Stare at this figure until a white subjective triangle appears. Can you see it? It is partially covering three circles. The circles look like PACMAN shapes don’t they?
Use the slider bar to adjust the amount of circle each corner of the triangle covers.
By changing the shape of the PACMAN mouths, you also change the shape of the subjective triangle. Can you see the shape change? You are adjusting the shape of a triangle that doesn’t exist!