three.js/examples/js/loaders/draco/geometry_helper.js

// Copyright 2017 The Draco Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//

/**
 * @fileoverview Helper class implementing various utilities for THREE.js
 * geometry.
 */

function GeometryHelper() {}

GeometryHelper.prototype = {
    constructor: GeometryHelper,

    // Computes vertex normals on THREE.js buffer geometry, even when the mesh
    // uses triangle strip connectivity.
    computeVertexNormals: function (bufferGeometry) {
      if (bufferGeometry.drawMode === THREE.TrianglesDrawMode) {
        bufferGeometry.computeVertexNormals();
        return;
      } else if (bufferGeometry.drawMode === THREE.TriangleStripDrawMode) {
        if (bufferGeometry.attributes.position === undefined) {
          return;
        }
        const inPositions = bufferGeometry.attributes.position.array;
        if (bufferGeometry.attributes.normal === undefined) {
          bufferGeometry.addAttribute(
              'normal',
              new THREE.BufferAttribute(new Float32Array(inPositions.length),
                                        3));
        } else {
          // Reset existing normals to zero.
          const array = bufferGeometry.attributes.normal.array;
          for (let i = 0; i < array.length; ++i) {
            array[ i ] = 0;
          }
        }
        let outNormals = bufferGeometry.attributes.normal.array;

        let pos0 = new THREE.Vector3();
        let pos1 = new THREE.Vector3();
        let pos2 = new THREE.Vector3();
        let posDif0 = new THREE.Vector3(), posDif1 = new THREE.Vector3();
        let localNormal = new THREE.Vector3();

        const stripIndices = bufferGeometry.index.array;
        for (let i = 2; i < stripIndices.length; ++i) {
          let index0 = stripIndices[i - 2] * 3;
          let index1 = stripIndices[i - 1] * 3;
          let index2 = stripIndices[i] * 3;
          // Skip degenerate triangles.
          if (index0 === index1 || index0 === index2 || index1 === index2) {
            continue;
          }
          if ((i & 1) !== 0) {
            // Swap index 1 and 0 on odd indexed triangles.
            const tmpIndex = index1;
            index1 = index2;
            index2 = tmpIndex;
          }

          // Get position values.
          pos0.fromArray(inPositions, index0);
          pos1.fromArray(inPositions, index1);
          pos2.fromArray(inPositions, index2);

          // Position differences
          posDif0.subVectors(pos2, pos0);
          posDif1.subVectors(pos1, pos0);

          // Weighted normal.
          localNormal.crossVectors(posDif1, posDif0);

          // Update normals on vertices
          outNormals[index0] += localNormal.x;
          outNormals[index0 + 1] += localNormal.y;
          outNormals[index0 + 2] += localNormal.z;

          outNormals[index1] += localNormal.x;
          outNormals[index1 + 1] += localNormal.y;
          outNormals[index1 + 2] += localNormal.z;

          outNormals[index2] += localNormal.x;
          outNormals[index2 + 1] += localNormal.y;
          outNormals[index2 + 2] += localNormal.z;
        }
        bufferGeometry.normalizeNormals();
        bufferGeometry.attributes.normal.needsUpdate = true;
      }
    },
};