What to observe
The animation above is immediately seen as a (blueish) cube, wiggling a bit. It is not a normal cube, but one corner is missing. If you work on it, you can see an alternative interpretation: there is a smaller cube attached to the corner, in front of the larger cube – and it rotates inversely to the large cube! There is a third alternative view: imagine you’re looking at a room corner, and a cube is placed in that corner.
It may take a while.
What to do
Press ‘Reset’. That will rotate the cube to a default and switch off the wiggle. Once you have seen the “switchover” effect, you can mentally flip cause it to occur. Interestingly, you can’t hold one interpretation for longer than, say, 10 s, another similarity to the Necker cube.
You also may want use the sliders to rotate the cube.
When you tick the ‘Perspective’ checkbox, parts farther away look smaller. Still you can switch between the three possible interpretations, though the cubes distort a little when mentally interpreted in reversed depth.
Without stereoscopic depth information and without clear illumination cues, all three interpretations (missing corner, daughter cube, or room corner) are geometrically possible.
Stong CL (1974) The Amateur Scientist – How to create visual illusions. Scientific American Nov, p. 126