Hypotrochoid

A circle of radius r rolls inside a fixed circle of radius R, carrying a pen at distance d from its centre. Brightness encodes distance from the origin: bright at the centre, darker toward the rim.

x(t) = (R − r)·cos t + d·cos((R − r)/r · t)
y(t) = (R − r)·sin t − d·sin((R − r)/r · t)