code: function (idx, arr) {
          var indices = this.indices;
          indices[idx * 3] = arr[0];
          indices[idx * 3 + 1] = arr[1];
          indices[idx * 3 + 2] = arr[2];
      }
docs: Set indices of triangle at given index.
  @param {number} idx
  @param {Array.<number>} arr