Custom Line Material for CAD-Style Edge Highlighting

Your approach to creating a custom line material for CAD-style edge highlighting is on the right track. Let me address your requirements and proposed solution.

Approach Analysis

Your current approach using LineSegments2, LineSegmentsGeometry, and LineMaterial is good for requirements 1-3:

• Displaying edges of object faces
• Configuring color and thickness
• Preventing Z-fighting

For requirement 4 (differentiating visible vs. occluded edges), your idea of using a DepthTexture with a custom shader is indeed a viable solution. This is a common technique in technical visualization.

Implementation Strategy

Here's how you could implement this:

1. Create a depth pass:

   • Render your scene to a DepthTexture using a WebGLRenderTarget
   • This will give you depth information for the entire scene

2. Create a custom ShaderMaterial:

   • Extend LineMaterial or create a new ShaderMaterial
   • Pass the depth texture to your shader as a uniform

3. In the fragment shader:

   • Compare the current fragment's depth with the value in the depth texture
   • If the current fragment is behind the stored depth value, it's occluded
   • Apply different colors/opacity based on this comparison

Code Structure

JAVASCRIPTCopied
// Create depth texture and render target
const depthTexture = new THREE.DepthTexture();
const renderTarget = new THREE.WebGLRenderTarget(
  window.innerWidth, 
  window.innerHeight,
  {
    depthTexture: depthTexture,
    depthBuffer: true
  }
);

// Custom shader material for edges
const edgeMaterial = new THREE.ShaderMaterial({
  uniforms: {
    depthTexture: { value: depthTexture },
    visibleColor: { value: new THREE.Color(0x0000ff) },
    occludedColor: { value: new THREE.Color(0xff0000) },
    opacity: { value: 1.0 },
    occludedOpacity: { value: 0.5 },
    lineWidth: { value: 2.0 },
    // Add camera near/far planes and resolution
    cameraNear: { value: camera.near },
    cameraFar: { value: camera.far },
    resolution: { value: new THREE.Vector2(window.innerWidth, window.innerHeight) }
  },
  vertexShader: /* your vertex shader */,
  fragmentShader: /* your fragment shader with depth comparison logic */,
  transparent: true
});

// Render function
function render() {
  // First pass: render scene to depth texture
  renderer.setRenderTarget(renderTarget);
  renderer.render(scene, camera);
  
  // Second pass: render scene with edge material using the depth info
  renderer.setRenderTarget(null);
  // Update uniforms if needed
  edgeMaterial.uniforms.cameraNear.value = camera.near;
  edgeMaterial.uniforms.cameraFar.value = camera.far;
  
  // Render your edges with the custom material
  renderer.render(scene, camera);
}

Alternative Approaches

1. Multiple Rendering Passes:

   • Render the model normally
   • Render the edges with depth test enabled (for visible edges)
   • Render the edges with depth test reversed (for occluded edges)

2. Using LineDashedMaterial:

   • For occluded edges, you could use dashed lines instead of solid ones
   • This doesn't require custom shaders but provides less flexibility

Your approach using a depth texture is not "hacky" - it's a standard technique for this kind of visualization problem. While it does require multiple render passes, the performance impact should be acceptable for most CAD applications.

The main challenge will be in the shader code to correctly interpret the depth values and handle edge cases properly.