1. Computer Graphics 3 Lecture 5: OpenGL Shading Language (GLSL)
Dr. Benjamin Mora
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Benjamin Mora

2. University of Wales Swansea
Content
Introduction.
How to include GLSL programs in your code.
GLEW, API
Variables and Types.
Vertex Processor.
Fragment Processor.
Samplers
Built-in functions.
Applications.
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Introduction
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4. Vertex and Fragment Programs
Vertices
Transform
And Lighting
Vertex Programs:
User-Defined Vertex Processing
Setup
Rasterization
Tests (z, stencil…)
Texture Fetch, Fragment Shading
Fragment Programs:
User-Defined
Per-Pixel Processing
Frame Buffer Blending
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Introduction
Introduced with OpenGL 2.0.
High Level Language.
Real shaders implementation hidden inside drivers.
C/C++ like coding.
Some differences.
Stronger typing.
Language still heavily influenced by current hardware design.
Still somehow messy…
Compatible with future pipelines.
Replaces fixed vertex and pixel pipelines.
Geometry shader available as an extension.
OpenGL 3.0 adds some functionalities.
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How to code/debug
Understanding of the whole pipeline needed.
Thorough analysis of the algorithm/solution first.
Favor simple solutions unless big performance issues.
Start with a very simple shader that achieves a simple task.
Test and iterate your code until program is finalized.
Frame rate proportional to code efficiency.
Thoroughly analyze your problem again…
Check for redundancies, memory access, etc…
Use textures for emulating/pre-computing complex functions
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How to code/debug
Debugging:
Again difficult.
Can test variables/temporary results by returning specific colors.
Tools:
RenderMonkey (ATI).
glslDevil
gDEBugger (30-days trial version).
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Shader Loading
Shaders should be ideally stored in a separate text file.
Needs to be loaded into your program as a string (char *) variable, and then sent to the API.
Several different shaders can be stored by the API and interchanged during rendering.
Cannot be changed between glBegin(…) and glEnd() calls.
VertexProgram.shader
myProgram.c
void main() {
gl_Position=gl_ModelviewProjectionMatrix
* gl_Vertex; } …
Char *myVertexProgram;
LoadText(myVertexProgram,
“VertexProgram.shader”);
//Use now the OpenGL 2.0 API
//to compile and enable the program
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Shader Loading
Loading a shader object (vertex or fragment) requires several steps:
Create a shader object
glCreateShader.
Associate the source code to the shader object
glShaderSource.
Compile the shader.
glCompileShader
Attach the shader to a program object (container for shaders)
glAttachShader
Link the compiled shader to a program.
glLinkShader
Replace the fixed pipeline with the program object.
glUseProgram.
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10. Shader Loading:Example
char *myVertexProgram;
char *myFragmentProgram;
GLuint vShader, fShader, program; …
vShader=glCreateShader(GL_VERTEX_SHADER);
fShader=glCreateShader(GL_FRAGMENT_SHADER);
glShaderSource(vShader, 1, & myVertexProgram, NULL);
glShaderSource(fShader, 1, & myFragmentProgram, NULL);
glCompileShader(vShader);
glCompileShader(fShader);
//Source strings can now be deleted
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11. Shader Loading:Example
program=glCreateProgram(); //Creates a program object
glAttachShader(program, vShader);
glAttachShader(program, fShader);
glLinkProgram(program);
glUseProgram(program);
//can come back to a fixed pipeline by passing NULL instead …
//Don’t forget :
//Objects must be deleted when not needed anymore
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Variables and Types
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Types
Simple types.
Structures (struct keyword) and arrays possible.
No Pointer!
Implicit conversion generally not possible.
Scalar types:
float, bool, int
int at least in the range [-65535, 65535]
Declaration:
float f,g=1.0;
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Types
Vector types:
vec2, vec3, vec4: Vectors of floats.
bvec2, bvec3, bvec4: Vectors of booleans.
ivec2, ivec3, ivec4: Vectors of integers.
Declaration: vec3 v=vec3(1.0,0.0,3.0);
Vector components:
.xyzw, for vectors representing positions.
.rgba, for vectors representing colors
.stqp, for vectors representing texture coordinates.
Designation not compulsory.
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Types
Swizzling examples:
float f;
vec4 v;
vec2 v2=v.ww;
vec3 v3=v.xzy;
v2=vec2(3.0,-1.0);
v2=texture1D(sampler,coordinate).xy;
v=v+f; //f is added to the 4 components of v!
v+=v; //Component-wise addition
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Types
Matrices (of floats, square):
mat2, mat3, mat4;
mat4 m;
vec4 v=m[2];
float f=m[2][2];
Row and columns inverted in OpenGL conventions!
m[2] is the third column of the matrix.
Don’t use oversized vector and matrices if not required.
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Types
Structure :
Struct light {
vec3 position;
vec3 color;
float watt; //could be actually stored with color }
light myLight;
No typedef!
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Types
Arrays:
vec3 vertices[20];
vec3 vertices2[];
//Also possible. Size must be determinable at compilation //time. See manual & specs.
Special case: texture coordinate array.
Internally declared as:
varying vec4 gl_TexCoord[];
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Types
Samplers
Texture variables.
sampler1D
sampler2D
sampler3D
samplerCube
sampler1DShadow
sampler2DShadow
Declaration:
Uniform sampler2D brick;
vec4 col=texture2D(brick, texCoordinate);
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Types: Samplers
Example
C/C++ core program:
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, marbleTex);
texLoc=glGetUniformLocation(myProgram, “marbleTexture”);
glUniform1i(texLoc,0);
Vertex Program:
varying vec2 coord; …
coord = gl_MultiTexCoord0.st; //Get the tex coordinates.
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Types: Samplers
Example
Fragment Program:
varying vec2 coord;
uniform sampler2D marbleTexture; //texture object. …
gl_FragColor = texture2D(marbleTexture, coord);
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Types
Qualifiers:
attribute
For frequently changing variables, typically what would be passed to OpenGL between glBegin(…) and glEnd().
Built-in attributes include gl_Vertex, gl_Normal,…
uniform
For not-so-frequently changing variables, typically what would be passed to OpenGL outside of a glBegin(…)/glEnd() section.
At most changed once per primitive.
Read-only.
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Types
Qualifiers:
varying
For variables passed from the vertex shader to the fragment shader. These variables undergo a linear interpolation.
Variable declation must be consistent across the vertex and fragment programs.
Perspectively correct.
const
Variable value fixed at compilation time. Cannot be modifier
The first 3 qualifiers must be global variables.
No qualifier means a read/write variable local to the shader.
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Types
Functions:
Functions can be written locally.
Call by value-return.
Parameter qualifiers: in
out
inout
const
In addition to previous qualifiers
Example:
float norm(in vec3 v) {…
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Built-In Variables
GLSL pre-declares many (useful) variables.
Input/Output variables are used for communication between programs.
Additional attributes can also be specified.
Implementation dependent.
A minimum number defined by OpenGL.
glGet(GL_MAX_VERTEX_ATTRIBS);
See later.
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26. Predefined Vertex Variables
attribute vec4 gl_Color;
attribute vec4 gl_SecondaryColor;
attribute vec3 gl_Normal;
attribute vec4 gl_MultiTexCoord0;
attribute vec4 gl_MultiTexCoord1;
const int gl_MaxTextureCoords; …
attribute vec4 gl_FogCoord;
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27. Vertex Ouput Variables
vec4 gl_Position;
vec4 gl_ClipVertex;
float gl_PointSize;
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28. Vertex Varying Ouput Variables
varying vec4 gl_FrontColor;
varying vec4 gl_BackColor;
varying vec4 gl_FrontSecondary;
varying vec4 gl_BackSecondary;
varying vec4 gl_TexCoord[];
float gl_FogFragCoord;
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29. Special Fragment Input Variables
varying vec4 gl_Color;
varying vec4 gl_SecondaryColor;
varying vec4 gl_TexCoord[];
varying float gl_FogFragCoord;
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30. Special Fragment Input Variables
bool gl_FrontFacing;
vec4 gl_FragCoord;
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31. Fragment Output Variables
vec4 gl_FragColor;
vec4 gl_FragData;
float gl_FragDepth;
//gl_FragCoord.z by default
These variables have a global scope.
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Built-In Constants
const int gl_MaxClipPlanes;
const int gl_MaxCombinedTextureImageUnits;
const int gl_MaxFragmentUniformComponents;
const int gl_MaxVertexAttribs;
const int gl_MaxVaryingFloats;
const int gl_MaxDrawBuffers;
const int gl_MaxTextureCoords;
const int gl_MaxTextureUnits;
const int gl_MaxTextureImageUnits;
const int gl_MaxVertexTextureImageUnits;
const int gl_MaxLights;
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33. Built-In Uniform Variables
uniform mat4 gl_ModelViewMatrix;
uniform mat4 gl_ModelViewProjectionMatrix;
uniform mat4 gl_ProjectionMatrix;
uniform mat4 gl_TextureMatrix[gl_MaxTextureCoords];
uniform mat4 gl_ModelViewMatrixInverse;
uniform mat4 gl_ModelViewProjectionMatrixInverse;
uniform mat4 gl_ProjectionMatrixInverse;
uniform mat4 gl_TextureMatrixInverse[gl_MaxTextureCoords];
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34. Built-In Uniform Variables
uniform mat4 gl_ModelViewMatrixTranspose;
uniform mat4 gl_ModelViewProjectionMatrixTranspose;
uniform mat4 gl_ProjectionMatrixTranspose;
uniform mat4 gl_TextureMatrixTranspose[gl_MaxTextureCoords];
uniform mat4 gl_ModelViewMatrixInverseTranspose;
uniform mat4 gl_ModelViewProjectionMatrixInverseTranspose;
uniform mat4 gl_ProjectionMatrixInverseTranspose;
uniform mat4 gl_TextureMatrixInverseTranspose[gl_MaxTextureCoords];
uniform mat3 gl_NormalMatrix;
uniform float gl_NormalScale;
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35. Built-In Uniform Variables
struct gl_LightSourceParameters {
vec4 ambient;
vec4 diffuse;
vec4 specular;
vec4 position;
vec4 halfVector;
vec3 spotDirection;
float spotExponent;
float spotCutoff;
float spotCosCutoff;
float constantAttenuation;
float linearAttenuation;
float quadraticAttenuation; };
uniform gl_LightSourceParameters gl_LightSource[gl_MaxLights];
etc…
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36. Vertex and Fragment Processors
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37. Vertex and Fragment Processors
Replaces the fixed pipeline.
Input/Ouput Data: Attribute or Uniform variables.
Built-In or User defined.
Uses “Varying Data” for the communication of Linearly interpolated Values between the vertex and the fragment program.
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Vertex Processor
Modelview and projection matrices not applied.
Normals not transformed to eye-coordinate.
Normals not normalized.
Texture coordinates not processed.
Lighting not performed.
Color material computations not performed. …
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Vertex Processor
After the vertex program, the following fixed functionalities are still applied:
Color clamping.
Perspective division.
Viewport mapping.
Depth range scaling.
Additional Vertex Attributes can be send from the main program.
Additional colors, tangents, curvatures…
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40. Passing More Vertex Attributes
Main C/C++ program:
Texture coordinates can be used.
Not best.
glVertexAttrib function.
void glVertexAttrib2dv(GLuint index, const GLdouble *v);
void glVertexAttrib4s(GLuint index, GLshort v0, GLshort v1, GLshort v2, GLshort v3) ;
void glVertexAttrib4fv(GLuint index, const GLfloat *v);
etc…
Index at least in the range [0..16]
Attrib 0 indicates the completion of a vertex.
Version for normalized data available…
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41. Passing More Vertex Attributes
How to associate a fragment program variable with an attribute index in the C/C++ program?
Use glBindAttribLocation function.
void glBindAttribLocation(GLuint program, GLuint index, const GLchar *name);
glBindAttribLocation(myProgram, 1, “objectTangent”);
Must be done before calling the linker.
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42. Passing More Vertex Attributes
Main C/C++ program:
glVertexAttribPointer.
void glVertexAttribPointer(GLuint index, GLint size, GLenum type, GLboolean normalized, GLsizei stride, const GLvoid *pointer);
Similar to vertex arrays. Arrays can now be stored in video memory and processed optimally.
Vertex attrib arrays possible.
Enabling/Disabling Attrib arrays:
void glEnableVertexAttribArray(GLuint index);
void glDisableVertexAttribArray(GLuint index);
Arrays used when making a call to glDrawArrays, glDrawElements, etc…
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43. Passing More Vertex Attributes
Uniform variables:
Setup in a different way than attribute variables.
After linking the program, the main application (C/C++) must query the location of the uniform variable, and then set its value.
GLint glGetUniformLocation (GLuint program, const GLchar *name) :
Look for a specific variable.
Returns the location.
void glUniform{1|2|3|4}{f|i} (Glint location, TYPE v);
Set the uniform value. Should not happen between glBegin/glEnd.
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Fragment Processor
The fragment program mainly processes interpolated information generated from the vertex program.
e.g. gl_Color.
The fragment program must replace/code:
Texture mapping environments & functions.
Texture application.
Color application/generation.
Shading.
Fog application.
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Built-In Functions
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Built-In Functions
Easy Shader Development.
Readability.
Simplicity.
Common functions needed for graphics.
Mask the actual hardware implementation.
The compiler has to be efficient/clever.
No warranty that a function is hardware accelerated.
Non-accelerated functions could be slower.
Most of them available from both programs.
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Built-In Functions
genType = float | vec2 | vec3 | vec4
Trigonometry Functions.
genType sin( genType );
genType cos( genType );
genType tan( genType );
genType asin( genType );
genType acos( genType );
genType atan( genType, genType );
genType atan( genType );
genType radians( genType );
genType degrees( genType );
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Built-In Functions
Inverse, Exponential and square root functions.
genType pow( genType, genType );
genType exp( genType );
genType log( genType );
genType exp2( genType );
genType log2( genType );
genType sqrt( genType );
genType inversesqrt( genType );
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Built-In Functions
Common functions
Min, Max, Clamping, Linear interpolation (Mix), modulo, floor, frac, step functions.
genType abs( genType );
genType ceil( genType );
genType clamp( genType, genType, genType );
genType clamp( genType, float, float );
genType floor( genType );
genType fract( genType );
genType max( genType, genType );
genType max( genType, float );
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Built-In Functions
Common functions
genType mix( genType, genType, genType );
genType mix( genType, genType, float );
genType mod( genType, genType );
genType mod( genType, float );
genType sign( genType );
genType smoothstep( genType, genType, genType );
genType smoothstep( float, float, genType );
genType step( genType, genType );
genType step( float, genType );
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Built-In Functions
3D functions and Matrix functions.
dot product, length, multiplications…
vec4 ftransform(); //Vertex ONLY. Same transform as //done with a fixed pipeline. A direct ModelviewProjection //multiplication may lead to a slightly different result.
vec3 cross( vec3, vec3 );
float distance( genType, genType );
float dot( genType, genType );
genType faceforward ( genType V, genType I, genType N );
float length( genType );
genType normalize( genType );
genType reflect( genType I, genType N );
genType refract( genType I, genType N, float eta );
mat matrixCompMult( mat, mat );
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Built-In Functions
Texture Lookup functions
//Optional bias term is Fragment ONLY
vec4 texture1D( sampler1D, float [,float bias] );
vec4 texture1DProj( sampler1D, vec2 [,float bias] );
vec4 texture1DProj( sampler1D, vec4 [,float bias] );
vec4 texture2D( sampler2D, vec2 [,float bias] );
vec4 texture2DProj( sampler2D, vec3 [,float bias] );
vec4 texture2DProj( sampler2D, vec4 [,float bias] );
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Built-In Functions
Texture Lookup functions
vec4 texture3D( sampler3D, vec3 [,float bias] );
vec4 texture3DProj( sampler3D, vec4 [,float bias] );
vec4 textureCube( samplerCube, vec3 [,float bias] );
vec4 shadow1D( sampler1DShadow, vec3 [,float bias] );
vec4 shadow2D( sampler2DShadow, vec3 [,float bias] );
vec4 shadow1DProj( sampler1DShadow, vec4 [,float bias] );
vec4 shadow2DProj( sampler2DShadow, vec4 [,float bias] );
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Built-In Functions
Texture Lookup functions
//Vertex ONLY; ensure //GL_MAX_VERTEX_TEXTURE_IMAGE_UNITS > 0
vec4 texture1DLod( sampler1D, float, float lod );
vec4 texture1DProjLod( sampler1D, vec2, float lod );
vec4 texture1DProjLod( sampler1D, vec4, float lod );
vec4 texture2DLod( sampler2D, vec2, float lod );
vec4 texture2DProjLod( sampler2D, vec3, float lod );
vec4 texture2DProjLod( sampler2D, vec4, float lod );
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Built-In Functions
Texture Lookup functions
//Vertex ONLY; ensure //GL_MAX_VERTEX_TEXTURE_IMAGE_UNITS > 0
vec4 texture3DProjLod( sampler3D, vec4, float lod );
vec4 textureCubeLod( samplerCube, vec3, float lod );
vec4 shadow1DLod( sampler1DShadow, vec3, float lod );
vec4 shadow2DLod( sampler2DShadow, vec3, float lod );
vec4 shadow1DProjLod( sampler1DShadow, vec4, float lod );
vec4 shadow2DProjLod( sampler2DShadow, vec4, float lod );
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Built-In Functions
Other functions:
float noise1( genType );
vec2 noise2( genType );
vec3 noise3( genType );
vec4 noise4( genType );
genType dFdx( genType );
genType dFdy( genType );
genType fwidth( genType ); …
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57. Application: Phong Shading
Show an extreme example with a specular lighting.
Ian Fergusson,
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