BTW, after just a few minutes thought on the quad/n-gon situation.... it seems to me that the line generated between some vertex of any n-gon and the intersection point can only intersect with one other edge of the n-gon (at least for convex polygons), so you just have to find the right edge.

Edit: but you probably have to worry about non-planar polygons once you get above triangles... otherwise, that line may not intersect with any edge :).

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