What to see
Above you see –most probably– an apparently random array of blocks. Blurring the image reveals an unexpected message of doubtful soundness.
There are several ways of blurring: You can squint your eyes, move away from the screen, or use the sliders “Size” or “Blur” or both of the latter two. The most famous version is on page 2 (it’s not true, BTW :), and the effect has been turned into an advert for glasses (page 3)
Comment
The strong spatial low-pass filtering removes the 3-dimensional masking cues, enabling a re-evaluation of figure and ground.
Sources
I once saw this when I visited Spillmann’s lab, the animation above is based on an image I found as “eye test”. The original source seems unknown. Please step forward so I can acknowledge!