Not sure of your requirement, but I would look at the relationship between the centres of your objects (or whatever 'handle' you are using to position the sprites).
I could certainly calculate the vector between the 2 sprites at the time of collision, but the problem is that either of them could be rotated at any angle during the collision. I've thought about a method something like this:
1) Save the position of each sprite at the time of impact
2) Use that to calculate the relative vectors of the 2 objects
3) Figure based on the rotation of the one sprite, and the vector of the other, calculate side of impact
Kindof like, "If Im here, and he's there, and im facing this way, it could only have collided with surface X"..
I got thinking about that kindof logic last night, and im not sure how accurate it would be. I'l give it a try, and see how it works out.
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