Fragment Shaders

Fragment Shaders
In "Modern" OpenGL

Computer Science Department

bernstdh@jmu.edu

Overview

Purpose:
- Determine the colors (e.g., the diffuse color, the specular color) and depth for a fragment (e.g., a pixel) in a raster
Structure:
- Programs written in a C-like language
- Invariant with respect to inputs (i.e., the same input will produce the same output)
Invocation Frequency:
- Essentially once per fragment generated by the rasterizer

User-Defined Inputs

Source:
- Prior stages in the pipeline
Process:
- User-defined inputs are commonly interpolated across the surface/interior of the graphics primitive/shape

Built-In Inputs

in vec4 gl_FragCoord:
- The location of the fragment in window space
in bool gl_frontFacing:
- TRUE if this fragment was generated by the front face of the primitive
in vec2 gl_PointCoord:
- The location within a point primitive that defines the position of the fragment relative to the size of the point

Uniforms

Purpose:
- A global variable that does not change from one invocation of the shader to the next (within a rendering call)
Tasks Before Invoking the Draw Command:
- glGetUniformLocation() to get a handle/name/ID
- glUniform____() to initialize
Declaration and Assignment Within a Shader:
- uniform type identifier;

Outputs

Purpose:
- Pass color and depth information to the next part of the pipeline
Specified Within the Shader:
- Declared using a layout(location=index) out type identifier statement (Note: 0 is the default index if the layout() clause is omitted.)
Specified Before Linking:
- void glBindFragDataLocation(GLuint programID, GLuint colorNumber, const char* name);
Automatic Specification (Not Recommended):
- If neither of the other two methods is used then the fragment color is automatically assigned when the program is linked