upstream-man-pages/manglsl/gl_Layer.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_Layer">
   5     <refmeta>
   6         <refmetainfo>
   7             <copyright>
   8                 <year>2012</year>
   9                 <holder>Khronos Group</holder>
  10             </copyright>
  11         </refmetainfo>
  12         <refentrytitle>gl_Layer</refentrytitle>
  13         <manvolnum>3G</manvolnum>
  14     </refmeta>
  15     <refnamediv>
  16         <refname>gl_Layer</refname>
  17         <refpurpose>contains the selected layer of a multi-layer framebuffer attachment</refpurpose>
  18     </refnamediv>
  19     <refsynopsisdiv><title>Declaration</title>
  20         <funcsynopsis>
  21             <function>out int gl_Layer; // Geometry stage</function>
  22         </funcsynopsis>
  23         <funcsynopsis>
  24             <function>in int gl_Layer; // Fragment stage</function>
  25         </funcsynopsis>
  26     </refsynopsisdiv>
  27     <refsect1 id="description"><title>Description</title>
  28         <para>
  29             In the geometry language, <function>gl_Layer</function> is used to select a specific
  30             layer (or face and layer of cube map) in a multi-layer framebuffer attachment. The actual
  31             layer used will come from one of the vertices in the primitive being shaded. Which vertex
  32             the layer comes from is undefined and as such it is recommended to write the same value
  33             to <function>gl_Layer</function> for all vertices in the primitive. If a shader statically
  34             assigns a value to <function>gl_Layer</function>, layered rendering mode is enabled.
  35             If a shader statically assigns a value to <function>gl_Layer</function>, and there is an execution path through
  36             the shader that does not set <function>gl_Layer</function>, then the value of <function>gl_Layer</function>
  37             is undefined for invocations of the shader that take that path.
  38         </para>
  39         <para>
  40             When used with an array of cube map textures, the <function>gl_Layer</function> output variable
  41             takes on a special value. Instead of referring to only the layer, it is used to select a cube map
  42             face and a layer. Setting <function>gl_Layer</function> to the value
  43             <inlineequation><mml:math><mml:mi>layer</mml:mi><mml:mo>*</mml:mo><mml:mn>6</mml:mn><mml:mo>+</mml:mo><mml:mi>face</mml:mi></mml:math></inlineequation>
  44             will render to <inlineequation><mml:math><mml:mi>face</mml:mi></mml:math></inlineequation>
  45             of the cube defined in <inlineequation><mml:math><mml:mi>layer</mml:mi></mml:math></inlineequation>.
  46             The face values are as follows:
  47         <informaltable frame="topbot">
  48             <tgroup cols="2" align="left">
  49                 <colspec colwidth="1.1*" />
  50                 <colspec colwidth="1*" align="center" />
  51                 <thead>
  52                     <row>
  53                         <entry rowsep="1" align="left"><emphasis role="bold">
  54                         Face Value
  55                         </emphasis></entry>
  56                         <entry rowsep="1" align="left"><emphasis role="bold">
  57                         Resulting Target
  58                         </emphasis></entry>
  59                     </row>
  60                     <row>
  61                         <entry>
  62                             0
  63                         </entry>
  64                         <entry>
  65                             <constant>GL_TEXTURE_CUBEMAP_POSITIVE_X</constant>
  66                         </entry>
  67                     </row>
  68                     <row>
  69                         <entry>
  70                             1
  71                         </entry>
  72                         <entry>
  73                             <constant>GL_TEXTURE_CUBEMAP_NEGATIVE_X</constant>
  74                         </entry>
  75                     </row>
  76                     <row>
  77                         <entry>
  78                             2
  79                         </entry>
  80                         <entry>
  81                             <constant>GL_TEXTURE_CUBEMAP_POSITIVE_Y</constant>
  82                         </entry>
  83                     </row>
  84                     <row>
  85                         <entry>
  86                             3
  87                         </entry>
  88                         <entry>
  89                             <constant>GL_TEXTURE_CUBEMAP_NEGATIVE_Y</constant>
  90                         </entry>
  91                     </row>
  92                     <row>
  93                         <entry>
  94                             4
  95                         </entry>
  96                         <entry>
  97                             <constant>GL_TEXTURE_CUBEMAP_POSITIVE_Z</constant>
  98                         </entry>
  99                     </row>
 100                     <row>
 101                         <entry>
 102                             5
 103                         </entry>
 104                         <entry>
 105                             <constant>GL_TEXTURE_CUBEMAP_NEGATIVE_Z</constant>
 106                         </entry>
 107                     </row>
 108                 </thead>
 109             </tgroup>
 110         </informaltable>
 111         </para>
 112         <para>
 113             For example, to render to the positive y cube map face located in the 5<superscript>th</superscript> layer of the
 114             cube map array, <function>gl_Layer</function> should be set to
 115             <inlineequation><mml:math><mml:mn>5</mml:mn><mml:mo>*</mml:mo><mml:mn>6</mml:mn><mml:mo>+</mml:mo><mml:mn>2</mml:mn></mml:math></inlineequation>.
 116         </para>
 117         <para>
 118             The input variable <function>gl_Layer</function> in the fragment language will have the same value that was written to the
 119             output variable <function>gl_Layer</function> in the geometry language. If the geometry stage does not dynamically assign a
 120             value to <function>gl_Layer</function>, the value of <function>gl_Layer</function> in the fragment stage will be undefined.  If the geometry stage
 121             makes no static assignment to <function>gl_Layer</function>, the input <function>gl_Layer</function> in the fragment stage will be zero.  Otherwise,
 122             the fragment stage will read the same value written by the geometry stage, even if that value is out of
 123             range.  If a fragment shader contains a static access to <function>gl_Layer</function>, it will count against the implementation
 124             defined limit for the maximum number of inputs to the fragment stage.
 125         </para>
 126     </refsect1>
 127     <refsect1 id="versions"><title>Version Support</title>
 128         <informaltable frame="topbot">
 129             #VARTABLECOLS#
 130                 <thead>
 131                     #FUNCTABLEHEADER#
 132                     <row>
 133                         <entry>gl_Layer (geometry stage)</entry>#newin15#
 134                     </row>
 135                     <row>
 136                         <entry>gl_Layer (fragment stage)</entry>#newin43#
 137                     </row>
 138                 </thead>
 139             </tgroup>
 140         </informaltable>
 141     </refsect1>
 142     <refsect1 id="seealso"><title>See Also</title>
 143         <para>
 144             <citerefentry><refentrytitle>gl_ViewportIndex</refentrytitle></citerefentry>
 145         </para>
 146     </refsect1>
 147     <refsect1 id="Copyright"><title>Copyright</title>
 148         <para>
 149             Copyright <trademark class="copyright"></trademark> 2011-2012 Khronos Group.
 150             This material may be distributed subject to the terms and conditions set forth in
 151             the Open Publication License, v 1.0, 8 June 1999.
 152             <ulink url="http://opencontent.org/openpub/">http://opencontent.org/openpub/</ulink>.
 153         </para>
 154     </refsect1>
 155 </refentry>