What you deem to be
RGB is actually drawn using
HSV. (Have a look
here, the upper right in the topmost image.) So you basically go through a bitmap pixel by pixel, determin the angle and distance to the bitmap's center pixel, and use these for hue and saturation respectively. Vaule is simply fixed at 1. It naturally becomes a circle, since distances > 1 from the center would result in invalid parameters for
HSV anyways.
"When one person suffers from a delusion, it is called insanity. When a million people suffer from a delusion, it is called religion." (Richard Dawkins)