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 (use the No ticks setting in the pop-up menu preset to Default.
Akiyoshi Kitaoka classifies the phenomenon as a “geometrical illusion with 3D impression”, I couldn’t agree more.
The present demonstration indeed was inspired by his “Bulge”.
You can use the pop-up menu, the colour pickers, or the #checks stepper to try out various combinations.
First, you might like to flip through the presets (below default) using the pop-up menu at the 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… But it’s probably the same mechanism at play as in “Skye’s Oblique Grating".
