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The existing algorithm had some corner cases (pun!), where it failed to
produce correct vertices in the right order. This appeared only when the
surface was transformed (rotated). It also produced degenerate polygons
(3 or more vertices with zero polygon area) for non-transformed cases
where the clipping and surface rectangles were adjacent but not
overlapping.
Introduce a new algorithm for finding the boundary vertices of the
intersection of a coordinate axis aligned rectangle and an arbitrary
polygon (here a quadrilateral). The code is based on the
Sutherland-Hodgman algorithm, where a polygon is clipped by infinite
lines one at a time.
This new algorithm should always produce the correct vertices in the
clockwise winding order, and discard duplicate vertices and degenerate
polygons. It retains the fast paths of the existing algorithm for the
no-hit and non-transformed cases.
Benchmarking with earlier versions showed that the new algorithm is
a little slower (56 vs. 68 us/call) than the existing algorithm, for
the transformed case. The 'cliptest f' command before and after this
commit can be used to compare the speed of the transformed case only.
Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>
Acked-by: Rob Clark <rob.clark@linaro.org>
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