static/js/workers/draco_worker.js

/**
 * Web Worker for Draco mesh decompression preprocessing
 * Prepares Draco data for GPU processing and provides CPU fallback
 */

// Import Draco decoder (would be loaded in a real implementation)
// importScripts('/static/js/libs/draco_decoder.js');

let decoderModule;
let decoder;
let attributeIdsMap;

// Handle messages from the main thread
self.addEventListener('message', async function(e) {
  const { type, buffer } = e.data;
  
  try {
    switch (type) {
      case 'initialize':
        await initDecoder();
        self.postMessage({ type: 'initialized' });
        break;
        
      case 'prepare':
        const preparedData = await prepareDracoData(buffer);
        self.postMessage({ type: 'prepared', result: preparedData }, [
          preparedData.indices.buffer,
          preparedData.positionData?.buffer,
          preparedData.normalData?.buffer,
          preparedData.uvData?.buffer
        ].filter(Boolean));
        break;
        
      case 'decompress':
        const decompressedData = await decompressDraco(buffer);
        self.postMessage({ type: 'decompressed', result: decompressedData }, [
          decompressedData.indices.buffer,
          decompressedData.positions.buffer,
          decompressedData.normals?.buffer,
          decompressedData.uvs?.buffer
        ].filter(Boolean));
        break;
        
      default:
        throw new Error(`Unknown command: ${type}`);
    }
  } catch (error) {
    self.postMessage({ type: 'error', result: { message: error.message } });
  }
});

/**
 * Initialize the Draco decoder
 */
async function initDecoder() {
  // In a real implementation, this would load the Draco decoder wasm module
  // Here we're simulating the module API for demonstration
  
  // Define attribute ID constants to match Draco's internal values
  attributeIdsMap = {
    POSITION: 0,
    NORMAL: 1,
    COLOR: 2,
    TEX_COORD: 3,
    GENERIC: 4
  };
  
  // Create mock decoder API
  decoderModule = {
    decoder: function() {
      return {
        GetEncodedGeometryType: function() { return 1; }, // 1 = triangular mesh
        DecodeBufferToMesh: function() { return { ptr: 123 }; }, // Mock mesh pointer
        GetAttribute: function() { return { ptr: 456 }; }, // Mock attribute pointer
        GetAttributeByType: function() { return { ptr: 456 }; }, // Mock attribute pointer
        GetFaceFromMesh: function() { return true; },
        GetAttributeFloat: function() { return 0.0; },
        GetAttributeIntForAllPoints: function() { return true; },
        GetAttributeFloatForAllPoints: function() { return true; },
        GetNumberOfFaces: function() { return 100; }, // Mock face count
        GetNumberOfPoints: function() { return 300; }, // Mock point count
        GetAttributeId: function() { return 0; },
        GetNumberOfComponents: function() { return 3; },
        GetPointMapSize: function() { return 300; }, // Mock point map size
        GetPointToPointMap: function() { return 0; }
      };
    },
    destroy: function() {}
  };
  
  decoder = decoderModule.decoder();
}

/**
 * Prepare Draco data for GPU processing
 * Extracts quantized attributes and metadata for GPU decompression
 * @param {ArrayBuffer} dracoData - Draco-encoded mesh data
 * @returns {Object} - Prepared data for GPU processing
 */
async function prepareDracoData(dracoData) {
  // Ensure decoder is initialized
  if (!decoder) {
    await initDecoder();
  }
  
  try {
    // This code simulates the data preparation process
    // In a real implementation, this would use the actual Draco API
    
    // In a real implementation, decode enough information for GPU processing
    // This is a simplified version for demonstration
    
    // Create a buffer view
    const dataView = new DataView(dracoData);
    
    // Parse simple header information (placeholder values)
    const vertexCount = 1000;
    const indexCount = 2994; // Assuming triangles, so multiple of 3
    
    // Create buffers to hold the quantized data for GPU processing
    const positionData = new Float32Array(vertexCount * 4); // RGBA format for textures
    const normalData = new Float32Array(vertexCount * 4);
    const uvData = new Float32Array(vertexCount * 4);
    const indices = new Uint32Array(indexCount);
    
    // In a real implementation, Draco methods would extract quantized data
    // Fill buffers with placeholder data for demonstration
    for (let i = 0; i < vertexCount; i++) {
      // Quantized positions (simulated)
      positionData[i * 4] = Math.random();     // Quantized X
      positionData[i * 4 + 1] = Math.random(); // Quantized Y
      positionData[i * 4 + 2] = Math.random(); // Quantized Z
      positionData[i * 4 + 3] = 0;             // Padding for RGBA texture
      
      // Octahedral encoded normals (simulated)
      normalData[i * 4] = Math.random();     // Octahedral X
      normalData[i * 4 + 1] = Math.random(); // Octahedral Y
      normalData[i * 4 + 2] = 0;             // Padding
      normalData[i * 4 + 3] = 0;             // Padding
      
      // Quantized UVs (simulated)
      uvData[i * 4] = Math.random();     // Quantized U
      uvData[i * 4 + 1] = Math.random(); // Quantized V
      uvData[i * 4 + 2] = 0;             // Padding
      uvData[i * 4 + 3] = 0;             // Padding
    }
    
    // Create mesh indices (simulated)
    for (let i = 0; i < indexCount; i += 3) {
      indices[i] = Math.floor(Math.random() * vertexCount);
      indices[i + 1] = Math.floor(Math.random() * vertexCount);
      indices[i + 2] = Math.floor(Math.random() * vertexCount);
    }
    
    // Dequantization information (simulated)
    const positionDequantizationFactors = [10, 10, 10]; // Scale factors for x, y, z
    const positionOffset = [0, 0, 0]; // Offset for positions
    const uvFactors = [1, 1]; // Scale factors for u, v
    
    return {
      vertexCount,
      indexCount,
      positionData,
      normalData,
      uvData,
      indices,
      positionDequantizationFactors,
      positionOffset,
      uvFactors
    };
    
  } catch (error) {
    console.error('Error preparing Draco data:', error);
    throw error;
  }
}

/**
 * Decompress Draco mesh data using CPU (fallback method)
 * @param {ArrayBuffer} dracoData - Draco-encoded mesh data
 * @returns {Object} - Decompressed mesh data
 */
async function decompressDraco(dracoData) {
  // Ensure decoder is initialized
  if (!decoder) {
    await initDecoder();
  }
  
  try {
    // This code simulates the CPU decompression process
    // In a real implementation, this would use the actual Draco API
    
    // In a real implementation, use the Draco decoder to extract mesh data
    // This is a simplified version for demonstration
    
    // Create a buffer view
    const dataView = new DataView(dracoData);
    
    // Parse header information (placeholder values)
    const vertexCount = 1000;
    const indexCount = 2994; // Assuming triangles, so multiple of 3
    
    // Create buffers to hold the decompressed data
    const positions = new Float32Array(vertexCount * 3);
    const normals = new Float32Array(vertexCount * 3);
    const uvs = new Float32Array(vertexCount * 2);
    const indices = new Uint32Array(indexCount);
    
    // In a real implementation, Draco methods would extract geometry data
    // Fill buffers with placeholder data for demonstration
    for (let i = 0; i < vertexCount; i++) {
      // Positions
      positions[i * 3] = (Math.random() * 2 - 1) * 5;     // X
      positions[i * 3 + 1] = (Math.random() * 2 - 1) * 5; // Y
      positions[i * 3 + 2] = (Math.random() * 2 - 1) * 5; // Z
      
      // Normals
      const nx = Math.random() * 2 - 1;
      const ny = Math.random() * 2 - 1;
      const nz = Math.random() * 2 - 1;
      const len = Math.sqrt(nx*nx + ny*ny + nz*nz);
      
      normals[i * 3] = nx / len;     // Normalized X
      normals[i * 3 + 1] = ny / len; // Normalized Y
      normals[i * 3 + 2] = nz / len; // Normalized Z
      
      // UVs
      uvs[i * 2] = Math.random();     // U
      uvs[i * 2 + 1] = Math.random(); // V
    }
    
    // Create mesh indices (simulated)
    for (let i = 0; i < indexCount; i += 3) {
      indices[i] = Math.floor(Math.random() * vertexCount);
      indices[i + 1] = Math.floor(Math.random() * vertexCount);
      indices[i + 2] = Math.floor(Math.random() * vertexCount);
    }
    
    return {
      vertexCount,
      indexCount,
      positions,
      normals,
      uvs,
      indices
    };
    
  } catch (error) {
    console.error('Error decompressing Draco data:', error);
    throw error;
  }
}