Intersecting Coplanar Box and Custom Buffer Geometries Yields Missing Surface in three-mesh-bvh

I need to clarify that your question is about the three-mesh-bvh library, which is a separate library built on top of Three.js for efficient Bounding Volume Hierarchy operations. While I can provide some general guidance based on Three.js geometries, the specific implementation details of three-mesh-bvh's CSG (Constructive Solid Geometry) operations may vary.

Potential Causes and Solutions

The issue you're describing with missing surfaces in the intersection operation is likely related to coplanarity, as you suspected. When two surfaces are exactly coplanar (sharing the same plane in 3D space), boolean operations can sometimes fail to produce the expected results.

Here are some potential workarounds:

1. Slight Offset: Apply a very small offset to one of the geometries to avoid exact coplanarity:

   JAVASCRIPTCopied
   // Apply a tiny offset to boxGeometry
   boxGeometry.translate(0, 0.0001, 0);

2. Epsilon Adjustment: If the three-mesh-bvh library has configuration options for epsilon values (tolerance thresholds), try adjusting these:

   JAVASCRIPTCopied
   // Example (check the actual API of the library you're using)
   const result = CSG.intersect(meshA, meshB, { 
     epsilon: 1e-5  // Adjust this value
   });

3. Pre-processing Geometries: Ensure both geometries are properly closed and manifold before performing the operation:

   JAVASCRIPTCopied
   // Make sure geometries are properly prepared
   boxGeometry.computeBoundingBox();
   customGeometry.computeBoundingBox();
   boxGeometry.computeVertexNormals();
   customGeometry.computeVertexNormals();

4. Post-processing: After the intersection, you might need to manually close the resulting geometry:

   JAVASCRIPTCopied
   // After intersection operation
   resultGeometry.computeVertexNormals();
   // You might need to add missing faces manually

5. Alternative Approach: Instead of relying on the exact intersection, you could slightly scale one of the geometries:

   JAVASCRIPTCopied
   boxGeometry.scale(1.001, 1.001, 1.001);

Since the specific implementation details of three-mesh-bvh's boolean operations aren't covered in the standard Three.js documentation, you might want to check the three-mesh-bvh repository issues or documentation for specific guidance on handling coplanarity issues in CSG operations.