Just a note as per Mateo_SanCarlos, Gimble lock! A long time ago some coder forgot that tan r = a/b is undefined at b = 0; as you can probably see if you have the math this could potentially happen at 0,90,180,270 degrees. (in my own code I think to make b = to 0.1 if b equals zero); you could adjust your rotation angles accordingly if it is a problem.