Oh, I see.
It would definitely be hard, if not impossible, to always pull the ball. The best example is, “what would happen when the ball changes direction?”. The arm can’t flip sides without moving the ball.
So each pattern needs to have one direction for the elbow. Either on the “clockwise” or the “counter clockwise” side.
I have not thought about this before, but I think having the elbow on the side it is makes sense, because most sandify patterns accidentally rotate counter clockwise.
But watching that video, it is clear that the star is rotating clockwise. Which makes me wonder if we have a mistake and it is flipping the pattern. It may not matter much for a design centered in the middle, but if it was text, would it be flipped? If we fix that, we can fix the push/pull.
This are the equations to go from theta, radius to the two unscaled motors:
m1 = float(theta) + math.acos(float(rho))
m2 = float(theta) - math.acos(float(rho))
You can change this equation to reverse the angle (change float(theta) to -float(theta)). This could also be achieved in the electronics, be swapping the motors (plugging motor X into port Y) and reversing each motor (by reversing the stepper plug).