I'm using the game engine and programming in Python. I have a mesh and I need only a part of it detect collisions, I mean only a vertex group of that mesh. How could I do this? I know how to detect collisions using the entire mesh.
To enable collision for the physics to certain faces you have to create a collision object.
To do this, add a new lowpoly mesh object which recreates the part which has to collide. Or enter edit mode with your object selected, select the collision faces (easy with pressing C or B), hit ShiftD to duplicate and RMB or Esc to cancel translation. Then hit P> Selection to separate the selected faces to a new object.
Finally parent the display object to the collision object (select the display object and then the collision object, hit CtrlP> Object). Go to the game properties, set the collision object to invisible and the Physics Type of the display object to No Collision.
To detect collision with/for logic bricks you must also create an other collision object.