# Weiler-Atherton algorithm: Determining holes arise from union

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Hi all, I just recently started looking at Weiler-Atherton algorithm for polygon boolean operation. In particular, I was looking at union between 2 polygons. Most references on the net I came across describe usage of the algorithm for clipping purpose. Though on 1 reference I found out to get the union I basically just need to traverse the 2 vertex lists in a different way. But what the reference didn't mention was how to figure out any holes that arises from the union. My guess is that given a polygon P and clipping polygon Q, if we start the traverse with an "out" intersection, and return to the intersection point without crossing over to Q, then the vertices found in the traverse forms a hole. Wondering if this is the case? Thuan

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