Handling Mesh Painting Projection in Three.js

You're facing a common challenge with texture painting in 3D space - the distortion that occurs when mapping 2D textures onto 3D surfaces with non-uniform UV mapping.

Understanding the Problem

When you draw directly on a texture using UV coordinates, the brush stroke appears distorted on the mesh because:

1. UV space is not necessarily uniform across the mesh surface
2. The projection from 3D world space to 2D texture space doesn't preserve area or shape

Possible Solutions

1. Displacement-Based Approach

One approach is to use a displacement map to create the illusion of paint thickness:

JAVASCRIPTCopied
// Set up a displacement map on your material
const material = new THREE.MeshStandardMaterial({
  map: diffuseTexture,
  displacementMap: displacementTexture,
  displacementScale: 0.1 // Adjust as needed
});

The displacementMap affects the actual vertex positions of your mesh, which can help create more realistic paint effects as it physically alters the mesh structure.

2. Local Space Projection

Instead of drawing directly in UV space, you could:

1. Create a temporary plane aligned with the mesh surface at the hit point
2. Project your brush stroke onto this plane
3. Transfer the result to your texture

JAVASCRIPTCopied
// On raycast hit
function onBrushStroke(intersection) {
  // Get the normal at the hit point
  const normal = intersection.face.normal.clone();
  normal.transformDirection(intersection.object.matrixWorld);
  
  // Create a projection matrix based on the hit point and normal
  const projectionMatrix = new THREE.Matrix4();
  projectionMatrix.makeRotationFromEuler(normal.toEuler());
  projectionMatrix.setPosition(intersection.point);
  
  // Use this to transform your brush stroke before applying to texture
  // ...
}

3. Multi-Resolution Approach

Another technique is to use multiple textures at different resolutions:

JAVASCRIPTCopied
// Set up a base texture and a detail texture
const baseMaterial = new THREE.MeshStandardMaterial({
  map: baseTexture,
  normalMap: baseNormalMap
});

// Add a detail layer where you paint
const detailTexture = new THREE.Texture(/* ... */);
material.onBeforeCompile = (shader) => {
  // Modify shader to blend in detail texture at the brush location
  // ...
};

4. Triplanar Mapping for Brush Application

You could implement a triplanar mapping approach just for the brush application:

JAVASCRIPTCopied
// In your shader or processing code
function applyBrushTriplanar(position, normal, brushTexture) {
  // Project brush onto three orthogonal planes and blend based on normal
  const xProjection = /* project onto YZ plane */;
  const yProjection = /* project onto XZ plane */;
  const zProjection = /* project onto XY plane */;
  
  // Blend based on normal direction
  const blendWeights = Math.abs(normal);
  blendWeights.normalize();
  
  return xProjection * blendWeights.x + 
         yProjection * blendWeights.y + 
         zProjection * blendWeights.z;
}

Recommendation

For the most accurate results without creating new geometry, I'd recommend a hybrid approach:

1. Use a temporary render target to project your brush stroke in 3D space
2. Sample this projection and apply it to your permanent texture
3. Consider using a displacement map alongside your color texture to add depth to the paint

This approach would give you the most control over how the brush appears on the surface while still permanently applying it to your texture.