I have a situation in which two rectangles collide, and I have to detect how much did they collide so so I can redraw the objects in a way that they are only touching each others edges.

It's a situation in which a moving ball should hit a completely unmovable wall and instantly stop moving. Since the ball sometimes moves multiple pixels per screen refresh, it it possible that it enters the wall with more that half its surface when the collision is detected, in which case i want to shift it position back to the point where it only touches the edges of the wall.

Here is the conceptual image it:

I decided to implement this with masks, and thought that i could supply the masks of both objects (wall and ball) and get the surface (as a square) of their intersection. However, there is also the offset parameter which i don't understand.

Here are the docs for the method:

Mask.overlap

    Returns the point of intersection if the masks 
    overlap with the given offset - or None if it does not overlap.
    Mask.overlap(othermask, offset) -> x,y

    The overlap tests uses the following offsets (which may be negative):

   +----+----------..
   |A   | yoffset
   |  +-+----------..
   +--|B
   |xoffset
   |  |
   :  :