For a good reason my collection is called “Visual Phenomena & Illusions” – my →latest addition is a strong phenomenon, but no illusion. It demonstrates the “pop-out” phenomenon with a simple, yet strong example.
The pop-out there stems from differences in local orientation. Orientation gradients are also the main aspect of the neighboring figure (from Bach & Meigen 1998, read →here), where we wrote in the legend of figure 2:
‘Cirque Invisible’ Victoria Chaplin & Jean-Baptiste Thierrée (photo: F. Simon, Hamburg 1994, with permission). Three Gestalten (clown, zebra and suitcase) segregate from each other and from the background by boundaries that are defined by gradients in luminance, orientation and spatial frequency…
Try the pop-out phenomenon →here and enjoy.