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[celloe]

I have a line segment defined by its points p0 and p1, and a polygon defined by a set of points, guaranteed to be convex. I followed this article to get my intersection test up and running okay - http://geomalgorithms.com/a13-_intersect-4.html and it was working great, until I started using it in different cases.

I'm using AS3, and currently the test fails in seemingly random cases, when my line segment is oriented in a particular way, and, oddly enough, sometimes only works when wrapped inside a time-based function such as an ENTER_FRAME call or Timer.


It's hard to show from a demo alone as it's purely based on tweaking values and seeing what works, but here's the code: http://pastebin.com/tejrZjUw

And my current results:


Any ideas?.. Been staring at this for quite a while and can't think of what causes it.

[Will Vale]

I'm not an AS3 person so I don't know what the implications are for unintialised variables, but it looks like you have a case where t can be uninitialised when you're computing the entry and exit points.

In your small D branch (close to edge?) you continue the loop skipping the calculation of t if N is positive. You could have this happen on all iterations, in which case you wind up exiting the loop and using the undefined value of t.

Not sure if that *is* happening in your case, but worth checking.

If AS3 initialises everything, this is a non-issue. Likewise if the maths mean this situation never comes up for all points it's a non-issue, but maybe it'll offer a clue.

W

[ChevyRay]

Default value for Number types is NaN, so that could be your problem.

[celloe]

Both of you lead me directly to the clue that helped me figure it out! Turns out it worked when called inside the enterFrame function as I'd defined SMALL_NUM below all my functions, so at the time of first calculation it was NaN. So I was getting a divide by zero.

Thanks for the help!