shading.html 6.09 KB
Newer Older
1 2 3 4 5 6 7 8
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html lang="en">
<head>
  <meta http-equiv="content-type" content="text/html; charset=utf-8">
  <title>Shading Language Support</title>
  <link rel="stylesheet" type="text/css" href="mesa.css">
</head>
<body>
9

Andreas Boll's avatar
Andreas Boll committed
10 11 12 13 14 15 16
<div class="header">
  <h1>The Mesa 3D Graphics Library</h1>
</div>

<iframe src="contents.html"></iframe>
<div class="content">

17
<h1>Shading Language Support</h1>
18 19 20

<p>
This page describes the features and status of Mesa's support for the
21
<a href="http://opengl.org/documentation/glsl/">
22 23 24
OpenGL Shading Language</a>.
</p>

Brian's avatar
Brian committed
25 26 27 28
<p>
Contents
</p>
<ul>
29
<li><a href="#envvars">Environment variables</a>
30
<li><a href="#support">GLSL 1.40 support</a>
Brian's avatar
Brian committed
31
<li><a href="#unsup">Unsupported Features</a>
Brian's avatar
Brian committed
32
<li><a href="#notes">Implementation Notes</a>
Brian's avatar
Brian committed
33
<li><a href="#hints">Programming Hints</a>
34
<li><a href="#standalone">Stand-alone GLSL Compiler</a>
Brian's avatar
Brian committed
35
<li><a href="#implementation">Compiler Implementation</a>
Brian's avatar
Brian committed
36
<li><a href="#validation">Compiler Validation</a>
Brian's avatar
Brian committed
37 38 39
</ul>


40
<h2 id="envvars">Environment Variables</h2>
41 42 43

<p>
The <b>MESA_GLSL</b> environment variable can be set to a comma-separated
44 45
list of keywords to control some aspects of the GLSL compiler and shader
execution.  These are generally used for debugging.
46 47
</p>
<ul>
48 49
<li><b>dump</b> - print GLSL shader code to stdout at link time
<li><b>log</b> - log all GLSL shaders to files.
50 51
    The filenames will be "shader_X.vert" or "shader_X.frag" where X
    the shader ID.
52 53 54 55
<li><b>nopt</b> - disable compiler optimizations
<li><b>opt</b> - force compiler optimizations
<li><b>uniform</b> - print message to stdout when glUniform is called
<li><b>nopvert</b> - force vertex shaders to be a simple shader that just transforms
56 57
    the vertex position with ftransform() and passes through the color and
    texcoord[0] attributes.
58
<li><b>nopfrag</b> - force fragment shader to be a simple shader that passes
59
    through the color attribute.
60
<li><b>useprog</b> - log glUseProgram calls to stderr
61 62 63 64 65
</ul>
<p>
Example:  export MESA_GLSL=dump,nopt
</p>

66 67 68 69 70 71 72 73 74 75 76 77 78 79
<p>
Shaders can be dumped and replaced on runtime for debugging purposes. Mesa 
needs to be configured with '--with-sha1' to enable this functionality. This 
feature is not currently supported by SCons build.

This is controlled via following environment variables:
<ul>
<li><b>MESA_SHADER_DUMP_PATH</b> - path where shader sources are dumped
<li><b>MESA_SHADER_READ_PATH</b> - path where replacement shaders are read
</ul>
Note, path set must exist before running for dumping or replacing to work. 
When both are set, these paths should be different so the dumped shaders do 
not clobber the replacement shaders.
</p>
80

81
<h2 id="support">GLSL Version</h2>
Brian Paul's avatar
Brian Paul committed
82 83

<p>
84
The GLSL compiler currently supports version 3.30 of the shading language.
85 86 87 88 89
</p>

<p>
Several GLSL extensions are also supported:
</p>
Brian Paul's avatar
Brian Paul committed
90
<ul>
91 92
<li>GL_ARB_draw_buffers
<li>GL_ARB_fragment_coord_conventions
93
<li>GL_ARB_shader_bit_encoding
Brian Paul's avatar
Brian Paul committed
94 95 96
</ul>


97
<h2 id="unsup">Unsupported Features</h2>
98

99 100
<p>XXX update this section</p>

101
<p>
102
The following features of the shading language are not yet fully supported
103 104 105 106
in Mesa:
</p>

<ul>
107 108 109
<li>Linking of multiple shaders does not always work.  Currently, linking
    is implemented through shader concatenation and re-compiling.  This
    doesn't always work because of some #pragma and preprocessor issues.
Brian's avatar
Brian committed
110 111
<li>The gl_Color and gl_SecondaryColor varying vars are interpolated
    without perspective correction
112 113 114 115 116 117 118
</ul>

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


119
<h2 id="notes">Implementation Notes</h2>
120 121 122 123

<ul>
<li>Shading language programs are compiled into low-level programs
    very similar to those of GL_ARB_vertex/fragment_program.
Brian's avatar
Brian committed
124
<li>All vector types (vec2, vec3, vec4, bvec2, etc) currently occupy full
125
    float[4] registers.
Brian's avatar
Brian committed
126 127
<li>Float constants and variables are packed so that up to four floats
    can occupy one program parameter/register.
128 129 130 131 132
<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.
Brian's avatar
Brian committed
133 134
<li>The ftransform() function doesn't necessarily match the results of
    fixed-function transformation.
135 136 137 138 139 140 141
</ul>

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


142
<h2 id="hints">Programming Hints</h2>
143 144 145 146 147 148 149 150 151 152 153

<ul>
<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>
Brian's avatar
Brian committed
154
</li>
155 156 157
</ul>


158
<h2 id="standalone">Stand-alone GLSL Compiler</h2>
Brian's avatar
Brian committed
159 160

<p>
161 162
The stand-alone GLSL compiler program can be used to compile GLSL shaders
into low-level GPU code.
Brian's avatar
Brian committed
163 164 165 166
</p>

<p>
This tool is useful for:
Andreas Boll's avatar
Andreas Boll committed
167
</p>
Brian's avatar
Brian committed
168 169 170 171 172 173 174
<ul>
<li>Inspecting GPU code to gain insight into compilation
<li>Generating initial GPU code for subsequent hand-tuning
<li>Debugging the GLSL compiler itself
</ul>

<p>
175
After building Mesa, the compiler can be found at src/glsl/glsl_compiler
Brian's avatar
Brian committed
176 177 178 179 180 181 182
</p>

<p>
Here's an example of using the compiler to compile a vertex shader and
emit GL_ARB_vertex_program-style instructions:
</p>
<pre>
Sam Hocevar's avatar
Sam Hocevar committed
183
    src/glsl/glsl_compiler --dump-ast myshader.vert
Brian's avatar
Brian committed
184 185
</pre>

186 187 188 189 190 191 192 193
Options include
<ul>
<li><b>--dump-ast</b> - dump GPU code
<li><b>--dump-hir</b> - dump high-level IR code
<li><b>--dump-lir</b> - dump low-level IR code
<li><b>--link</b> - ???
</ul>

Brian's avatar
Brian committed
194

195
<h2 id="implementation">Compiler Implementation</h2>
Brian's avatar
Brian committed
196 197 198

<p>
The source code for Mesa's shading language compiler is in the
199
<code>src/glsl/</code> directory.
Brian's avatar
Brian committed
200 201 202
</p>

<p>
203
XXX provide some info about the compiler....
Brian's avatar
Brian committed
204 205 206 207 208 209 210 211
</p>

<p>
The final vertex and fragment programs may be interpreted in software
(see prog_execute.c) or translated into a specific hardware architecture
(see drivers/dri/i915/i915_fragprog.c for example).
</p>

212
<h2 id="validation">Compiler Validation</h2>
Brian's avatar
Brian committed
213 214

<p>
215
Developers working on the GLSL compiler should test frequently to avoid
Brian's avatar
Brian committed
216 217
regressions.
</p>
218

Brian's avatar
Brian committed
219
<p>
220
The <a href="http://piglit.freedesktop.org/">Piglit</a> project
221
has many GLSL tests.
Brian's avatar
Brian committed
222 223
</p>

224 225 226
<p>
The Mesa demos repository also has some good GLSL tests.
</p>
Brian's avatar
Brian committed
227

Andreas Boll's avatar
Andreas Boll committed
228
</div>
229 230
</body>
</html>