upstream-doc/manglsl/gl_FrontFacing.xml

   1 <?xml version="1.0" encoding="UTF-8"?>
   2 <!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook MathML Module V1.1b1//EN"
   3               "http://www.oasis-open.org/docbook/xml/mathml/1.1CR1/dbmathml.dtd">
   4 <refentry id="gl_FrontFacing">
   5     <refmeta>
   6         <refmetainfo>
   7             <copyright>
   8                 <year>2011</year>
   9                 <holder>Khronos Group</holder>
  10             </copyright>
  11         </refmetainfo>
  12         <refentrytitle>gl_FrontFacing</refentrytitle>
  13         <manvolnum>3G</manvolnum>
  14     </refmeta>
  15     <refnamediv>
  16         <refname>gl_FrontFacing</refname>
  17         <refpurpose>indicates whether a primitive is front or back facing</refpurpose>
  18     </refnamediv>
  19     <refsynopsisdiv><title>Declaration</title>
  20         <funcsynopsis>
  21             <function>in bool gl_FrontFacing;</function>
  22         </funcsynopsis>
  23     </refsynopsisdiv>
  24     <refsect1 id="description"><title>Description</title>
  25         <para>
  26             Available only in the fragment language, <function>gl_FrontFacing</function> is an input variable
  27             whose value is <function>true</function> if the fragment belongs to a front-facing primitive and
  28             false otherwise. The determination of whether a triangle primitive is front-facing is made by
  29             examining the sign of the area of the triangle, including a possible reversal of this sign
  30             as controlled by <function>glFrontFace</function>. One way to compute this area is:
  31         </para>
  32         <para>
  33             <inlineequation><mml:math>
  34                 <mml:mi>a</mml:mi><mml:mo>=</mml:mo><mml:mfrac><mml:mrow><mml:mn>1</mml:mn></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:mfrac>
  35                 <mml:msubsup><mml:mo>&#x2211;</mml:mo><mml:mrow>
  36                     <mml:mi>j</mml:mi><mml:mo>=</mml:mo><mml:mn>0</mml:mn>
  37                 </mml:mrow>
  38                 <mml:mrow>
  39                     <mml:mi>n</mml:mi><mml:mo>-</mml:mo><mml:mn>1</mml:mn>
  40                 </mml:mrow></mml:msubsup>
  41                 <mml:msubsup><mml:mi>x</mml:mi><mml:mrow>w</mml:mrow><mml:mrow><mml:mi>i</mml:mi></mml:mrow></mml:msubsup>
  42                 <mml:msubsup><mml:mi>y</mml:mi><mml:mrow>w</mml:mrow><mml:mrow><mml:mi>i</mml:mi><mml:mo>+</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msubsup>
  43                 <mml:mo>-</mml:mo>
  44                 <mml:msubsup><mml:mi>x</mml:mi><mml:mrow>w</mml:mrow><mml:mrow><mml:mi>i</mml:mi><mml:mo>+</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msubsup>
  45                 <mml:msubsup><mml:mi>y</mml:mi><mml:mrow>w</mml:mrow><mml:mrow><mml:mi>i</mml:mi></mml:mrow></mml:msubsup>
  46             </mml:math></inlineequation>
  47         </para>
  48         <para>
  49             where <inlineequation><mml:math><mml:msubsup><mml:mi>x</mml:mi><mml:mrow>w</mml:mrow><mml:mrow><mml:mi>i</mml:mi></mml:mrow></mml:msubsup></mml:math></inlineequation> and
  50             <inlineequation><mml:math><mml:msubsup><mml:mi>y</mml:mi><mml:mrow>w</mml:mrow><mml:mrow><mml:mi>i</mml:mi></mml:mrow></mml:msubsup></mml:math></inlineequation> are
  51             the x and y window coordinates of the <emphasis>i</emphasis>th vertex of the n-vertex polygon.
  52         </para>
  53     </refsect1>
  54     <refsect1 id="versions"><title>Version Support</title>
  55         <informaltable frame="topbot">
  56             <tgroup cols="3" align="left">
  57                 <colspec colwidth="1.1*" />
  58                 <colspec colwidth="1*" align="center" />
  59                 <colspec colwidth="1*" align="center" />
  60                 <colspec colwidth="1*" align="center" />
  61                 <colspec colwidth="1*" align="center" />
  62                 <colspec colwidth="1*" align="center" />
  63                 <colspec colwidth="1*" align="center" />
  64                 <colspec colwidth="1*" align="center" />
  65                 <colspec colwidth="1*" align="center" />
  66                 <thead>
  67                     <row>
  68                         <entry rowsep="1" align="left"><emphasis role="bold">
  69                         Variable
  70                         </emphasis></entry>
  71                         <entry rowsep="1" align="left"><emphasis role="bold">
  72                         Version 1.10
  73                         </emphasis></entry>
  74                         <entry rowsep="1" align="left"><emphasis role="bold">
  75                         Version 1.20
  76                         </emphasis></entry>
  77                         <entry rowsep="1" align="left"><emphasis role="bold">
  78                         Version 1.30
  79                         </emphasis></entry>
  80                         <entry rowsep="1" align="left"><emphasis role="bold">
  81                         Version 1.40
  82                         </emphasis></entry>
  83                         <entry rowsep="1" align="left"><emphasis role="bold">
  84                         Version 1.50
  85                         </emphasis></entry>
  86                         <entry rowsep="1" align="left"><emphasis role="bold">
  87                         Version 3.30
  88                         </emphasis></entry>
  89                         <entry rowsep="1" align="left"><emphasis role="bold">
  90                         Version 4.00
  91                         </emphasis></entry>
  92                         <entry rowsep="1" align="left"><emphasis role="bold">
  93                         Version 4.10
  94                         </emphasis></entry>
  95                     </row>
  96                     <row>
  97                         <entry>
  98                             gl_FrontFacing
  99                         </entry>
 100                         <entry>
 101                             <emphasis>Y</emphasis>  <!-- 1.10, OpenGL 2.0 -->
 102                         </entry>
 103                         <entry>
 104                             <emphasis>Y</emphasis>  <!-- 1.20, OpenGL 2.1 -->
 105                         </entry>
 106                         <entry>
 107                             <emphasis>Y</emphasis>  <!-- 1.30, OpenGL 3.0 -->
 108                         </entry>
 109                         <entry>
 110                             <emphasis>Y</emphasis>  <!-- 1.40, OpenGL 3.1 -->
 111                         </entry>
 112                         <entry>
 113                             <emphasis>Y</emphasis>  <!-- 1.50, OpenGL 3.2 -->
 114                         </entry>
 115                         <entry>
 116                             <emphasis>Y</emphasis>  <!-- 3.30, OpenGL 3.3 -->
 117                         </entry>
 118                         <entry>
 119                             <emphasis>Y</emphasis>  <!-- 4.00, OpenGL 4.0 -->
 120                         </entry>
 121                         <entry>
 122                             <emphasis>Y</emphasis>  <!-- 4.10, OpenGL 4.1 -->
 123                         </entry>
 124                     </row>
 125                 </thead>
 126             </tgroup>
 127         </informaltable>
 128     </refsect1>
 129     <refsect1 id="seealso"><title>See Also</title>
 130         <para>
 131             <citerefentry><refentrytitle>gl_FragCoord</refentrytitle></citerefentry>
 132         </para>
 133     </refsect1>
 134     <refsect1 id="Copyright"><title>Copyright</title>
 135         <para>
 136             Copyright <trademark class="copyright"></trademark> 2011 Khronos Group.
 137             This material may be distributed subject to the terms and conditions set forth in
 138             the Open Publication License, v 1.0, 8 June 1999.
 139             <ulink url="http://opencontent.org/openpub/">http://opencontent.org/openpub/</ulink>.
 140         </para>
 141     </refsect1>
 142 </refentry>