Lengths of short bars: a
Lengths of long bars: b
Pink, green bars and a yellow bar create a parallelogram mechanism.
Pink, violet bars and a yellow bar create an antiparallelogram mechanism.
Orange pen draws orange circle of radius a.
If the pink crank rotates regularly, the pen moves irregularly on the circle.
Circle center is on the line that connects centers of the fixed pivots.
Distance between fixed pivot center of the pink bar and the circle center is 2b.
This avoids the need to arrange a pivot at the circle center.
The mechanism has unstable positions when all bars (except the pink one) are in line.