Vertex Shader

Purpose

The vertex shader processes each vertex individually:

• Transform vertex position to clip space
• Pass attributes to fragment shader
• Calculate lighting values (if using per-vertex lighting)

Inputs (Vertex Attributes)

Attribute	Description
Position	Local coordinates (vec3)
Normal	Surface normal for lighting (vec3)
Texture coordinates	UV mapping (vec2)
Colours	Vertex colours (vec3/vec4)

Outputs

• gl_Position: Clip-space position (required)
• Varying/out variables: Interpolated values for fragment shader

Example GLSL

#version 330
in vec3 position;
in vec3 normal;
out vec3 frag_normal;
uniform mat4 mvp_matrix;

void main() {
    frag_normal = normal;
    gl_Position = mvp_matrix * vec4(position, 1.0);
}

Coordinate Transformations

Model-View-Projection Matrix

The MVP matrix transforms vertices through coordinate spaces:

$\mathbf{M}_{\text{MVP}} = \mathbf{M}_{\text{projection}} \cdot \mathbf{M}_{\text{view}} \cdot \mathbf{M}_{\text{model}}$

Coordinate Spaces

Space	Description
Model/Object	Original coordinates
World	After model transformation
View/Camera	Relative to camera
Clip	After projection, before perspective divide
NDC	Normalised Device Coordinates (−1 to 1)
Screen	Final pixel coordinates

Vertex Attribute Interpolation

Attributes are interpolated across the triangle using barycentric coordinates:

$\mathbf{v} = \alpha \mathbf{v}_0 + \beta \mathbf{v}_1 + \gamma \mathbf{v}_2$

where $\alpha + \beta + \gamma = 1$.

CPU-Side Drawing Sequence

1. glUseProgram(shaderProgram): Activate shaders
2. glBindVertexArray(vao): Bind vertex configuration
3. glUniformMatrix4fv(...): Upload transformation matrices
4. glDrawElements(...): Issue draw call

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Summary

• Vertex shader processes each vertex once
• Transforms position to clip space via gl_Position
• Passes interpolated attributes to fragment shader
• MVP matrix combines model, view, and projection transforms

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Past Paper Questions

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

2023 Paper 3 Question 4: What is the purpose of a vertex shader? What are its inputs and outputs?

2022 Paper 3 Question 4: Explain the role of the vertex shader in the OpenGL pipeline.