Accessing instanceMatrix in Three.js TSL for InstancedMesh

Based on the available documentation, there's limited information specifically about TSL (Three.js Shader Language) and its handling of the instanceMatrix property for InstancedMesh.

However, I can suggest a potential workaround based on how Three.js typically handles instanced meshes:

Possible Workaround

When working with custom shaders for instanced meshes, you typically need to manually pass the instance matrix data to your shader. This is often done by creating a custom attribute or uniform.

JAVASCRIPTCopied
// Create a custom attribute for instance matrices
const instanceMatrices = new Float32Array(count * 16); // 16 values per 4x4 matrix
// Fill the array with your matrix data

// Create the attribute and add it to your geometry
geometry.setAttribute('aInstanceMatrix', new THREE.InstancedBufferAttribute(instanceMatrices, 16));

// In your vertex shader, you would then use:

GLSLCopied
// In your vertex shader
attribute mat4 aInstanceMatrix;

void main() {
    // Use the instance matrix
    vec4 instancePosition = aInstanceMatrix * vec4(position, 1.0);
    gl_Position = projectionMatrix * modelViewMatrix * instancePosition;
}

Alternative Approach

If you're using the standard Three.js materials with custom modifications, you might be able to extend the built-in shaders:

JAVASCRIPTCopied
// Modify the shader chunks
THREE.ShaderChunk.instancedMatrix = /* your custom implementation */;

// Or create a custom material that extends MeshStandardMaterial
const material = new THREE.MeshStandardMaterial();
material.onBeforeCompile = (shader) => {
    // Modify the shader to handle instanceMatrix as needed
    shader.vertexShader = shader.vertexShader.replace(
        '#include <begin_vertex>',
        `
        #include <begin_vertex>
        // Your custom instanceMatrix handling
        `
    );
};

Since the documentation doesn't provide specific details about TSL's handling of instanceMatrix, you might need to experiment with these approaches or check the Three.js GitHub repository or forums for more recent information on this specific limitation.