Brian | 9595d19 | 2007-01-20 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> |
| 6 | |
| 7 | <BODY> |
| 8 | |
| 9 | <H1>Shading Language Support</H1> |
| 10 | |
| 11 | <p> |
| 12 | This page describes the features and status of Mesa's support for the |
| 13 | <a href="http://opengl.org/documentation/glsl/" target="_parent"> |
| 14 | OpenGL Shading Language</a>. |
| 15 | </p> |
| 16 | |
| 17 | <p> |
Brian | 61d31ae | 2007-02-17 09:41:19 -0700 | [diff] [blame] | 18 | Last updated on 17 Feb 2007. |
Brian | 9595d19 | 2007-01-20 13:40:57 -0700 | [diff] [blame] | 19 | </p> |
| 20 | |
Brian | 61d31ae | 2007-02-17 09:41:19 -0700 | [diff] [blame] | 21 | <p> |
| 22 | Contents |
| 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"> |
Brian | 9595d19 | 2007-01-20 13:40:57 -0700 | [diff] [blame] | 33 | <h2>Unsupported Features</h2> |
| 34 | |
| 35 | <p> |
| 36 | The following features of the shading language are not yet supported |
| 37 | in Mesa: |
| 38 | </p> |
| 39 | |
| 40 | <ul> |
Brian | 6d4cf6b | 2007-02-21 16:07:03 -0700 | [diff] [blame] | 41 | <li>Dereferencing arrays with non-constant indexes |
| 42 | <li>User-defined structs |
Brian | 9595d19 | 2007-01-20 13:40:57 -0700 | [diff] [blame] | 43 | <li>Linking of multiple shaders is not supported |
Brian | 9595d19 | 2007-01-20 13:40:57 -0700 | [diff] [blame] | 44 | <li>Integer operations are not fully implemented (most are implemented |
| 45 | as floating point). |
| 46 | </ul> |
| 47 | |
| 48 | <p> |
| 49 | All other major features of the shading language should function. |
| 50 | </p> |
| 51 | |
| 52 | |
Brian | 61d31ae | 2007-02-17 09:41:19 -0700 | [diff] [blame] | 53 | <a name="impl"> |
Brian | 9595d19 | 2007-01-20 13:40:57 -0700 | [diff] [blame] | 54 | <h2>Implementation Notes</h2> |
| 55 | |
| 56 | <ul> |
| 57 | <li>Shading language programs are compiled into low-level programs |
| 58 | very similar to those of GL_ARB_vertex/fragment_program. |
Brian | bbec2fd | 2007-01-28 12:11:10 -0700 | [diff] [blame] | 59 | <li>All vector types (vec2, vec3, vec4, bvec2, etc) currently occupy full |
Brian | 9595d19 | 2007-01-20 13:40:57 -0700 | [diff] [blame] | 60 | float[4] registers. |
Brian | bbec2fd | 2007-01-28 12:11:10 -0700 | [diff] [blame] | 61 | <li>Float constants and variables are packed so that up to four floats |
| 62 | can occupy one program parameter/register. |
Brian | 9595d19 | 2007-01-20 13:40:57 -0700 | [diff] [blame] | 63 | <li>All function calls are inlined. |
| 64 | <li>Shaders which use too many registers will not compile. |
| 65 | <li>The quality of generated code is pretty good, register usage is fair. |
| 66 | <li>Shader error detection and reporting of errors (InfoLog) is not |
| 67 | very good yet. |
| 68 | <li>There are massive memory leaks in the compiler. |
| 69 | </ul> |
| 70 | |
| 71 | <p> |
| 72 | These issues will be addressed/resolved in the future. |
| 73 | </p> |
| 74 | |
| 75 | |
Brian | 61d31ae | 2007-02-17 09:41:19 -0700 | [diff] [blame] | 76 | <a name="hints"> |
Brian | 9595d19 | 2007-01-20 13:40:57 -0700 | [diff] [blame] | 77 | <h2>Programming Hints</h2> |
| 78 | |
| 79 | <ul> |
Brian | bbec2fd | 2007-01-28 12:11:10 -0700 | [diff] [blame] | 80 | <li>Declare <em>in</em> function parameters as <em>const</em> whenever possible. |
Brian | 9595d19 | 2007-01-20 13:40:57 -0700 | [diff] [blame] | 81 | This improves the efficiency of function inlining. |
| 82 | </li> |
| 83 | <br> |
| 84 | <li>To reduce register usage, declare variables within smaller scopes. |
| 85 | For example, the following code: |
| 86 | <pre> |
| 87 | void main() |
| 88 | { |
| 89 | vec4 a1, a2, b1, b2; |
| 90 | gl_Position = expression using a1, a2. |
| 91 | gl_Color = expression using b1, b2; |
| 92 | } |
| 93 | </pre> |
| 94 | Can be rewritten as follows to use half as many registers: |
| 95 | <pre> |
| 96 | void main() |
| 97 | { |
| 98 | { |
| 99 | vec4 a1, a2; |
| 100 | gl_Position = expression using a1, a2. |
| 101 | } |
| 102 | { |
| 103 | vec4 b1, b2; |
| 104 | gl_Color = expression using b1, b2; |
| 105 | } |
| 106 | } |
| 107 | </pre> |
| 108 | Alternately, rather than using several float variables, use |
| 109 | a vec4 instead. Use swizzling and writemasks to access the |
| 110 | components of the vec4 as floats. |
| 111 | </li> |
| 112 | <br> |
| 113 | <li>Use the built-in library functions whenever possible. |
| 114 | For example, instead of writing this: |
| 115 | <pre> |
| 116 | float x = 1.0 / sqrt(y); |
| 117 | </pre> |
| 118 | Write this: |
| 119 | <pre> |
| 120 | float x = inversesqrt(y); |
| 121 | </pre> |
| 122 | </ul> |
| 123 | |
| 124 | |
Brian | 61d31ae | 2007-02-17 09:41:19 -0700 | [diff] [blame] | 125 | <a name="standalone"> |
| 126 | <h2>Stand-alone Compiler</h2> |
| 127 | |
| 128 | <p> |
| 129 | A unique stand-alone GLSL compiler driver has been added to Mesa. |
| 130 | <p> |
| 131 | |
| 132 | <p> |
| 133 | The stand-alone compiler (like a conventional command-line compiler) |
| 134 | is a tool that accepts Shading Language programs and emits low-level |
| 135 | GPU programs. |
| 136 | </p> |
| 137 | |
| 138 | <p> |
| 139 | This tool is useful for: |
| 140 | <p> |
| 141 | <ul> |
| 142 | <li>Inspecting GPU code to gain insight into compilation |
| 143 | <li>Generating initial GPU code for subsequent hand-tuning |
| 144 | <li>Debugging the GLSL compiler itself |
| 145 | </ul> |
| 146 | |
| 147 | <p> |
Brian | ff0cc92 | 2007-02-22 16:29:48 -0700 | [diff] [blame^] | 148 | To build the glslcompiler program (this will be improved someday): |
Brian | 61d31ae | 2007-02-17 09:41:19 -0700 | [diff] [blame] | 149 | </p> |
Brian | ff0cc92 | 2007-02-22 16:29:48 -0700 | [diff] [blame^] | 150 | <pre> |
| 151 | cd src/mesa |
| 152 | make libmesa.a |
| 153 | cd drivers/glslcompiler |
| 154 | make |
| 155 | </pre> |
| 156 | |
Brian | 61d31ae | 2007-02-17 09:41:19 -0700 | [diff] [blame] | 157 | |
| 158 | <p> |
| 159 | Here's an example of using the compiler to compile a vertex shader and |
| 160 | emit GL_ARB_vertex_program-style instructions: |
| 161 | </p> |
| 162 | <pre> |
| 163 | glslcompiler --arb --linenumbers --vs vertshader.txt |
| 164 | </pre> |
| 165 | <p> |
| 166 | The output may look similar to this: |
| 167 | </p> |
| 168 | <pre> |
| 169 | !!ARBvp1.0 |
| 170 | 0: MOV result.texcoord[0], vertex.texcoord[0]; |
| 171 | 1: DP4 temp0.x, state.matrix.mvp.row[0], vertex.position; |
| 172 | 2: DP4 temp0.y, state.matrix.mvp.row[1], vertex.position; |
| 173 | 3: DP4 temp0.z, state.matrix.mvp.row[2], vertex.position; |
| 174 | 4: DP4 temp0.w, state.matrix.mvp.row[3], vertex.position; |
| 175 | 5: MOV result.position, temp0; |
| 176 | 6: END |
| 177 | </pre> |
| 178 | |
| 179 | <p> |
| 180 | Note that some shading language constructs (such as uniform and varying |
| 181 | variables) aren't expressible in ARB or NV-style programs. |
| 182 | Therefore, the resulting output is not always legal by definition of |
| 183 | those program languages. |
| 184 | </p> |
| 185 | <p> |
| 186 | Also note that this compiler driver is still under development. |
| 187 | Over time, the correctness of the GPU programs, with respect to the ARB |
| 188 | and NV languagues, should improve. |
| 189 | </p> |
| 190 | |
Brian | 9595d19 | 2007-01-20 13:40:57 -0700 | [diff] [blame] | 191 | </BODY> |
| 192 | </HTML> |