External Material Loading in Three.js

Your approach for separating 3D geometry data from materials in Three.js is sound and follows good practices for asset management. Using the userData property to store external material references is a valid approach since it's designed for custom data storage.

Existing Patterns in Three.js

Three.js already has some established patterns for handling external materials:

1. MTLLoader: Used with OBJLoader to load materials separately from geometry
2. GLTFLoader: Supports external references to textures
3. MaterialLoader: Specifically designed to load materials from JSON

Your Approach Analysis

Your proposed solution has several advantages:

• Clear separation of concerns (geometry vs. materials)
• Potential for material reuse across models
• Easier material updates without reloading geometry

The implementation using userData and a custom ExtMaterialLoader is a reasonable approach. Here's how you might refine it:

Suggested Implementation

JAVASCRIPTCopied
class ExtMaterialLoader {
  constructor(baseUrl = '') {
    this.baseUrl = baseUrl;
    this.materialLoader = new THREE.MaterialLoader();
    this.textureLoader = new THREE.TextureLoader();
    this.loadedMaterials = new Map(); // Cache for materials
  }

  async loadMaterialsForObject(object) {
    if (!object) return;
    
    // Process this object if it has a material
    if (object.userData && object.userData.extMaterial) {
      const materialData = object.userData.extMaterial;
      const url = this.baseUrl + materialData.url;
      
      // Check cache first
      if (this.loadedMaterials.has(url)) {
        object.material = this.loadedMaterials.get(url);
      } else {
        // Load the material
        const material = await this.loadMaterial(url, materialData);
        object.material = material;
        this.loadedMaterials.set(url, material);
      }
    }
    
    // Process children recursively
    if (object.children) {
      for (const child of object.children) {
        await this.loadMaterialsForObject(child);
      }
    }
  }

  async loadMaterial(url, materialData) {
    // Fetch the material JSON
    const response = await fetch(url);
    const json = await response.json();
    
    // Apply any additional metadata from userData
    if (materialData.textureScale) {
      // Store this to apply to textures after loading
      json.userData = json.userData || {};
      json.userData.textureScale = materialData.textureScale;
    }
    
    // Load the material using MaterialLoader
    const material = this.materialLoader.parse(json);
    
    // Apply any texture modifications
    if (material.map && materialData.textureScale) {
      material.map.repeat.set(
        materialData.textureScale.x || 1,
        materialData.textureScale.y || 1
      );
    }
    
    return material;
  }
}

Improvements to Consider

1. Caching: The implementation above includes a simple cache to avoid loading the same material multiple times.

2. Parallel Loading: Consider loading materials in parallel for better performance:

JAVASCRIPTCopied
async loadAllMaterialsForObject(object) {
  const promises = [];
  this.collectMaterialPromises(object, promises);
  await Promise.all(promises);
}

collectMaterialPromises(object, promises) {
  if (object.userData?.extMaterial) {
    promises.push(this.loadAndAssignMaterial(object));
  }
  
  if (object.children) {
    for (const child of object.children) {
      this.collectMaterialPromises(child, promises);
    }
  }
}

3. Integration with Three.js Loaders: Consider extending the ObjectLoader class to handle your external material references automatically.

Your approach is well-aligned with Three.js patterns and should work effectively for separating geometry and material data.