Vertex Shading

Vertex shaders are the first programmable stage in the modern graphics pipeline. They are responsible for processing individual vertices.

Introduction

Vertex shading is a crucial part of modern 3D graphics rendering. It's the stage where each vertex of a 3D model is individually processed by a small program called a vertex shader. These shaders run on the graphics processing unit (GPU) and are responsible for transforming vertex positions, calculating lighting, and preparing data for subsequent stages of the graphics pipeline.

Purpose of Vertex Shading

The primary responsibilities of a vertex shader include:

• Vertex Transformation: Applying transformations such as translation, rotation, and scaling to bring vertices from model space to world space, then to view space, and finally to clip space.
• Lighting Calculations: Computing per-vertex lighting based on vertex normals, light positions, and material properties.
• Texture Coordinate Generation: Generating or transforming texture coordinates.
• Passing Data to the Next Stage: Preparing and outputting data (attributes) that will be interpolated across the primitive (triangle) and used by the pixel shader. This can include world-space position, color, normals, texture coordinates, and custom attributes.

Vertex Shader Inputs and Outputs

A vertex shader typically receives per-vertex attributes as input, such as:

• Vertex position (e.g., float4)
• Vertex normal (e.g., float3)
• Texture coordinates (e.g., float2 or float3)
• Vertex color (e.g., float4)

These inputs are often passed as arguments or fetched from vertex buffer objects (VBOs).

The essential output of a vertex shader is the transformed vertex position in clip space (usually a float4). Additionally, vertex shaders can output various other varying variables that are interpolated across the face of the primitive for use in other shader stages, most notably the pixel shader.

Example: A Simple Vertex Shader (HLSL-like pseudocode)

This example demonstrates a basic vertex shader that transforms vertex position using model-view-projection matrices and passes a color through.

// Uniforms (data passed from CPU)
matrix WorldViewProjection : WORLDVIEWPROJECTION; // Combined matrix

// Input structure for vertex data
struct VS_INPUT
{
    float4 Position : POSITION;       // Vertex position in model space
    float4 Color    : COLOR0;         // Vertex color
};

// Output structure for data passed to the rasterizer/pixel shader
struct VS_OUTPUT
{
    float4 Position : SV_POSITION;    // Clip-space position (required)
    float4 Color    : COLOR0;         // Interpolated color
};

// The vertex shader function
VS_OUTPUT main(VS_INPUT input)
{
    VS_OUTPUT output;

    // Transform vertex position from model space to clip space
    output.Position = mul(input.Position, WorldViewProjection);

    // Pass the color through without modification
    output.Color = input.Color;

    return output;
}
        

Explanation:

• WorldViewProjection: A uniform variable representing the combined transformation matrix from model space to clip space. This is typically set by the application on the CPU.
• VS_INPUT: Defines the data received for each vertex.
• VS_OUTPUT: Defines the data output by the vertex shader. SV_POSITION is a special semantic that indicates the clip-space position, which is mandatory.
• mul(input.Position, WorldViewProjection): Performs a matrix-vector multiplication to transform the vertex position.
• output.Color = input.Color;: The color attribute is simply passed through, to be interpolated later.

Advanced Vertex Shading Techniques

Vertex shaders are capable of much more complex operations:

• Skinning and Animation: Deforming a mesh based on skeletal animation.
• Per-Vertex Normalization: Recalculating normals after transformations to ensure correct lighting.
• Procedural Geometry Generation: Creating or modifying geometry directly on the GPU.
• Vertex Animation: Animating vertices without a skeleton, e.g., for wind effects on foliage.
• Tessellation Control: In more advanced pipelines, vertex shaders can interact with tessellation stages to dynamically add detail to geometry.

Performance Considerations

While powerful, vertex shaders are executed for every vertex. Optimizations include:

• Minimizing the number of vertices processed.
• Using efficient matrix operations.
• Reducing the number of texture lookups or complex calculations.
• Sharing common computations across vertices where possible (though many modern pipelines prefer to do this in other stages if applicable).