text: // 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;
          }
      },
  };