I bet you didn’t know the most famous plummer of all time, Mario has his own optical illusion? This cool illusion showcases the relative motion effect. In the animation below it appears as if Mario is jumping vertically and the shell is flying horizontally. But in reality, both are moving in circles!
So how does this visual optical illusion work? The first thing to note is that, although Mario and the shell are following a similar circular trajectory, they are in antiphase. Which is just a fancy way of saying that when our Italian plummer is at 12 o’clock, the shell is at 6 o’clock. The background image also follows a circular trajectory. But it goes clockwise, so the rotation of the background is opposite to that of Mario and the shell.
The crux of visual optical illusions like this one, is that the rotations of the three elements are aligned in a very specific way. The horizontal component of the background motion is the same as Mario’s: When the background moves to the right, so does Mario. Mario’s horizontal movement is therefore cancelled out, and consequently Mario appears to move only vertically. Conversely, the vertical component of the background motion is matched to that of the shell. Therefore, the shell appears to move only horizontally.
This illusion is a very salient demonstration of how we use background information to make judgements about positions and movement. We don’t perceive the actual movement of Mario and the shell, we perceive their movement relative to the background. For more amazing optical illusions check out 101 Optical Illusions.com.