On the left is two multi-surfaces. The surface orientations and counter-clockwise boundaries for each surface is shown. Red "+" signs mark where a boundary intersects a surface in the preferred direction, while teal "-" signs mark where a boundary intersects a surface in the opposite direction. On the right is the multi-path that forms the intersection of the two multi-surfaces, and the endpoints of the intersection multi-path are also shown. The multi-point that forms the endpoints of the intersection consists of: the intersection points of the boundary of the blue surface with the orange surface, minus the intersection points of the boundary of the orange surface with the blue surface ("minus" means reverse polarity).
Own work
Math buff