Transmission Material and Mask Pass Compatibility Issue

The smearing effect you're experiencing when using transmission properties with a mask pass is related to how Three.js handles transmission materials. When you enable transmission, IOR, and thickness on a MeshPhysicalMaterial, Three.js creates a separate render target to handle the refraction effects, which can conflict with your masking approach.

The Issue

The transmissionResolutionScale property of the WebGLRenderer is particularly relevant here. When using transmission materials, Three.js creates a separate render target to calculate how light passes through the material. This process doesn't always play well with post-processing effects like your mask pass.

Potential Solutions

1. Adjust the transmission resolution scale:

   JAVASCRIPTCopied
   renderer.transmissionResolutionScale = 0.5; // Try different values between 0.25-1

   Lowering this value might reduce the smearing effect while maintaining acceptable visual quality.

2. Modify your rendering approach:
   Instead of using the built-in transmission effect with post-processing, consider:

   • Using a custom shader material that simulates the transmission effect but works better with your masking setup
   • Rendering the masked background and the transmission object in separate passes

3. Alternative material setup:

   JAVASCRIPTCopied
   const material = new THREE.MeshPhysicalMaterial({
     color: params.color,
     metalness: params.metalness,
     roughness: params.roughness,
     // Reduce or eliminate these problematic properties
     ior: 1.0, // Try lower values closer to 1.0
     thickness: 0.1, // Try lower values
     transmission: params.transmission,
     specularIntensity: params.specularIntensity,
     specularColor: params.specularColor,
     opacity: params.opacity,
     transparent: true // Enable transparency
   });

4. Render order adjustment:
   Make sure your objects have the correct render order:

   JAVASCRIPTCopied
   backgroundPlane.renderOrder = 0;
   transmissionSphere.renderOrder = 1;

Additional Considerations

• The transmission effect in Three.js is designed primarily for realistic glass/liquid rendering, not necessarily for masking effects
• You might need to choose between realistic transmission effects or clean masking, as combining both can be challenging
• Consider using a simpler approach with transparent materials and alpha blending if the transmission effects aren't critical to your visual requirements

If you need the exact masking effect without the smearing, you might need to implement a custom solution that bypasses Three.js's built-in transmission rendering pipeline.

Transmission Material and Mask Pass Compatibility Issue

The smearing effect you're experiencing when using transmission properties with a mask pass is related to how Three.js handles transmission materials. When you enable transmission, IOR, and thickness on a MeshPhysicalMaterial, Three.js creates a separate render target to handle the refraction effects, which can conflict with your masking approach.

The Issue

The transmissionResolutionScale property in the WebGLRenderer controls the resolution of the transmission render target. By default, this is set to 1, which means full resolution. When the camera moves, the transmission material is recalculating the refraction based on the new camera position, causing the smearing effect you're seeing.

Potential Solutions

1. Adjust the transmission resolution scale:

   JAVASCRIPTCopied
   renderer.transmissionResolutionScale = 0.5; // Try different values

2. Simplify your material properties:
   Since you mentioned it works when transmission, IOR, and thickness are off, consider using a simpler approach:

   JAVASCRIPTCopied
   const material = new THREE.MeshPhysicalMaterial({
       color: params.color,
       metalness: params.metalness,
       roughness: params.roughness,
       transmission: params.transmission,
       transparent: true,
       // Remove IOR and thickness if possible
   });

3. Use a custom shader material instead of MeshPhysicalMaterial to create your own transmission effect that works better with the mask pass.

4. Render order adjustment:

   JAVASCRIPTCopied
   // Set a specific render order for your objects
   transmissionSphere.renderOrder = 1;
   backgroundPlane.renderOrder = 0;

5. Alternative approach: Instead of using the mask pass, consider using a different technique like render targets or clipping planes to achieve the masked effect.

Implementation Example

Here's how you might modify your renderer setup:

JAVASCRIPTCopied
// Configure the renderer for better transmission handling
renderer.physicallyCorrectLights = true;
renderer.transmissionResolutionScale = 1.0; // Try adjusting this value
renderer.outputEncoding = THREE.sRGBEncoding;

// You might also need to update your material
material.thickness = 0.5; // Try a lower value
material.ior = 1.2; // Try a value closer to 1.0

The fundamental challenge is that transmission materials with physical properties (IOR, thickness) are designed to work with the standard rendering pipeline, while the mask pass is part of the post-processing pipeline. These two approaches can sometimes conflict with each other, especially during camera movement.

If the above solutions don't resolve the issue, you might need to reconsider your approach to achieve the desired visual effect without relying on both transmission properties and mask passes simultaneously.