I wrote this during lunch to kill some time before a conference call. I liked the effect on the Moiré Pattern page and thought I would replicate a variation of it.
Controls
Left/Right Arrows or Mouse Buttons: Reverse direction or perform a step in that direction when in Step Mode.
Space or Middle Mouse Buttons: Pause and enter Step mode
Note that after the circles pass each other, the left/right controls may seem counter-intuitive.