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Asked: 2026-05-25T06:02:44+00:00

I’ve got two polygons defined as a list of Vectors, I’ve managed to write

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I’ve got two polygons defined as a list of Vectors, I’ve managed to write routines to transform and intersect these two polygons (seen below Frame 1). Using line-intersection I can figure out whether these collide, and have written a working Collide() function.

This is to be used in a variable step timed game, and therefore (as shown below) in Frame 1 the right polygon is not colliding, it’s perfectly normal for on Frame 2 for the polygons to be right inside each other, with the right polygon having moved to the left.

My question is, what is the best way to figure out the moment of intersection? In the example, let’s assume in Frame 1 the right polygon is at X = 300, Frame 2 it moved -100 and is now at 200, and that’s all I know by the time Frame 2 comes about, it was at 300, now it’s at 200. What I want to know is when did it actually collide, at what X value, here it was probably about 250.

Polygon intersect

I’m preferably looking for a C# source code solution to this problem.
Maybe there’s a better way of approaching this for games?

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1 Answer

  1. Editorial Team

    I would use the separating axis theorem, as outlined here:

    Then I would sweep test or use multisampling if needed.

    GMan here on StackOverflow wrote a sample implementation over at gpwiki.org.

    This may all be overkill for your use-case, but it handles polygons of any order. Of course, for simple bounding boxes it can be done much more efficiently through other means.

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