Fragment Shader

Purpose

The fragment shader computes the final colour for each pixel candidate:

• Called once per fragment generated by rasterisation
• Computes colour, depth, and other per-pixel values
• Handles texturing, lighting, and procedural effects

Inputs

Input	Source
Interpolated attributes	Vertex shader outputs
Uniforms	Global parameters (matrices, lights)
Textures	Image data sampled by UV coordinates

Outputs

• gl_FragColor or out vec4 colour: Final pixel colour
• Optionally: modified depth value

Example GLSL: Simple Lighting

#version 330
in vec3 frag_normal;
uniform vec3 light_dir;
out vec4 frag_colour;

void main() {
    float intensity = max(dot(normalize(frag_normal), 
                              normalize(light_dir)), 0.0);
    frag_colour = vec4(vec3(intensity), 1.0);
}

Procedural Shading: Checkerboard Shader

A checkerboard shader demonstrates procedural texturing—computing patterns mathematically without image textures:

#version 330
in vec2 tex_uv;
out vec4 frag_colour;

void main() {
    const float scale = 10.0;
    vec2 tiled = floor(tex_uv * scale);
    float checker = mod(tiled.x + tiled.y, 2.0);
    
    vec3 col = (checker < 1.0) ? vec3(1.0, 1.0, 0.0) : vec3(0.0, 0.0, 0.0);
    frag_colour = vec4(col, 1.0);
}

How It Works

1. floor(tex_uv * scale): Discretise UV into grid cells
2. mod(tiled.x + tiled.y, 2.0): Alternate between 0 and 1
3. Ternary operator: Assign colours based on cell parity

Fragment vs Pixel

• Fragment: A sample generated by rasterisation, may be discarded
• Pixel: Final written value after all tests pass (depth, stencil)

Common Fragment Shader Operations

Operation	Description
Texture lookup	Sample from texture using UV
Lighting	Compute diffuse, specular contributions
Blending	Combine with existing framebuffer colour
Discarding	Conditionally reject fragments

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Summary

• Fragment shader computes final colours for each pixel
• Receives interpolated attributes from vertex shader
• Can perform procedural shading without textures
• Outputs colour that may be written to framebuffer

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Past Paper Questions

2025 Paper 3 Question 4(b)(ii): Give 4 examples of uniform variables. [2 marks]

2024 Paper 3 Question 4: Write a fragment shader that samples a texture and applies basic lighting.

2022 Paper 3 Question 4(b): Write a GLSL fragment shader that produces a yellow and black checkerboard pattern using texture coordinates.