Per-Instance Polygon Offset in InstancedMesh

Unfortunately, the standard approach of using InstancedBufferAttribute for per-instance properties won't work directly for polygon offset settings. Unlike colors or transforms which can be passed as attributes to shaders, polygon offset is a rendering state that's set at the material level, not a shader attribute.

Why This Won't Work Directly

The polygon offset properties (polygonOffsetFactor and polygonOffsetUnit) are material-level properties that affect how WebGL handles depth testing during rendering. These aren't shader attributes that can be varied per instance through the standard instancing mechanism.

Possible Alternatives

1. Multiple Materials: Create multiple materials with different polygon offset settings and use multiple InstancedMesh objects, each with a different material.

2. Custom Shader Approach: Implement a custom depth offset in your fragment shader. This would require:

   • Adding custom attributes for offset values
   • Modifying the fragment shader to adjust the fragment depth

Here's a simplified example of the custom shader approach:

JAVASCRIPTCopied
// Create instanced buffer attribute for polygon offset values
const offsetValues = new Float32Array(instanceCount * 2); // 2 values per instance
// Fill with your desired offset values for each instance
// ...

// Add to geometry
geometry.setAttribute(
  'instanceOffset', 
  new THREE.InstancedBufferAttribute(offsetValues, 2)
);

// Modify material's shader
const material = new THREE.MeshStandardMaterial();
material.onBeforeCompile = (shader) => {
  // Add attribute
  shader.vertexShader = shader.vertexShader.replace(
    'void main() {',
    `
    attribute vec2 instanceOffset;
    varying vec2 vInstanceOffset;
    void main() {
      vInstanceOffset = instanceOffset;
    `
  );
  
  // Pass to fragment shader
  shader.vertexShader = shader.vertexShader.replace(
    '#include <project_vertex>',
    `
    #include <project_vertex>
    vInstanceOffset = instanceOffset;
    `
  );
  
  // Add varying to fragment shader
  shader.fragmentShader = shader.fragmentShader.replace(
    'void main() {',
    `
    varying vec2 vInstanceOffset;
    void main() {
    `
  );
  
  // Modify gl_FragDepth
  shader.fragmentShader = shader.fragmentShader.replace(
    '#include <output_fragment>',
    `
    #include <output_fragment>
    gl_FragDepth = gl_FragCoord.z + vInstanceOffset.x * gl_FragCoord.w + vInstanceOffset.y;
    `
  );
};

This is a complex approach that may not perfectly replicate the behavior of the built-in polygon offset, but it's the closest you can get to per-instance polygon offset control.