I don''t know the FPS game use quaternion or matrix to perform transformation? ~~~~~~~~~~~~~~~~~~~

I don''t know about the latest games, but Quake 1 used simple Euler angles for rotations. (BAD!!! )

These days people use quaternions in order to manipulate or store orientations. But when the time comes that you want to transform a whole lot of vertices, you temporarily convert the quaternion to a matrix, transform all the vertices by that matrix, then throw away the matrix.

-Jonathan.