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Clipping

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Clipping Summary It’s the process of finding the exact part of a polygon lying inside the view volume To maintain consistency, clipping of a polygon should result in a polygon, not a sequence of partially unconnected lines We will first look at 2 different 2D solutions and then extend one to 3D

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Sutherland-Hodgman Algorithm Clip the polygon against each boundary of the clip region successively Result is possibly NULL if polygon is outside Can be generalised to work for any polygonal clip region, not just rectangular

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Clipping To A Region To find the new polygon iterate through each of the polygon edges and construct a new sequence of points starting with an empty sequence for each edge there are 4 possible cases to consider

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Clipping a polygon edge against the boundary Given an edge P0,P1 we have 4 cases: entering the clipping region add P and P1 leaving the region add only P entirely outside do nothing entirely inside add only P1 Where P is the point of intersection

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Still the Sutherland-Hodgman We can determine which of the 4 cases and also the point of intersection with just if statements To sum it up, an example:

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Weiler-Atherton Algorithm When we have non-convex polygons then the algorithm above might produce polygons with coincident edges This is fine for rendering but maybe not for other applications (eg shadows) The Weiler-Atherton algorithm produces separate polygons for each visible fragment

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Weiler-Atherton Algorithm

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Find the intersection vertices and connect them in the two lists

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Find the intersection vertices and connect them in the two lists

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Find the intersection vertices and connect them in the two lists

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Completed Loop

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Classify each intersection vertex as Entering or Leaving

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Capture clipped polygons Start at an entering vertex If you encounter a leaving vertex swap to right hand (clip polygon) loop If you encounter an entering vertex swap to left hand (polygon) loop A loop is finished when you arrive back at start Repeat whilst there are entering vertices

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Capture clipped polygons Loop 1: L, 4, 5, K Loop 2: J, 9, 0, i

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Clipping Polygons in 3D The Sutherland-Hodgman can easily be extended to 3D the clipping boundaries are 6 planes instead of 4 lines intersection calculation is done by comparing an edge to a plane instead of edge to edge It can either be done in Projection Space or in Canonical Perspective

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Clipping in Projection Space The view volume is defined by: Testing for the 4 cases is fast, for example for the x = 1 (right) clip plane: x0  1 and x1  1 entirely inside x0  1 and x1 > 1 leaving x0 > 1 and x1  1 entering x0 > 1 and x1 > 1 entirely outside

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Clipping in Canonical Perspective When we have an edge that extends from the front to behind the COP, then if we clip after projection (which in effect is what the PS does) we might get wrong results

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Clipping in Homogeneous Coord. The Sutherland-Hodgman can also be used for clipping in 4D before dividing the points by the w This can have the advantage that is even more general, it even allows for the front clip plane to be behind the COP

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Clipping Recap Sutherland-Hodgman is simple to describe but fails in certain cases Weiler-Atherton clipping is more robust but considerably harder Both extend to 3D but we need to consider projection and end up clipping in 4D

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