code: protected final int index (int tx, int ty)
      {
  //         if (!_bounds.contains(tx, ty)) {
  //             String errmsg = "Coordinates out of bounds: +" + tx + "+" + ty +
  //                 " not in " + StringUtil.toString(_bounds);
  //             throw new IllegalArgumentException(errmsg);
  //         }
          return (ty-_bounds.y)*_bounds.width + (tx-_bounds.x);
      }
docs: Returns the index into our arrays of the specified tile.