Understanding Vertex Shaders in Three.js

Introduction

Vertex shaders are a fundamental component of modern graphics programming. In Three.js, vertex shaders are used to process the vertices of 3D models before they are rendered. This article provides an overview of vertex shaders, their role in the graphics pipeline, and how to use them in Three.js.

What is a Vertex Shader?

A vertex shader is a small program that runs on the GPU and processes each vertex of a 3D model. Its main tasks include:

Transforming vertex positions from object space to screen space.
Applying vertex-level effects, such as displacement or animation.
Passing data to the fragment shader, which handles pixel-level rendering.

How Vertex Shaders Work

Input Stage
The vertex shader receives input data from the vertex buffer, which includes the position, normal, and texture coordinates of each vertex.
Transformation Stage
The vertex shader transforms the vertex positions from local coordinates to world coordinates, and then to screen coordinates. This involves matrix operations, typically using the model-view and projection matrices.
Output Stage
The vertex shader outputs transformed vertex positions and additional data (such as colors or texture coordinates) to the fragment shader.

Setting Up a Vertex Shader in Three.js

Create a Shader Material
To use a custom vertex shader in Three.js, you need to create a ShaderMaterial. Here’s a basic example:

const vertexShader = `
    varying vec3 vPosition;

    void main() {
        vPosition = position;
        gl_Position = projectionMatrix * modelViewMatrix * vec4(position, 1.0);
    }
`;

const fragmentShader = `
    varying vec3 vPosition;

    void main() {
        gl_FragColor = vec4(abs(vPosition), 1.0);
    }
`;

const material = new THREE.ShaderMaterial({
    vertexShader: vertexShader,
    fragmentShader: fragmentShader
});

Apply the Shader Material to a Mesh
Create a mesh and apply the shader material:

const geometry = new THREE.BoxGeometry();
const cube = new THREE.Mesh(geometry, material);
scene.add(cube);

Customizing the Vertex Shader

Transforming Vertices
You can modify the vertex shader to apply custom transformations or animations:

uniform float time;
varying vec3 vPosition;

void main() {
    vec3 animatedPosition = position + vec3(sin(time), 0.0, 0.0);
    vPosition = animatedPosition;
    gl_Position = projectionMatrix * modelViewMatrix * vec4(animatedPosition, 1.0);
}

Adding Custom Attributes
You can pass additional attributes to the vertex shader and use them for various effects:

attribute vec3 customAttribute;
varying vec3 vCustom;

void main() {
    vCustom = customAttribute;
    gl_Position = projectionMatrix * modelViewMatrix * vec4(position, 1.0);
}

Testing and Debugging

Use WebGL Debugging Tools
Tools like WebGL Insights or the Three.js Inspector can help you debug shader programs.
Check Shader Errors
Ensure you handle and log errors in your shaders to diagnose issues:

const shaderMaterial = new THREE.ShaderMaterial({
    vertexShader: vertexShader,
    fragmentShader: fragmentShader
});

shaderMaterial.onBeforeCompile = (shader) => {
    console.log(shader.vertexShader);
    console.log(shader.fragmentShader);
};

Conclusion

Vertex shaders are a powerful tool in Three.js for creating custom visual effects and animations. By understanding and utilizing vertex shaders, you can enhance the rendering of your 3D models and create more dynamic and interactive graphics.

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