[Second Try] |4*atan2(y,x)-pi|<pi or equivalently, |4*atan2(y,x)/pi-1|<1 Don't laugh too hard; as floating point and GPU's become ubiquitous, this approach may actually be as fast as some other approaches -- particularly in very deeply pipelined units. Of course, it would work somewhat more reliably if atan2(y,x)/pi were the primitive instead of atan2(y,x) itself.