What to observe
The neighbouring image is immediately seen as a cube, wiggling a bit. It is not a normal cube, but one corner is missing (gray faces). If you work on it, you can see an alternative interpretation: there is a smaller gray 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
Once you have seen the effect, you can mentally flip it over. Interestingly, you can’t hold one interpretation for longer than, say, 10 s, another similarity to the Necker cube.
You may want to press the button ‘stop’ and use the sliders to rotate the cube (the angular control is currently suboptimal, though).
Without stereoscopic depth information and without clear illumination cues, all three interpretations (missing corner, daughter cube, or room corner) are geometrically possible.
Programmed with “Processing”.
Stong CL (1974) The Amateur Scientist – How to create visual illusions. Scientific American Nov, p. 126