Hello!
Your spirograph page http://wordsmith.org/anu/java/spirograph.html is very
interesting.

I am trying to understand "moving circle radius" and "moving circle
offset." I accidentally set the moving circle radius to a negative number
and got an interesting picture, but I don't know what a negative radius
represents. I am guessing that "offset" is a way to place the pen away from
the edge of the rolling circle, but I don't know what it is measuring.

Thank you,
Paul Boisen