Correct. In MonoGame/XNA BlendState.AlphaBlend is pre-multiplied alpha and BlendState.NonPremultiplied is not. It seems that the default HLSL alpha blending uses the pre-multiplied alpha formula.

Oh, wait now I remember. Clipping is connected to rendering and I already have that in my game. Unfortunately, it has nothing to do with my collision-detection problem. I'm talking about the physics.

What is clipping? I do have the AABB of P (the bounds), but I don't see how my goal is achieved by comparing the box with the AABB surrounding P. Also I want to check if R overlaps P, which is a lot harder than checking if all vertices of P are inside R.

Could you recommend any algorithms? Should I use it in-game, or in my level editor?

I know, but that wasn't my question. I don't want to make sure every single polygon in the game is cut into convex polygons, it's prone to human error and is bound to leave a small bug that is hard for me to spot that will break the game. It's more unnecessary work.