i want to draw shadow by projecting models to the ground plane. so i used the projection matrix (row-major order here) 1 0 0 0 0 1 0 0 -x/z -y/z 0 -1/z 0 0 0 1 it works well when i multiply vectors with the matrix myself, but doesn''t work when i use glMultMatrix. Could anybody tell me why, or does anybody has better ideas for making shadow on a plane?