From: Brian Date: Sat, 20 Jan 2007 20:40:57 +0000 (-0700) Subject: Added a page with shading language status, tips, etc. X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=9595d1935c9934da60a09e659b6ea7d3b7d226e2;p=mesa.git Added a page with shading language status, tips, etc. --- diff --git a/docs/shading.html b/docs/shading.html new file mode 100644 index 00000000000..e7c63d1dd26 --- /dev/null +++ b/docs/shading.html @@ -0,0 +1,116 @@ + + +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);
+
```
+

+ + + +