Jim Hurley wrote:
Take a look at:
go stack url "http://home.infostations.net/jhurley/CollidingPolygons.rev";
Sorry, Jim, but that doesn't work for all polygons. For example (excuse
the ascii drawing ....)
111111111
1 1
1 1
22222222222222222222222
2 2
22222222222222222222222
1 1
111111111
Even for 2 rectangles, it doesn't always work. If I remember
correctly, the most effective algorithm for overlap depends on edge
intersections (with a special case test for one polygon entirely within
the other). Any two polygons that overlap will have at least one case
where their edges intersect - though you need to deal with the special
cases where the edges touch; you can then test the mid-point of the
edges adjacent to the point of touch to determine if they are within or
outwith the other polygon (or alternatively, test the points 1 delta
away from the point of touch - but then you need to be very careful
about precision and rounding errors).
And even then, you have some weird corner cases involved in
self-reflexive polygons - far less Rev's "invisible edge" polygons which
can produce disjoint areas.
(Is self-reflexive the right term ? polygons that overlap themselves, like
11111
1 1
111111111111111
1 1
111111111111 1
1 1 1 1
1 1 1 1
1 11111 1
1 1
11111111111
You need to determine whether the points in the self-overlap area are
within the polygon or not ... and I can't remember which is the "right"
decision, i.e. the one that makes the problem easier.
--
Alex Tweedly http://www.tweedly.net
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