Shadow Rendering Issue with Multiple Materials in Three.js

Based on your description of the shadow rendering issue with multiple materials, I can provide some insights on how to approach this problem.

The issue you're experiencing appears to be related to how Three.js handles shadow casting when using multiple materials with different visibility settings and side properties. This is particularly problematic when you have a combination of FrontSide and BackSide materials where one is visible and the other is not.

Potential Solutions

1. Use Shadow Material Properties:
   Instead of making the material invisible with visible=false, you could set the material's castShadow property to control shadow behavior:

   JAVASCRIPTCopied
   const sphereMaterial = new THREE.MeshStandardMaterial({ 
     side: THREE.FrontSide, 
     color: 0xff0000 
   });
   const cloneMaterial = new THREE.MeshStandardMaterial({ 
     side: THREE.BackSide, 
     color: 0x0000ff 
   });
   
   // Instead of cloneMaterial.visible=false
   // Try controlling shadow properties
   const sphere = new THREE.Mesh(geometry, [sphereMaterial, cloneMaterial]);
   sphere.castShadow = true;

2. Adjust Shadow Bias:
   You mentioned wanting to keep a non-zero bias. Try adjusting the shadow bias to minimize shadow artifacts:

   JAVASCRIPTCopied
   directionalLight.shadow.bias = -0.001; // Adjust this value as needed

3. Use a Single Material with Custom Rendering:
   Instead of using two materials, consider using a single material with custom shader modifications to achieve the desired visual effect without affecting shadow rendering.

4. Group Objects:
   Split your object into two separate meshes - one for the front side and one for the back side. This way you can control visibility and shadow casting independently:

   JAVASCRIPTCopied
   const frontMesh = new THREE.Mesh(geometry, sphereMaterial);
   frontMesh.castShadow = true;
   
   const backMesh = new THREE.Mesh(geometry, cloneMaterial);
   backMesh.visible = false; // This won't affect the shadow of frontMesh

5. Material Shadow Properties:
   Explore using the material's shadow-specific properties:

   JAVASCRIPTCopied
   cloneMaterial.shadowSide = THREE.FrontSide;

Without being able to directly examine your full implementation, these approaches represent general strategies to address shadow rendering issues with multiple materials. You may need to experiment with combinations of these solutions to find what works best for your specific case.