Diamonds or squares?

from Michael’s   Visual Phenomena & Optical Illusions


What to see

On the right is an arrangement of sharp grey diamonds or are these blurred concentric squares?

If you are watching this with AutoRun it will slowly change from one percept to the other. When the amount of blur is minimal (Blur slider at bottom) the diamonds dominate, with increasing blur the blurry squares will dominate.


This phenomenon was constructed by Arthur Shapiro and published in his 2021 paper (here re-imagined with kind permission). It rests on the fact that blur acts as a high spatial frequency filter – a fancy way to say that fine / sharp structures are reduced, but slowly changing features are little affected. With a lot of blur, the edges defining the diamonds are no longer visible and only the blurry squares remain to be seen. Less clear is the other extreme: Zero blur reveals the diamond edges, ok, but the blurry squares are still in the image; so why are they no longer seen? Because they are “masked”: the high-contrast diamond edges carry higher signal energy and swamp the lower spatial frequencies.

Thus this phenomenon is rather well understood: No shortcomings of our visual system, but a change of actual content in the image, if arranged somewhat strikingly.

Masking is seen at work also in the Grid Masking demo. Blur revealing different shapes due to their different spatial frequency range subserve also these Blur Illusions, Blur & figure-ground reversal, the “Ghostly Gaze”, Dr. Angry and Mr. Smile, and Face in blocks.


Shapiro AG (2021) Hybrid motion illusions as examples of perceptual conflict