CSE 381 – Advanced Game Programming GLSL

Rendering Revisited

In a vertex shader You can write code for tasks such as: –Vertex position transformation using the modelview and projection matrices –Normal transformation, and if required its normalization –Texture coordinate generation and transformation –Lighting per vertex or computing values for lighting per pixel –Color computation

GLSL Vertex Shader Responsibility The vertex shader is responsible for at least writing a variable: gl_Position –usually transforming the vertex with the modelview and projection matrices. A vertex processor has access to OpenGL state –so it can perform operations that involve lighting for instance, and use materials. –It can also access textures (only available in the newest hardware). –There is no access to the frame buffer.

Shader Beware There is no requirement to perform all the necessary operations in your shader –your application may not use lighting for instance. Warning: –once you write a vertex shader you are replacing the full functionality of the vertex processor you can't perform normal transformation and expect the fixed functionality to perform texture coordinate generation –When a vertex shader is used it becomes responsible for replacing all the needed functionality of this stage of the pipeline

The fragment (pixel) shader Is responsible for operations like: –Computing colors, and texture coordinates per pixel –Texture application –Fog computation –Computing normals if you want lighting per pixel

Pixel Shader Responsibilities A fragment shader has two output options: –to discard the fragment, hence outputting nothing –to compute either gl_FragColor (the final color of the fragment), or gl_FragData when rendering to multiple targets. Depth can also be written but it is not required since the previous stage already has computed it. Note: –the fragment shader has no access to the frame buffer. –operations such as blending occur only after the fragment shader has run.

GLSL Use Overview

A note about programs A GLSL “Program” does not refer to your game What is it? –a GLSL “program” –basically a set of shaders

Creating a Shader 1.Create a shader container 2.Define the contents of your shader 3.Compile your shader

Creating a Shader Container First, create the shader container Use: GLhandleARB glCreateShaderObjectARB(GLenum shaderType); Ex: GLhandleARB shaderContainer; shaderContainer = glCreateShaderObjectARB(GL_VERTEX_SHADER_ARB); … shaderContainer = glCreateShaderObjectARB(GL_FRAGMENT_SHADER_ARB);

How many shaders can you create? As many as you want to add to a program, NOTE: there can only be: –one main function for the set of vertex shaders AND –one main function for the set of fragment shaders in each single program.

Defining your shader The source code for a shader is a string array, although you can use a pointer to a single string. To set the source code for a shader use: void glShaderSourceARB(GLhandleARB shader, int numOfStrings, const char **strings, int *lenOfStrings); Parameters: –shader - the handle to the shader. –numOfStrings - the number of strings in the array. –strings - the array of strings. –lenOfStrings - an array with the length of each string, or NULL, meaning that the strings are NULL terminated.

Compile the shader Use: void glCompileShaderARB(GLhandleARB program); Parameter: –program - the handle to the program.

Creating a Program 1.Create a program container 2.Attach the shaders to the program 3.Link the program 4.Use the shader

Create a program container What’s a program container? Use: GLhandleARB glCreateProgramObjectARB(void); Returns: –a handle for the container

How many programs? As many programs as you want Once rendering: –you can switch from program to program –go back to fixed functionality during a single frame Ex: –you may want to draw a teapot with refraction and reflection shaders WHILE –having a cube map displayed for background using OpenGL's fixed functionality

Attach the shaders to the program The shaders: –do not need to be compiled at this time –they don't even have to have source code Why is that good? –platform independence To attach a shader to a program: void glAttachObjectARB(GLhandleARB program, GLhandleARB shader); Parameters: –program - the handle to the program –shader - the handle to the shader you want to attach

Dealing with multiple shaders If you have a pair vertex/fragment shaders, you'll need to attach both to the program You can have many shaders of the same type (vertex or fragment) attached to the same program, –just like a C program can have many modules. For each type of shader there can only be one shader with a main function, also as in C. You can attach a shader to multiple programs, for instance if you plan to use the same vertex shader in several programs.

Link the program In order to carry out this step the shaders must be compiled as described previously Use: void glLinkProgramARB(GLhandleARB program); Parameters: –program - the handler to the program. After the link operation, the shader's source can be modified, and the shaders recompiled without affecting the program.

Use a shader program To actually load and use a shader program, use: void glUSeProgramObjectARB(GLhandleARB prog); Parameters: –prog – either: the handle to the program you want to use zero to return to fixed functionality

Notes on using a program Each program is assigned a handle You can have as many programs linked and ready to use as you want –and your hardware allows If a program is in use, and it is linked again, it will automatically be placed in use again –so in this case you don't need to call this function again. –if the parameter is zero then the fixed functionality is activated.

Let’s open up an example Next time: –we’ll examine how to write the shaders

References GLSL Tutorial – OpenGL Shading Language (the orange book) –