Implementing Geometric Specular Antialiasing in Three.js

Your approach to implementing Valve's Geometric Specular Antialiasing (GSAA) technique in Three.js is on the right track, but there are a few considerations that might help improve your results.

Analysis of Your Implementation

Your shader code implementation looks generally correct in terms of calculating the geometric roughness factor:

GLSLCopied
vec3 normalW = inverseTransformDirection(normal, viewMatrix);
vec3 nDfdx = dFdx(normalW);
vec3 nDfdy = dFdy(normalW);
float geometricRoughnessFactor = pow(saturate(max(dot(nDfdx, nDfdx), dot(nDfdy, nDfdy))), 0.333);

material.roughness = max(roughnessFactor, 0.0525);
material.roughness = min(max(material.roughness, geometricRoughnessFactor), 1.0);

Potential Issues and Improvements

1. Shader Injection Point: The location where you're injecting this code in the Three.js shader is critical. It should be after normal mapping but before the roughness is used in BRDF calculations.

2. Normal Space: Ensure you're using the correct normal space. In your code, you're transforming to world space, but depending on the shader, you might need view space or another space.

3. Exponent Value: The 0.333 exponent (cube root) in Valve's implementation is somewhat arbitrary. You might experiment with different values (0.25-0.5) to find what works best for your specific case.

4. Roughness Blending: Consider a more gradual blending approach rather than just taking the max:

   GLSLCopied
   float blendFactor = 0.75; // Adjust as needed
   material.roughness = mix(material.roughness, 
                           max(material.roughness, geometricRoughnessFactor),
                           blendFactor);

5. Scale Factor: You might need to scale the geometric roughness factor:

   GLSLCopied
   float scale = 1.5; // Adjust based on your scene scale
   geometricRoughnessFactor *= scale;

Regarding Centroid Interpolation

The centroid interpolation part of Valve's technique is more challenging to implement in Three.js because it requires modifications at the vertex shader level and potentially changes to how Three.js handles attribute interpolation.

For WebGL/Three.js, you might need to:

1. Create a custom shader material
2. Implement a version of the technique that works within WebGL's constraints
3. Consider that WebGL might not support centroid interpolation directly

Alternative Approaches

1. Roughness Mapping: Create a roughness map that increases roughness at edges and high-curvature areas.

2. Normal Map Filtering: Implement a technique to filter normal maps based on viewing angle and distance.

3. Temporal Anti-Aliasing (TAA): If your application allows for it, TAA can help with specular aliasing by accumulating samples over time.

4. Specular Lobe Adjustment: Modify the specular BRDF to widen the lobe based on geometric factors.

Implementation Strategy

To fully implement GSAA in Three.js:

1. Create a custom material by extending MeshStandardMaterial
2. Override the onBeforeCompile method to inject your GSAA code
3. Place the code after normal mapping but before BRDF calculations
4. Experiment with the parameters mentioned above

JAVASCRIPTCopied
// Example of extending MeshStandardMaterial
class GSAAMaterial extends THREE.MeshStandardMaterial {
  constructor(parameters) {
    super(parameters);
    this.gsaaIntensity = parameters.gsaaIntensity || 1.0;
  }

  onBeforeCompile(shader) {
    shader.uniforms.gsaaIntensity = { value: this.gsaaIntensity };
    
    // Find where roughness is calculated
    const roughnessIndex = shader.fragmentShader.indexOf('material.roughness = max( roughnessFactor, 0.0525 );');
    
    if (roughnessIndex !== -1) {
      // Insert GSAA code
      const gsaaCode = `
        vec3 normalW = inverseTransformDirection(normal, viewMatrix);
        vec3 nDfdx = dFdx(normalW);
        vec3 nDfdy = dFdy(normalW);
        float geometricRoughnessFactor = pow(saturate(max(dot(nDfdx, nDfdx), dot(nDfdy, nDfdy))), 0.333) * gsaaIntensity;
        
        material.roughness = max(roughnessFactor, 0.0525);
        material.roughness = min(max(material.roughness, geometricRoughnessFactor), 1.0);
      `;
      
      shader.fragmentShader = shader.fragmentShader.replace(
        'material.roughness = max( roughnessFactor, 0.0525 );',
        gsaaCode
      );
    }
    
    super.onBeforeCompile(shader);
  }
}

This approach should give you more control over the implementation and allow you to fine-tune the parameters for your specific use case.