In the neighbouring checkerboard you can see a central “bulge”. This bulge is illusionary: the checkerboard is fully regular and each check is square, as becomes obvious when the small ticks are not present.
Akiyoshi Kitaoka classifies the phenomenon as a “geometrical illusion with 3D impression”, I could not agree more, and the present demonstration indeed was inspired by his “Bulge” and also by this cute movie.
What to do
You can use the sliders or the colour pickers to try out various combination. This also stops the automatic slider change.
First you might like to flip through the presets (below “default”) using the pop-up menu at top right. The “no ticks” setting shows the raw checkerboard. “Low contrast” does not impair the 3D impression. Interestingly, with the right relative lightness, settings one can change the bulge into a “pinch”.
How does it work? I do not know…