Warning: (X1,Y1) contains no external contours. Function POLYBOOL assumes that that external contours have clockwise-ordered vertices, and all contours in (X1,Y1) have counterclockwise-ordered vertices. Use POLY2CW to reverse the vertex order if necessary.
the vertices describing the outer edge of a polygon–the outside of a donut, for example. These vertices must be given in CLOCKWISE order.
the vertices describing an area in which the convex hull is hollow (the donut hole). These vertices must be given in COUNTERclockwise order.