From: <pierloic@fr...>  20050526 13:21:23

Hello, I am implementing the Weiler Atherton algorithm as it really matches my n= eeds for some physic dev:  concave with holes polygon clipped with concave with holes polygon  the algorithm outputs 2 lists: inside polygons and outside polygons  ability to use results as inputs for several passes You can find the original article here : http://www.cs.drexel.edu/~david/Classes/CS430/HWs/p214weiler.pdf The general cases works really good but it is getting harder with the pathological cases when some intersections points happens to be the same = as some polygon points:  2 non parallel edges intersect on one end of one edge  2 non parallel edges intersect on one end of one edge which is also an = end of the other edge  2 parallel edges intersect on two points which are end of edges I tried different ways to treat them (insert intersection nodes or not) b= ut nothing really works for the moment. The paper is quite "vague" about it : "If care is taken in placement of intersections where the subject and cli= p polygon contours are identical in the xy plane, no degenerate polygons w= ill be produced by the clipping process" Then the text makes two assertions : "These two types of intersections will be found to alternate along any gi= ven contour and the number of intersections will always be even" I cant satisfy these two assertions with the different approaches i tried= for the pathological cases... Anyone has experimented these issues ? Any help is welcome. Thanks,  Pierre Loic Herve 