computer graphics 3 lecture 5: opengl shading language (glsl)

Computer Graphics 3Lecture 5:OpenGL

Shading Language(GLSL)

Benjamin Mora 1University of Wales

Swansea

Dr. Benjamin Mora

Content

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• Introduction.

• How to include GLSL programs in your code.– GLEW, API

• Variables and Types.

• Vertex Processor.

• Fragment Processor.

• Samplers

• Built-in functions.

• Applications.

Introduction

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Vertex and Fragment Programs

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Setup

Rasterization

Frame Buffer Blending

Texture Fetch, Fragment Shading

Tests (z, stencil…)

Vertices

Transform And Lighting

Vertex Programs:User-Defined Vertex

Processing

Fragment Programs:User-Defined

Per-Pixel Processing

Introduction

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• 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.

How to code/debug

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• 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

How to code/debug

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• Debugging: – Again difficult.– Can test variables/temporary results by returning specific

colors.

• Tools:– RenderMonkey (ATI).– glslDevil

• http://www.vis.uni-stuttgart.de/glsldevil/– gDEBugger (30-days trial version).

Shader Loading

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• 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.

…Char *myVertexProgram;LoadText(myVertexProgram,

“VertexProgram.shader”);//Use now the OpenGL 2.0 API //to compile and enable the program…

myProgram.c

void main(){ gl_Position=gl_ModelviewProjectionMatrix

* gl_Vertex;}

VertexProgram.shader

Shader Loading

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• 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.

Shader Loading:Example

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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

Shader Loading:Example

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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

Variables and Types

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Types

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• 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;

Types

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• 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.

Types

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• 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

Types

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• 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.

Types

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• Structure :– Struct light {

vec3 position;

vec3 color;

float watt; //could be actually stored with color

}

light myLight;

• No typedef!

Types

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• 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[];

Types

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• Samplers– Texture variables.

• sampler1D• sampler2D• sampler3D• samplerCube• sampler1DShadow• sampler2DShadow

– Declaration:• Uniform sampler2D brick;

vec4 col=texture2D(brick, texCoordinate);

Types: Samplers

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• 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.

Types: Samplers

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• Example– Fragment Program:

varying vec2 coord;

uniform sampler2D marbleTexture; //texture object.

…

gl_FragColor = texture2D(marbleTexture, coord);

Types

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• 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.

Types

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• 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.

Types

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• 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) {…

Built-In Variables

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• 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.

Predefined Vertex Variables

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• 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;

Vertex Ouput Variables

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• vec4 gl_Position;

• vec4 gl_ClipVertex;

• float gl_PointSize;

Vertex Varying Ouput Variables

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• varying vec4 gl_FrontColor;

• varying vec4 gl_BackColor;

• varying vec4 gl_FrontSecondary;

• varying vec4 gl_BackSecondary;

• varying vec4 gl_TexCoord[];

• float gl_FogFragCoord;

Special Fragment Input Variables

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• varying vec4 gl_Color;

• varying vec4 gl_SecondaryColor;

• varying vec4 gl_TexCoord[];

• varying float gl_FogFragCoord;

Special Fragment Input Variables

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• bool gl_FrontFacing;

• vec4 gl_FragCoord;

Fragment Output Variables

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• vec4 gl_FragColor;

• vec4 gl_FragData;

• float gl_FragDepth;– //gl_FragCoord.z by default

• These variables have a global scope.

Built-In Constants

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• 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;

Built-In Uniform Variables

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• 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];

Built-In Uniform Variables

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• 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;

Built-In Uniform Variables

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• 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…

Vertex and FragmentProcessors

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Vertex and Fragment Processors

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• 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.

Vertex Processor

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• 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.

• …

Vertex Processor

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• 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…

Passing More Vertex Attributes

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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…

Passing More Vertex Attributes

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• 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.

Passing More Vertex Attributes

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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…

Passing More Vertex Attributes

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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.

Fragment Processor

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• 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.

Built-In Functions

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Built-In Functions

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• 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.

Built-In Functions

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• 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 );

Built-In Functions

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• 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 );

Built-In Functions

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• 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 );

Built-In Functions

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• 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 );

Built-In Functions

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• 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 );

Built-In Functions

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• 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] );

Built-In Functions

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• 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] );

Built-In Functions

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• 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 );

Built-In Functions

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• 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 );

Built-In Functions

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• Other functions:– float noise1( genType );– vec2 noise2( genType );– vec3 noise3( genType );– vec4 noise4( genType );– genType dFdx( genType );– genType dFdy( genType );– genType fwidth( genType );– …

Application: Phong Shading

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Ian Fergusson, https://www.cis.strath.ac.uk/teaching/ug/classes/52.359/lect13.pdf