On 14 Jul '08, at 5:26 AM, Robert Clair wrote:
The basic algorithm is conceptually simple: 1.) find all the intersections 2.) break into pieces at the intersections3.) decide which pieces you need on the basis of some in/out tests and which set operation you are doing.4.) assemble the final resultThe hitch is that #3 is a bunch of yes/no decisions that you must make on the basis of imprecise floating point operations in #1. And all the results of #3 have to be consistent or #4 won't work properly. This is trivial in the easy cases (curves intersecting at reasonable angles, no tiny pieces) but *VERY* hard to get right for the arbitrary case
That was my experience exactly :) Task #3 is one of those things that seems obvious to a human, but is surprisingly complex to describe to a computer. Unfortunately I don't remember the name of the algorithm I used, but it took a good-sized academic article to describe. It was very prone to problems with edge [sic] cases, like two intersections at almost but not quite the same point, or intersections of almost- parallel lines. (An earlier algorithm I tried was simpler but turned out to fail on vertical lines!)
I don't mean to suggest this is impossible, but everyone's development resources are limited, and there are other features you [the OP] could implement instead that would give you more bang for the buck.
—Jens
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