Which way is the cube oriented? Is it facing right-and-down or left-and-up? (If you are not sure what we mean, change the “lighting bias” setting.) If you would keep looking at it, you will eventually “switch” and see it in an alternative orientation. After some time, it will “switch” again, back to the original. And it will keep switching between these alternative for as long as you care to look at it.

What is going on?

The wireframe figure you see, does not tell you which “side” of the cube is in front (try rotating it using sliders). Because both alternatives are equally plausable, they compete in your perceptual system but are able to gain an upper hand only for some time. You can “help” on of the alternatives by concentrating on it and trying to ignore the other possibility. Or by fixating at different vertices. Or by using “Lighting bias” slider, which adds opacity cues to tell you visual system which edges are “behind”.

If you are looking at Necker cube together with someone else, try telling what you see and when you switch. You will discover that sometimes you see it differently and that you are switching between alternatives at different times as well.

Necker cube was accidentaly discovered in 1832 by Swiss crystallographer Louis Albert Necker. He observed that crystals in published figures and engraved plates would sometimes suddenly and involuntary change their apparent position (orientation). He also found that he could (to a certain extent) control his perception by focusing on a particular vertex of the cube.

The switching phenomenon you experience is called multi-stable perception. For more example like this, take a look Shape-from-shading and Kinetic-depth effect displays.

References