Clarification of collisions in pymunk

Question

I've been investigating Pymunk as a possible platform for a spatial chemistry simulation. I naively thought that when two shapes collide there would be a single contact point. In fact that can be up to two contact points. I also find that each contact point has two points, point_a and point_b. The documentation says

point_a and point_b are the contact position on the surface of each shape

I wonder if someone less naive than I am could explain:

• Why can the ContactPointSet contain one or two contact points only and not three or more?
• Why are point_a and point_b for a given ContactPoint not necessarily the same point, since they are the point at which contact occurs between two shapes?
• Is the shape that point_a is on guaranteed to be arbiter.shapes[0] and the shape that point_b is on guaranteed to be arbiter.shapes[1], or do I need to check that?

Thanks.

Answer 1

Im dont know enough of the collision algo to answer from the mathematical background, but I can give some help on the way:

1. This is from the collision algorithm used. For easy (e.g. Circle to circle) collisions only one point is found, as you can see here: https://github.com/slembcke/Chipmunk2D/blob/master/src/cpCollision.c#L526 In more advanced cases, e.g. Poly to Poly, then the GJK & EPA algorithms are used, https://github.com/slembcke/Chipmunk2D/blob/master/src/cpCollision.c#L607 and this can produce one or two points.

2. I think the reasoning is shown in the below picture. The points are located on each shape, and since they can be overlapping it becomes natural with one for each.

3. I suspect so, but am not 100% sure.

If you are intersted about the GJK & EPA implementation, there's a intro blog post from when it was first introduced into Chipmunk here: http://howlingmoonsoftware.com/wordpress/enhanced-collision-algorithms-for-chipmunk-6-2/