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 <TITLE>Shading Language Support</TITLE>

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 <H1>Shading Language Support</H1>

 <p>
 This page describes the features and status of Mesa's support for the
 <a href="http://opengl.org/documentation/glsl/" target="_parent">
 OpenGL Shading Language</a>.
 </p>

 <p>
 Last updated on 20 Jan 2007.
 </p>

 <h2>Unsupported Features</h2>

 <p>
 The following features of the shading language are not yet supported
 in Mesa:
 </p>

 <ul>
 <li>Arrays
 <li>Structs
 <li>Linking of multiple shaders is not supported
 <li>Noise functions
 <li>Not all built-in OpenGL state variables are supported yet.
    Common variables such as gl_ModelViewMatrix and gl_NormalMatrix
    are supported.
 <li>Integer operations are not fully implemented (most are implemented
    as floating point).
 </ul>

 <p>
 All other major features of the shading language should function.
 </p>


 <h2>Implementation Notes</h2>

 <ul>
 <li>Shading language programs are compiled into low-level programs
    very similar to those of GL_ARB_vertex/fragment_program.
 <li>All float/int/bool and vector types currently occupy full
    float[4] registers.
 <li>Float constants are packed so that up to four floats can occupy one
    program parameter/register.
 <li>All function calls are inlined.
 <li>Shaders which use too many registers will not compile.
 <li>The quality of generated code is pretty good, register usage is fair.
 <li>Shader error detection and reporting of errors (InfoLog) is not
    very good yet.
 <li>There are massive memory leaks in the compiler.
 </ul>

 <p>
 These issues will be addressed/resolved in the future.
 </p>


 <h2>Programming Hints</h2>

 <ul>
 <li>Always declare <em>in</em> function parameters as <em>const</em>.
    This improves the efficiency of function inlining.
 </li>
 <br>
 <li>To reduce register usage, declare variables within smaller scopes.
    For example, the following code:
 <pre>
    void main()
    {
       vec4 a1, a2, b1, b2;
       gl_Position = expression using a1, a2.
       gl_Color = expression using b1, b2;
    }
 </pre>
    Can be rewritten as follows to use half as many registers:
 <pre>
    void main()
    {
       {
          vec4 a1, a2;
          gl_Position = expression using a1, a2.
       }
       {
          vec4 b1, b2;
          gl_Color = expression using b1, b2;
       }
    }
 </pre>
    Alternately, rather than using several float variables, use
    a vec4 instead.  Use swizzling and writemasks to access the
    components of the vec4 as floats.
 </li>
 <br>
 <li>Use the built-in library functions whenever possible.
    For example, instead of writing this:
 <pre>
        float x = 1.0 / sqrt(y);
 </pre>
    Write this:
 <pre>
        float x = inversesqrt(y);
 </pre>
 </ul>


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