Handling Transparency Issues in Overlapping ThreeJS Meshes

The issue you're experiencing with transparent materials rendering incorrectly when overlapping is a common challenge in ThreeJS and 3D rendering in general. This is related to how the depth buffer works with transparent objects.

The Problem

When rendering transparent objects, the standard depth-based rendering pipeline can cause visual artifacts because:

1. Objects are typically rendered in an arbitrary order
2. The depth buffer doesn't handle partial transparency well
3. Overlapping transparent surfaces can create z-fighting or incorrect blending

Potential Solutions

Based on the information available in ThreeJS documentation, here are some approaches that might help:

1. Proper Render Order

The most reliable solution is to ensure proper render order of transparent objects:

JAVASCRIPTCopied
// Sort objects from back to front (relative to camera)
scene.traverse(function(object) {
  if (object.isMesh && object.material.transparent) {
    // Calculate distance to camera
    object.renderOrder = camera.position.distanceTo(object.position);
  }
});

// Update this on camera movement

2. Material Settings Combination

Try this specific combination of settings:

JAVASCRIPTCopied
material = new THREE.MeshBasicMaterial({
  transparent: true,
  opacity: 0.5,
  depthWrite: false,  // Don't write to depth buffer
  depthTest: true     // But still test against it
});

3. Alpha Settings

For textures with sharp transparency (like UI elements):

JAVASCRIPTCopied
material = new THREE.MeshBasicMaterial({
  transparent: true,
  alphaTest: 0.5,     // Discard pixels below this alpha threshold
  depthWrite: true    // Write to depth buffer for fully opaque pixels
});

4. Manual Sorting

If your scene is relatively static, you can manually set render order:

JAVASCRIPTCopied
// Furthest objects first
backgroundMesh.renderOrder = 0;
middlegroundMesh.renderOrder = 1;
foregroundMesh.renderOrder = 2;

5. Consider Using Layers

For complex UI elements, consider using separate render passes or layers:

JAVASCRIPTCopied
// Create a separate scene for UI elements
const uiScene = new THREE.Scene();
// Render main scene first, then UI scene
renderer.render(mainScene, camera);
renderer.render(uiScene, camera);

Additional Considerations

• Since you're using an older version of ThreeJS (v101/v111), consider upgrading to the latest version as transparency handling has improved
• For UI elements specifically, consider using CSS3DRenderer or HTML overlays instead of 3D meshes with transparency
• If possible, redesign your UI to minimize overlapping transparent elements

Without seeing your specific implementation, these are general approaches based on common ThreeJS transparency issues. The combination of proper render order and carefully tuned material settings is usually the most effective solution.