bitmap & bitmap comparison is just waste of CPU imo. because a bitmap is basicly a very huge array each cell with a plain information (hit or not)- there are so many more effective ways as this brute force method (its almost like any other method is more effective as this one).

Perhaps you are searching for octree techniques:
http://en.wikipedia.org/wiki/Octree
this budy shows very good what it´s benfits are:

insead of computing each pixel of the complex character shape you analyze before the shape and devide it into different block sizes. Once done you only check collision with those blocks and once hit do a brute force collision only on that block part of the detailed shape.

some links explaining better what as I can do atm:
http://www.gamasutra.com/features/19970801/octree.htm
http://www.flipcode.com/articles/art...ooctrees.shtml