box2d collision groups

Question

does anyone know whether bodies only collide when (body1.categoryBits & body2.maskBits) && (body1.maskBits & body2.categoryBits) ? or do they already collid

Answer 1

If anyone is still looking for more explanations, I made a detailed tutorial here :
http://aurelienribon.wordpress.com/2011/07/01/box2d-tutorial-collision-filtering/