Handling Depth Sorting with InstancedBufferGeometry

When rendering a large number of instances like your galaxy of 200,000 stars, depth sorting can indeed become an issue with InstancedBufferGeometry. The problem you're experiencing is related to how WebGL handles depth testing and transparency.

Solutions for Depth Sorting

Since you're using ShaderMaterial with InstancedBufferGeometry, here are some approaches to address your depth sorting issue:

1. Enable Alpha Blending and Depth Testing

Make sure your ShaderMaterial has the appropriate transparency settings:

JAVASCRIPTCopied
const material = new THREE.ShaderMaterial({
  transparent: true,
  depthTest: true,
  depthWrite: false, // Important for transparent objects
  // Your shader code and other properties
});

2. Sort Instances by Distance

While InstancedBufferGeometry doesn't natively support sorting instances, you can:

1. Calculate distances from camera to each instance on the CPU
2. Sort your instance data by distance
3. Update your instanced buffer attributes accordingly

Example implementation:

JAVASCRIPTCopied
// Assuming you have position data for each instance
function sortInstancesByDistance(camera, positions) {
  const cameraPosition = camera.position;
  
  // Create array of indices and distances
  const indexDistancePairs = [];
  for (let i = 0; i < positions.length / 3; i++) {
    const x = positions[i * 3];
    const y = positions[i * 3 + 1];
    const z = positions[i * 3 + 2];
    
    const dx = x - cameraPosition.x;
    const dy = y - cameraPosition.y;
    const dz = z - cameraPosition.z;
    
    // Calculate squared distance (faster than using Math.sqrt)
    const distSq = dx * dx + dy * dy + dz * dz;
    
    indexDistancePairs.push({ index: i, distance: distSq });
  }
  
  // Sort by distance (farthest to nearest for correct rendering)
  indexDistancePairs.sort((a, b) => b.distance - a.distance);
  
  // Reorder your instance data based on sorted indices
  // This depends on how your data is structured
  return indexDistancePairs.map(pair => pair.index);
}

You would then need to reorder all your instance attributes based on this sorting.

3. Use Alpha Testing Instead of Blending

If your stars have a distinct shape, you can use alpha testing instead of blending:

JAVASCRIPTCopied
const material = new THREE.ShaderMaterial({
  transparent: false,
  depthTest: true,
  depthWrite: true,
  uniforms: {
    alphaTest: { value: 0.5 }
  },
  fragmentShader: `
    // Your existing shader code
    void main() {
      // Your existing code to calculate color
      if (gl_FragColor.a < alphaTest) discard;
    }
  `
});

4. Render in Multiple Passes

For complex scenes, you might need to:

1. Render opaque parts first with depth writing enabled
2. Sort transparent parts by distance
3. Render transparent parts from back to front

Performance Considerations

With 200,000 instances, sorting on every frame could be expensive. Consider:

• Sorting less frequently (e.g., every few frames)
• Using frustum culling to reduce the number of instances that need sorting
• Implementing a spatial partitioning structure to optimize distance calculations

Remember that any CPU-based sorting solution will add overhead, so you'll need to balance visual quality with performance.

The most efficient approach would likely be to use alpha testing if your star visuals allow for it, as this avoids the need for sorting altogether.