docs/shading.html

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   9 <H1>Shading Language Support</H1>
  10
  11 <p>
  12 This page describes the features and status of Mesa's support for the
  13 <a href="http://opengl.org/documentation/glsl/" target="_parent">
  14 OpenGL Shading Language</a>.
  15 </p>
  16
  17 <p>
  18 Last updated on 20 Jan 2007.
  19 </p>
  20
  21 <h2>Unsupported Features</h2>
  22
  23 <p>
  24 The following features of the shading language are not yet supported
  25 in Mesa:
  26 </p>
  27
  28 <ul>
  29 <li>Arrays
  30 <li>Structs
  31 <li>Linking of multiple shaders is not supported
  32 <li>Noise functions
  33 <li>Not all built-in OpenGL state variables are supported yet.
  34     Common variables such as gl_ModelViewMatrix and gl_NormalMatrix
  35     are supported.
  36 <li>Integer operations are not fully implemented (most are implemented
  37     as floating point).
  38 </ul>
  39
  40 <p>
  41 All other major features of the shading language should function.
  42 </p>
  43
  44
  45 <h2>Implementation Notes</h2>
  46
  47 <ul>
  48 <li>Shading language programs are compiled into low-level programs
  49     very similar to those of GL_ARB_vertex/fragment_program.
  50 <li>All vector types (vec2, vec3, vec4, bvec2, etc) currently occupy full
  51     float[4] registers.
  52 <li>Float constants and variables are packed so that up to four floats
  53     can occupy one program parameter/register.
  54 <li>All function calls are inlined.
  55 <li>Shaders which use too many registers will not compile.
  56 <li>The quality of generated code is pretty good, register usage is fair.
  57 <li>Shader error detection and reporting of errors (InfoLog) is not
  58     very good yet.
  59 <li>There are massive memory leaks in the compiler.
  60 </ul>
  61
  62 <p>
  63 These issues will be addressed/resolved in the future.
  64 </p>
  65
  66
  67 <h2>Programming Hints</h2>
  68
  69 <ul>
  70 <li>Declare <em>in</em> function parameters as <em>const</em> whenever possible.
  71     This improves the efficiency of function inlining.
  72 </li>
  73 <br>
  74 <li>To reduce register usage, declare variables within smaller scopes.
  75     For example, the following code:
  76 <pre>
  77     void main()
  78     {
  79        vec4 a1, a2, b1, b2;
  80        gl_Position = expression using a1, a2.
  81        gl_Color = expression using b1, b2;
  82     }
  83 </pre>
  84     Can be rewritten as follows to use half as many registers:
  85 <pre>
  86     void main()
  87     {
  88        {
  89           vec4 a1, a2;
  90           gl_Position = expression using a1, a2.
  91        }
  92        {
  93           vec4 b1, b2;
  94           gl_Color = expression using b1, b2;
  95        }
  96     }
  97 </pre>
  98     Alternately, rather than using several float variables, use
  99     a vec4 instead.  Use swizzling and writemasks to access the
 100     components of the vec4 as floats.
 101 </li>
 102 <br>
 103 <li>Use the built-in library functions whenever possible.
 104     For example, instead of writing this:
 105 <pre>
 106         float x = 1.0 / sqrt(y);
 107 </pre>
 108     Write this:
 109 <pre>
 110         float x = inversesqrt(y);
 111 </pre>
 112 </ul>
 113
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