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+
+
+Shading Language Support
+
+Shading Language Support
+
+
+This page describes the features and status of Mesa's support for the
+
+OpenGL Shading Language .
+
+
+
+Last updated on 20 Jan 2007.
+
+
+Unsupported Features
+
+
+The following features of the shading language are not yet supported
+in Mesa:
+
+
+
+Arrays
+ Structs
+ Linking of multiple shaders is not supported
+ Noise functions
+ Not all built-in OpenGL state variables are supported yet.
+ Common variables such as gl_ModelViewMatrix and gl_NormalMatrix
+ are supported.
+ Integer operations are not fully implemented (most are implemented
+ as floating point).
+
+
+
+All other major features of the shading language should function.
+
+
+
+Implementation Notes
+
+
+Shading language programs are compiled into low-level programs
+ very similar to those of GL_ARB_vertex/fragment_program.
+ All float/int/bool and vector types currently occupy full
+ float[4] registers.
+ Float constants are packed so that up to four floats can occupy one
+ program parameter/register.
+ All function calls are inlined.
+ Shaders which use too many registers will not compile.
+ The quality of generated code is pretty good, register usage is fair.
+ Shader error detection and reporting of errors (InfoLog) is not
+ very good yet.
+ There are massive memory leaks in the compiler.
+
+
+
+These issues will be addressed/resolved in the future.
+
+
+
+Programming Hints
+
+
+Always declare in function parameters as const .
+ This improves the efficiency of function inlining.
+
+To reduce register usage, declare variables within smaller scopes.
+ For example, the following code:
+
+ void main()
+ {
+ vec4 a1, a2, b1, b2;
+ gl_Position = expression using a1, a2.
+ gl_Color = expression using b1, b2;
+ }
+
+ Can be rewritten as follows to use half as many registers:
+
+ void main()
+ {
+ {
+ vec4 a1, a2;
+ gl_Position = expression using a1, a2.
+ }
+ {
+ vec4 b1, b2;
+ gl_Color = expression using b1, b2;
+ }
+ }
+
+ Alternately, rather than using several float variables, use
+ a vec4 instead. Use swizzling and writemasks to access the
+ components of the vec4 as floats.
+
+Use the built-in library functions whenever possible.
+ For example, instead of writing this:
+
+ float x = 1.0 / sqrt(y);
+
+ Write this:
+
+ float x = inversesqrt(y);
+
+
+
+
+
+