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{{Short description|Polygon clipping algorithm}}
The '''Weiler–Atherton''' is a polygon-[[Clipping (computer graphics)|clipping]] [[algorithm]]. It is used in areas like [[computer graphics]] and games development where clipping of polygons is needed. It allows clipping of a ''subject or candidate polygon'' by an arbitrarily shaped ''clipping polygon/area/region''.
It is generally applicable only in [[2D computer graphics|2D]]. However, it can be used in [[3D computer graphics|3D]] through visible surface determination and with improved efficiency through [[Z-order]]ing.<ref>Foley, James, Andries van Dam, Steven Feiner, and John Hughes. "Computer Graphics: Principle and Practice". Addison-Wesley Publishing Company. Reading, Massachusetts: 1987. pages 689-693</ref>
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[[image:Weiler-Atherton subdivision.svg|thumb|upright=1.2|Subdivision with the Weiler-Atherton algorithm]]
Before being applied to a polygon, the algorithm requires several preconditions to be fulfilled:
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* The algorithm can support holes (as counter-clockwise polygons wholly inside their parent polygon), but requires additional algorithms to decide which polygons are holes, after which merging of the polygons can be performed using a variant of the algorithm.
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Given polygon A as the clipping region and polygon B as the subject polygon to be clipped, the algorithm consists of the following steps:
# List the vertices of the clipping-region polygon A and those of the subject polygon B.
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