Either I have a blackout, our I need some help.

I'm using pushMatrix and popMatrix to rotate and place images I draw in a graphic buffer using rotate(), but now I would like to FLIP the graphic buffer, but I can't find a working function to do that. I would like a matrix-operation which maps either X or Y to -X or -Y, but not both. Any ideas how to do that? (Other than loading the graphic buffer as pixel array.)