Kanizsa Illusion
from Michael’s Visual Phenomena & Optical Illusions
The neighbouring picture displays a variation of the well-known “Kanizsa’s Triangle”: The blue “Pack-people” define by their cutout wedges an apparent triangle, which seems to hover above the background. If you place the mouse over it (or tap it), the apparent triangle is no longer aligned with the background texture, and the effect all but vanishes.
Kok P, Bains LJ, van Mourik T, Norris DG, de Lange FP (2016) Selective Activation of the Deep Layers of the Human Primary Visual Cortex by Top-Down Feedback. Current Biol
Cats see this illusion too:
Smith GE, Chouinard PA, Byosiere S-E (2021) If I fits I sits: A citizen science investigation into illusory contour susceptibility in domestic cats (Felis silvestris catus). Applied Animal Behaviour Science 240:105338