code: function (idx, out) {
          if (idx < this.triangleCount && idx >= 0) {
              if (!out) {
                  out = [];
              }
              var indices = this.indices;
              out[0] = indices[idx * 3];
              out[1] = indices[idx * 3 + 1];
              out[2] = indices[idx * 3 + 2];
              return out;
          }
      }
docs: Get indices of triangle at given index.
  @param {number} idx
  @param {Array.<number>} out
  @return {Array.<number>}