Transparency Ordering Issues in Three.js

This is a common issue when working with transparent materials in Three.js. Based on the information you've provided, you're experiencing problems with the rendering order of transparent objects, specifically with hair materials that use transparency maps.

Understanding the Problem

When rendering transparent objects in Three.js, the default behavior is to sort objects based on their distance from the camera (painter's algorithm). However, this sorting doesn't always work perfectly, especially with complex geometries like hair.

Possible Solutions

1. Ensure WebGLRenderer's sortObjects is enabled

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renderer.sortObjects = true; // This is true by default

2. Use a custom transparent sort function

You can define a custom sorting function for transparent objects:

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renderer.setTransparentSort(function(a, b) {
  // Custom sorting logic here
  // Return negative if a should render before b
  // Return positive if b should render before a
  // Return 0 if order doesn't matter
});

3. Manual ordering of transparent objects

If automatic sorting doesn't work well for your case, you might need to manually control the rendering order:

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// Set renderOrder property on your objects
hairMesh.renderOrder = 1; // Higher numbers render last
otherMesh.renderOrder = 0;

4. Adjust material settings

Make sure your material settings are correctly configured:

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const hairMaterial = new THREE.MeshStandardMaterial({
  map: hairTexture,
  transparent: true,
  alphaTest: 0.5, // Adjust this value to control transparency threshold
  side: THREE.DoubleSide // May be needed depending on your model
});

5. Consider using alphaTest instead of transparency

For hair-like structures, sometimes using alphaTest with a higher value works better than full transparency:

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hairMaterial.alphaTest = 0.5; // Discard pixels with alpha below this value

If you need more specific advice, please share more details about your implementation, such as your material setup code and how your scene is structured.