GPU-Based Matrix Transformations for InstancedMesh

Based on the available documentation, I can provide some guidance on working with InstancedMesh while keeping matrix computations on the GPU.

Understanding InstancedMesh

InstancedMesh is a special type of Mesh in Three.js that allows you to render multiple instances of the same geometry with different transformations, using GPU instancing for better performance. It's available in the objects category of Three.js:

• InstancedMesh

Working with Instance Matrices

When working with InstancedMesh, you typically need to set the transformation matrix for each instance. The traditional approach involves setting these matrices on the CPU and then uploading them to the GPU, which can be costly for large numbers of instances.

To keep matrix computations on the GPU, you'll need to work with:

1. InstancedBufferGeometry
2. InstancedBufferAttribute
3. Custom shaders

Implementation Approach

Here's a general approach to implement GPU-based matrix transformations:

1. Create an InstancedBufferGeometry
2. Add instance-specific attributes using InstancedBufferAttribute
3. Use these attributes in a custom shader to compute transformation matrices on the GPU

JAVASCRIPTCopied
// Create instanced buffer geometry
const geometry = new THREE.InstancedBufferGeometry();
geometry.copy(new THREE.BoxGeometry(1, 1, 1));

// Create instance attributes (position, rotation, scale)
const instancePositions = new THREE.InstancedBufferAttribute(
  new Float32Array(instanceCount * 3), 3
);
const instanceRotations = new THREE.InstancedBufferAttribute(
  new Float32Array(instanceCount * 4), 4 // Using quaternions
);
const instanceScales = new THREE.InstancedBufferAttribute(
  new Float32Array(instanceCount * 3), 3
);

// Set instance data
for (let i = 0; i < instanceCount; i++) {
  // Set position, rotation, scale for each instance
  // ...
}

// Add attributes to geometry
geometry.setAttribute('instancePosition', instancePositions);
geometry.setAttribute('instanceRotation', instanceRotations);
geometry.setAttribute('instanceScale', instanceScales);

// Create material with custom shader
const material = new THREE.ShaderMaterial({
  vertexShader: `
    attribute vec3 instancePosition;
    attribute vec4 instanceRotation;
    attribute vec3 instanceScale;
    
    // Function to create rotation matrix from quaternion
    mat4 quatToMatrix(vec4 q) {
      // Implementation here
    }
    
    void main() {
      // Compute transformation matrix on GPU
      mat4 instanceMatrix = mat4(1.0);
      
      // Apply scale
      instanceMatrix[0][0] = instanceScale.x;
      instanceMatrix[1][1] = instanceScale.y;
      instanceMatrix[2][2] = instanceScale.z;
      
      // Apply rotation (quaternion to matrix)
      instanceMatrix = instanceMatrix * quatToMatrix(instanceRotation);
      
      // Apply position
      instanceMatrix[3].xyz = instancePosition;
      
      // Apply to vertex
      gl_Position = projectionMatrix * modelViewMatrix * instanceMatrix * vec4(position, 1.0);
    }
  `,
  fragmentShader: `
    void main() {
      gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);
    }
  `
});

// Create mesh
const mesh = new THREE.Mesh(geometry, material);
scene.add(mesh);

This approach allows you to update the instance attributes (position, rotation, scale) without having to compute full 4x4 matrices on the CPU, and the actual matrix computation happens in the vertex shader.

For more complex transformations or animations, you might want to use compute shaders or render targets to update the transformation data entirely on the GPU.