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Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]1<HTML>
2
3<TITLE>Shading Language Support</TITLE>
4
5<link rel="stylesheet" type="text/css" href="mesa.css"></head>
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7<BODY>
8
9<H1>Shading Language Support</H1>
10
11<p>
12This page describes the features and status of Mesa's support for the
13<a href="http://opengl.org/documentation/glsl/" target="_parent">
14OpenGL Shading Language</a>.
15</p>
16
17<p>
Brian61d31ae2007-02-17 09:41:19 -0700[diff] [blame^]18Last updated on 17 Feb 2007.
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]19</p>
20
Brian61d31ae2007-02-17 09:41:19 -0700[diff] [blame^]21<p>
22Contents
23</p>
24<ul>
25<li><a href="#unsup">Unsupported Features</a>
26<li><a href="#impl">Implementation Notes</a>
27<li><a href="#hints">Programming Hints</a>
28<li><a href="#standalone">Stand-alone Compiler</a>
29</ul>
30
31
32<a name="unsup">
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]33<h2>Unsupported Features</h2>
34
35<p>
36The following features of the shading language are not yet supported
37in Mesa:
38</p>
39
40<ul>
41<li>Arrays
42<li>Structs
43<li>Linking of multiple shaders is not supported
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]44<li>Not all built-in OpenGL state variables are supported yet.
45 Common variables such as gl_ModelViewMatrix and gl_NormalMatrix
46 are supported.
47<li>Integer operations are not fully implemented (most are implemented
48 as floating point).
49</ul>
50
51<p>
52All other major features of the shading language should function.
53</p>
54
55
Brian61d31ae2007-02-17 09:41:19 -0700[diff] [blame^]56<a name="impl">
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]57<h2>Implementation Notes</h2>
58
59<ul>
60<li>Shading language programs are compiled into low-level programs
61 very similar to those of GL_ARB_vertex/fragment_program.
Brianbbec2fd2007-01-28 12:11:10 -0700[diff] [blame]62<li>All vector types (vec2, vec3, vec4, bvec2, etc) currently occupy full
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]63 float[4] registers.
Brianbbec2fd2007-01-28 12:11:10 -0700[diff] [blame]64<li>Float constants and variables are packed so that up to four floats
65 can occupy one program parameter/register.
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]66<li>All function calls are inlined.
67<li>Shaders which use too many registers will not compile.
68<li>The quality of generated code is pretty good, register usage is fair.
69<li>Shader error detection and reporting of errors (InfoLog) is not
70 very good yet.
71<li>There are massive memory leaks in the compiler.
72</ul>
73
74<p>
75These issues will be addressed/resolved in the future.
76</p>
77
78
Brian61d31ae2007-02-17 09:41:19 -0700[diff] [blame^]79<a name="hints">
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]80<h2>Programming Hints</h2>
81
82<ul>
Brianbbec2fd2007-01-28 12:11:10 -0700[diff] [blame]83<li>Declare <em>in</em> function parameters as <em>const</em> whenever possible.
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]84 This improves the efficiency of function inlining.
85</li>
86<br>
87<li>To reduce register usage, declare variables within smaller scopes.
88 For example, the following code:
89<pre>
90 void main()
91 {
92 vec4 a1, a2, b1, b2;
93 gl_Position = expression using a1, a2.
94 gl_Color = expression using b1, b2;
95 }
96</pre>
97 Can be rewritten as follows to use half as many registers:
98<pre>
99 void main()
100 {
101 {
102 vec4 a1, a2;
103 gl_Position = expression using a1, a2.
104 }
105 {
106 vec4 b1, b2;
107 gl_Color = expression using b1, b2;
108 }
109 }
110</pre>
111 Alternately, rather than using several float variables, use
112 a vec4 instead. Use swizzling and writemasks to access the
113 components of the vec4 as floats.
114</li>
115<br>
116<li>Use the built-in library functions whenever possible.
117 For example, instead of writing this:
118<pre>
119 float x = 1.0 / sqrt(y);
120</pre>
121 Write this:
122<pre>
123 float x = inversesqrt(y);
124</pre>
125</ul>
126
127
Brian61d31ae2007-02-17 09:41:19 -0700[diff] [blame^]128<a name="standalone">
129<h2>Stand-alone Compiler</h2>
130
131<p>
132A unique stand-alone GLSL compiler driver has been added to Mesa.
133<p>
134
135<p>
136The stand-alone compiler (like a conventional command-line compiler)
137is a tool that accepts Shading Language programs and emits low-level
138GPU programs.
139</p>
140
141<p>
142This tool is useful for:
143<p>
144<ul>
145<li>Inspecting GPU code to gain insight into compilation
146<li>Generating initial GPU code for subsequent hand-tuning
147<li>Debugging the GLSL compiler itself
148</ul>
149
150<p>
151(compiler build instructions TBD)
152</p>
153
154<p>
155Here's an example of using the compiler to compile a vertex shader and
156emit GL_ARB_vertex_program-style instructions:
157</p>
158<pre>
159 glslcompiler --arb --linenumbers --vs vertshader.txt
160</pre>
161<p>
162The output may look similar to this:
163</p>
164<pre>
165!!ARBvp1.0
166 0: MOV result.texcoord[0], vertex.texcoord[0];
167 1: DP4 temp0.x, state.matrix.mvp.row[0], vertex.position;
168 2: DP4 temp0.y, state.matrix.mvp.row[1], vertex.position;
169 3: DP4 temp0.z, state.matrix.mvp.row[2], vertex.position;
170 4: DP4 temp0.w, state.matrix.mvp.row[3], vertex.position;
171 5: MOV result.position, temp0;
172 6: END
173</pre>
174
175<p>
176Note that some shading language constructs (such as uniform and varying
177variables) aren't expressible in ARB or NV-style programs.
178Therefore, the resulting output is not always legal by definition of
179those program languages.
180</p>
181<p>
182Also note that this compiler driver is still under development.
183Over time, the correctness of the GPU programs, with respect to the ARB
184and NV languagues, should improve.
185</p>
186
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