Vertex Shaders

Vertex shaders are the first programmable stage in the DirectX graphics pipeline. They are responsible for processing individual vertices, transforming them from model space to clip space, and calculating various attributes that will be passed to subsequent pipeline stages, such as pixel shaders.

What is a Vertex Shader?

A vertex shader operates on each vertex of a geometric primitive (like a triangle or line). For each vertex, it receives input data and produces output data. The primary tasks of a vertex shader include:

Vertex Transformation: Applying world, view, and projection matrices to transform vertex coordinates from object space to screen space (clip space).
Per-Vertex Calculations: Computing lighting, normals, texture coordinates, and other vertex attributes.
Vertex Output: Outputting the transformed vertex position and any other calculated attributes to be interpolated across the primitive.

Input and Output

Vertex shaders take vertex data as input, which typically includes:

Position (POSITION semantic)
Normal (NORMAL semantic)
Texture Coordinates (TEXCOORD[n] semantics)
Color (COLOR[n] semantics)

The essential output is the transformed vertex position in clip space, usually tagged with the SV_POSITION semantic.

HLSL for Vertex Shaders

High-Level Shader Language (HLSL) is commonly used to write DirectX shaders. Here's a basic example of a vertex shader:


struct VS_INPUT {
    float4 position : POSITION;
    float2 texcoord : TEXCOORD0;
};

struct VS_OUTPUT {
    float4 position : SV_POSITION;
    float2 texcoord : TEXCOORD0;
};

cbuffer WorldViewProjection : register(b0) {
    matrix WVP;
};

VS_OUTPUT main(VS_INPUT input) {
    VS_OUTPUT output;
    output.position = mul(WVP, input.position);
    output.texcoord = input.texcoord;
    return output;
}

Key Concepts

Semantics: These (e.g., POSITION, SV_POSITION) link shader inputs and outputs to the graphics pipeline.
Matrices: World, View, and Projection matrices are fundamental for transforming vertices.
mul() function: Used for matrix-vector and matrix-matrix multiplication.
SV_POSITION: The semantic for the final clip-space position, which is mandatory for the vertex shader output.

Vertex Shader's Place in the Pipeline:

Diagram showing Vertex Shader in Graphics Pipeline

Conceptual representation of vertex shader stage.

Performance Considerations

Vertex shaders are executed once per vertex. Optimizing them for speed is crucial:

Minimize complex calculations.
Reduce the number of instructions.
Leverage hardware capabilities.
Process only necessary data.

Understanding vertex shaders is fundamental to creating complex and visually rich 3D graphics applications with DirectX.