Rotary encoders are electromechanical transducers that convert angular displacement into corresponding electrical signals ...

An illusion is when we see and perceive an object that doesn't match the sensory input that reaches our eyes. In the case of the image below, the sensory input is four Pac Man–like black figures.

While public nuisance is one reason, messed up rooms, and living rooms can be another. So if you want to have something which hides out all this in a subtle way, then background blur is the way to go.