized by section.
* GL Enumerations::             Enumerated values.
* Low-Level GL::                Primitive interface to OpenGL.
* GL Extensions::               Beyond core OpenGL.
@end menu


@node About OpenGL
@section About OpenGL

OpenGL is a standard API for drawing three-dimensional graphics.  From
its origin in Silicon Graphics's workstations the early 1990s, today
it has become ubiquitous, with implementations on mobile phones,
televisions, tablets, desktops, and even web browsers.

OpenGL has been able to achieve such widespread adoption not just
because it co-evolved with powerful graphics hardware, but also
because it was conceived of as an interface specification and not a
piece of source code.  In fact, these days it is a family of APIs,
available in several flavors and versions:

@table @asis
@item OpenGL 1.x
This series of specifications started with the original releases in
1992, and ended with OpenGL 1.5 in 2003.  This era corresponds to a
time when graphics cards were less powerful and more special-purpose,
with dedicated hardware to handle such details as fog and lighting.
As such the OpenGL 1.x API reflects the capabilities of these special
units.

@item OpenGL 2.x
By the early 2000s, graphics hardware had become much more
general-purpose and needed a more general-purpose API.  The so-called
@dfn{fixed-function rendering pipeline} of the earlier years was
replaced with a @dfn{programmable rendering pipeline}, in which
effects that would have required special hardware were instead
performed by custom programs running on the graphics card.  OpenGL
added support for allocating @dfn{buffer objects} on the graphics
card, and for @dfn{shader programs}, which did the actual rendering.
In time, this buffer-focused API came to be the preferred form of
talking to the GL.

@item OpenGL ES
OpenGL ES was a ``cut-down'' version of OpenGL 2.x, designed to be
small enough to appeal to embedded device vendors.  OpenGL ES 1.x
removed some of the legacy functionality from OpenGL, while adding
interfaces to use fixed-point math, for devices without floating-point
units.  OpenGL ES 2.x went farther still, removing the fixed-function
pipeline entirely.  OpenGL ES 2.x is common on current smart phone
platforms.

@item OpenGL 3.x and above
The OpenGL 3.x series followed the lead of OpenGL ES, first
deprecating (in 3.0) and then removing (in 3.1) the fixed-function
pipeline.  OpenGL 3.0 was released in 2008, but the free Mesa
impementation only began supporting it in 2012, so it is currently
(@value{UPDATED}) less common.
@end table

Figl wraps the OpenGL 2.1 API.  It's a ubiquitous subset of the OpenGL
implementations that are actually deployed in the wild; its legacy API
looks back to OpenGL 1.x, while the buffer-oriented API is compatible
with OpenGL ES.

The full OpenGL 2.1 specification is available at
@uref{http://www.opengl.org/registry/doc/glspec21.20061201.pdf}.


@node GL Contexts
@section GL Contexts

All this talk about drawing is very well and good, but how do you
actually get a canvas?  Interestingly enough, this is outside the
purview of the OpenGL specification.  There are specific ways to get
an @dfn{OpenGL context} for each different windowing system that is
out there.  OpenGL is all crayons and no paper.

For the X window system, there is a standard API for creating a GL
context given a window (or a drawable), @dfn{GLX}.  @xref{GLX}, for
more information on its binding in Guile.

Bseides creating contexts from native windows or drawables, each
backend also supports functions to make a context @dfn{current}.  The
OpenGL API is stateful; you can think of each call as taking an
implicit @dfn{current context} parameter, which holds the current
state of the GL and is operated on by the function in question.
Contexts are thread-specific, and one context should not be active on
more than one thread at a time.

All calls to OpenGL functions must be made while a context is active;
otherwise the result is undefined.  Hopefully while you are getting
used to this rule, your driver is nice enough not to crash on you if
you call a function outside a GL context, but it's not even required
to do that.  Backend-specific functions may or may not require a
context to be current; for example, Windows requires a context to be
current, wheras GLX does not.

There have been a few attempts at abstracting away the need for
calling API specific to a given windowing system, notably GLUT and
EGL.  GLUT is the older of the two, and though it is practically
unchanged since the mid-1990s, it is still widely used on desktops.
@xref{GLUT}, for more on GLUT.

EGL is technically part of OpenGL ES, and was designed with the modern
OpenGL API and mobile hardware in mind, though it also works on the
desktop.  Figl does not yet have an EGL binding.


@node Rendering
@section Rendering

To draw with OpenGL, you obtain a drawing context (@pxref{GL
Contexts}) and send @dfn{the GL} some geometry.  (You can think of the
GL as a layer over your graphics card.)  You can give the GL points,
lines, and triangles in three-dimensional space.  You configure your
GL to render a certain part of space, and it takes your geometry,
rasterizes it, and writes it to the screen (when you tell it to).

That's the basic idea.  You can customize most parts of this
@dfn{rendering pipeline}, by specifying attributes of your geometry
with the OpenGL API, and by programmatically operating on the geometry
and the pixels with programs called @dfn{shaders}.

GL is an @dfn{immediate-mode} graphics API, which is to say that it
doesn't keep around a scene graph of objects.  Instead, at every frame
you as the OpenGL user have to tell the GL what is in the world, and
how to paint it.  It's a fairly low-level interface, but a powerful
one. See
@uref{http://www.opengl.org/wiki/Rendering_Pipeline_Overview}, for
more details.

In the old days of OpenGL 1.0, it was common to call a function to
paint each individual vertex.  You'll still see this style in some old
tutorials.  This quickly gets expensive if you have a lot of vertexes,
though.  This style, known as @dfn{Legacy OpenGL}, was deprecated and
even removed from some versions of OpenGL.  See
@uref{http://www.opengl.org/wiki/Legacy_OpenGL}, for more on the older
APIs.

Instead, the newer thing to do is to send the geometry to the GL in a
big array buffer, and have the GL draw geometry from the buffer.  The
newer functions like @code{glGenBuffers} allocate buffers, returning
an integer that @dfn{names} a buffer managed by the GL.  You as a user
can update the contents of the buffer, but when drawing you reference
the buffer by name.  This has the advantage of reducing the chatter
and data transfer between you and the GL, though it can be less
convenient to use.

So which API should you use?  Use what you feel like using, if you
have a choice.  Legacy OpenGL isn't going away any time soon on the
desktop.  Sometimes you don't have a choice, though; for example, when
targeting a device that only supports OpenGL ES 2.x, legacy OpenGL is
unavailable.

But if you want some advice, we suggest that you use the newer APIs.
Not only will your code be future-proof and more efficient on the GL
level, reducing the number of API calls improves performance, and it
can reduce the amount of heap allocation in your program.  All
floating-point numbers are currently allocated on the heap in Guile,
and doing less floating-point math in tight loops can only be a good
thing.


@node GL API
@section GL API

The procedures exported from the @code{(figl gl)} module are
documented below, organized by their corresponding section in the
OpenGL 2.1 specification.

@example
(use-modules (figl gl))
@end example

See @uref{http://www.opengl.org/registry/doc/glspec21.20061201.pdf},
for more information.

@menu
* OpenGL Operation::
* Rasterization::
* Per Fragment Operations::
* Special Functions::
* State and State Requests::
@end menu


@node OpenGL Operation
@subsection OpenGL Operation

@subsubsection Begin/End Paradigm

@defmac gl-begin begin-mode body ...
Begin immediate-mode drawing with @var{begin-mode}, evaluate
the sequence of @var{body} expressions, and then end drawing (as with
@code{glBegin} and @code{glEnd}).

The values produced by the last @var{body} expression are returned to
the continuation of the @code{gl-begin}.
@end defmac

@defun gl-edge-flag boundary?
Flag edges as either boundary or nonboundary.  Note that the edge mode
is only significant if the @code{polygon-mode} is @code{line} or
@code{point}.
@end defun

@subsubsection Vertex Specification

@defun gl-vertex x y [z=0.0] [w=1.0]
Draw a vertex at the given coordinates.
@end defun

The following procedures modify the current per-vertex state.  Drawing
a vertex captures the current state and associates it with the
vertex.

@defun gl-texture-coordinate s [t=0.0] [r=0.0] [q=1.0]
Set the current texture coordinate.
@end defun

@defun gl-multi-texture-coordinate texture s [t=0.0] [r=0.0] [q=1.0]
Set the current texture coordinate for a specific texture unit.
@end defun

@defun gl-color red green blue [alpha=1.0]
Set the current color.
@end defun

@defun gl-vertex-attribute index x [y=0.0] [z=0.0] [w=1.0]
Set the current value of a generic vertex attribute.
@end defun

@defun gl-normal x y z
Set the current normal vector.  By default the normal should have unit
length, though setting @code{(enable-cap rescale-normal)} or
@code{(enable-cap normalize)} can change this.
@end defun

@defun gl-fog-coordinate coord
Set the current fog coordinate.
@end defun

@defun gl-secondary-color red green blue
Set the current secondary color.
@end defun

@defun gl-index c
Set the current color index.
@end defun

@subsubsection Rectangles

@defun gl-rectangle x1 y1 x2 y2
Draw a rectangle in immediate-mode with a given pair of corner
points.
@end defun

@subsubsection Coordinate Transformation

@defun gl-depth-range near-val far-val
Specify the mapping of the near and far clipping planes, respectively,
to window coordinates.
@end defun

@defun gl-viewport x y width height
Set the viewport: the pixel position of the lower-left corner of the
viewport rectangle, and the width and height of the viewport.
@end defun

@defun gl-load-matrix m [#:transpose=#f]
Load a matrix.  @var{m} should be a packed vector in column-major
order.

Note that Guile's two-dimensional arrays are stored in row-major
order, so you might need to transpose the matrix as it is loaded (via
the @code{#:transpose} keyword argument).
@end defun

@defun gl-multiply-matrix m [#:transpose=#f]
Multiply the current matrix by @var{m}.  As with
@code{gl-load-matrix}, you might need to transpose the matrix first.
@end defun

@defun set-gl-matrix-mode matrix-mode
Set the current matrix mode.  See the @code{matrix-mode} enumerator.
@end defun

@defmac with-gl-push-matrix body ...
Save the current matrix, evaluate the sequence of @var{body}
expressions, and restore the saved matrix.
@end defmac

@defun gl-load-identity
Load the identity matrix.
@end defun

@defun gl-rotate angle x y z
Rotate the current matrix about the vector
@code{(@var{x},@var{y},@var{z})}.  @var{angle} should be specified in
degrees.
@end defun

@defun gl-translate x y z
Translate the current matrix.
@end defun

@defun gl-scale x y z
Scale the current matrix.
@end defun

@defun gl-frustum left right bottom top near-val far-val
Multiply the current matrix by a perspective matrix.  @var{left},
@var{right}, @var{bottom}, and @var{top} are the coordinates of the
corresponding clipping planes.  @var{near-val} and @var{far-val}
specify the distances to the near and far clipping planes.
@end defun

@defun gl-ortho left right bottom top near-val far-val
Multiply the current matrix by a perspective matrix.  @var{left},
@var{right}, @var{bottom}, and @var{top} are the coordinates of the
corresponding clipping planes.  @var{near-val} and @var{far-val}
specify the distances to the near and far clipping planes.
@end defun

@defun set-gl-active-texture texture
Set the active texture unit.
@end defun

@defun gl-enable enable-cap
@defunx gl-disable enable-cap
Enable or disable server-side GL capabilities.
@end defun

@subsubsection Colors and Coloring

@defun set-gl-shade-model mode
Select flat or smooth shading.
@end defun


@node Rasterization
@subsection Rasterization


@node Per Fragment Operations
@subsection Per-Fragment Operations

@defun set-gl-stencil-function stencil-function k [#:mask] [#:face]
Set the front and/or back function and the reference value @var{k} for
stencil testing.  Without the @var{face} keyword argument, both
functions are set.  The default @var{mask} is all-inclusive.
@end defun

@defun set-gl-stencil-operation stencil-fail depth-fail depth-pass [#:face]
Set the front and/or back stencil test actions.  Without the
@var{face} keyword argument, both stencil test actions are set.  See
the @code{stencil-op} enumeration for possible values for
@var{stencil-fail}, @var{depth-fail}, and @var{depth-pass}.
@end defun

@defun set-gl-blend-equation mode-rgb [mode-alpha=mode-rgb]
Set the blend equation.  With one argument, set the same blend
equation for all components.  Pass two arguments to specify a separate
equation for the alpha component.
@end defun

@defun set-gl-blend-function src-rgb dest-rgb [src-alpha=src-rgb] [dest-alpha=dest-rgb]
Set the blend function.  With two arguments, set the same blend
function for all components.  Pass an additional two arguments to
specify separate functions for the alpha components.
@end defun

@defun set-gl-scissor x y width height
Define the scissor box.  The box is defined in window coordinates,
with (@var{x},@var{y}) being the lower-left corner of the box.
@end defun

@defun set-gl-sample-coverage value invert
Specify multisample coverage parameters.
@end defun

@defun set-gl-alpha-function func ref
Specify the alpha test function.  See the @code{alpha-function}
enumerator.
@end defun

@defun set-gl-depth-function func
Specify the depth test function.  See the @code{depth-function}
enumerator.
p@end defun

@defun set-gl-blend-color r g b a
Specify the blend color.
@end defun

@defun set-gl-logic-operation opcode
Specify a logical pixel operation for color index rendering.
@end defun

@subsubsection Whole Framebuffer Operations

@defun set-gl-draw-buffers buffers
Specify a list of color buffers to be drawn into.  @var{buffers}
should be a list of @code{draw-buffer-mode} enumerated values.
@end defun

@defun set-gl-stencil-mask mask [#:face]
Control the writing of individual bits into the front and/or back
stencil planes.  With one argument, the stencil mask for both states
are set.
@end defun

@defun set-gl-draw-buffer mode
Specify the buffer or buffers to draw into.
@end defun

@defun set-gl-index-mask mask
Control the writing of individual bits into the color index buffers.
@end defun

@defun set-gl-color-mask red? green? blue? alpha?
Enable and disable writing of frame buffer color components.
@end defun

@defun set-gl-depth-mask enable?
Enable and disable writing into the depth buffer.
@end defun

@defun gl-clear mask
Clear a set of buffers to pre-set values.  Use the
@code{clear-buffer-mask} enumerator to specify which buffers to clear.
@end defun

@defun set-gl-clear-color r g b a
Set the clear color for the color buffers.
@end defun

@defun set-gl-clear-index c
Set the clear index for the color index buffers.
@end defun

@defun set-gl-clear-depth depth
Set the clear value for the depth buffer.
@end defun

@defun set-gl-clear-stencil-value s
Set the clear value for the stencil buffer.
@end defun

@defun set-gl-clear-accumulation-color r g b a
Set the clear color for the accumulation buffer.
@end defun

@defun set-gl-accumulation-buffer-operation op value
Operate on the accumulation buffer.  @var{op} may be one of the
@code{accum-op} enumerated values.  The interpretation of @var{value}
depends on @var{op}.
@end defun

@subsubsection Drawing, Reading and Copying Pixels

@defun set-gl-read-buffer mode
Select a color buffer source for pixels.  Use @code{read-buffer-mode}
to select a mode.
@end defun

@defun gl-copy-pixels x y width height type
Copy pixels from a screen-aligned rectangle in the frame buffer to a
region relative to the current raster position.  @var{type} selects
which buffer to copy from.
@end defun


@node Special Functions
@subsection Special Functions


@node State and State Requests
@subsection State and State Requests

@subsubsection Querying GL State

@defmac with-gl-push-attrib bits body ...
Save part of the current state, evaluation the sequence of @var{body}
expressions, then restore the state.  Use @code{attrib-mask} to
specify which parts of the state to save.
@end defmac


@node GL Enumerations
@section GL Enumerations
@include low-level-gl-enums.texi


@node Low-Level GL
@section Low-Level GL
@include low-level-gl.texi


@node GL Extensions
@section GL Extensions

@quotation
The future is already here -- it's just not very evenly distributed.

-- William Gibson
@end quotation

Before interfaces end up in core OpenGL, the are usually present as
vendor-specific or candidate extensions.  Indeed, the making of an
OpenGL standard these days seems to be a matter of simply collecting a
set of mature extensions and making them coherent.

Figl doesn't currently provide specific interfaces for extensions.
Perhaps it should, but that's a lot of work that we haven't had time
to do.  Contributions are welcome.

In the meantime, if you know enough about GL to know that you need an
extension, you can define one yourself -- after all, Figl is all a
bunch of Scheme code anyway.

For example, let's say you decide that you need to render to a
framebuffer object.  You go to @uref{http://www.opengl.org/registry/}
and pick out an extension, say
@uref{http://www.opengl.org/registry/specs/ARB/framebuffer_object.txt}.

This extension defines a procedure, @code{GLboolean
glIsRenderBuffer(GLuint)}.  So you define it:

@example
(use-modules (figl gl runtime) (figl gl types))
(define-gl-procedure (glIsRenderBuffer (buf GLuint) -> GLboolean)
  "Render buffer predicate.  Other docs here.")
@end example

And that's that.  It's a low-level binding, but what did you expect?

Note that you'll still need to check for the availability of this
extension at runtime with @code{(glGetString GL_EXTENSIONS)}.