Foxfire,Thank for that,I will be glad to do that.But i dont like msn i use yahoo instead.

we can colaborate (the 3 of us) to figure out a working implementation in GS.Its good idea to do that. I am waiting for you Foxfire and CItrok

I found that my approach may not work when knife(B) is completely inside base(A). in this case wrong results may appear.Some other cases that wrong results appear :
1- when we handle concave-shaped models (as both knife and base)
2- when the camera (eye) is inside either base or knife (which cause the stencil buffer to completely filed)

It has some shadowing and collission detection knot. So for now it is not practical unless we work on it to figure it out.