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);
    }
protected final int index ( int tx , int ty ) { return ( ty - _bounds . y ) * _bounds . width + ( tx - _bounds . x ) ; }
Returns the index into our arrays of the specified tile.