These are the top three winners in the 2021 Best Illusion of the Year contest, orchestrated by the Neural Correlate Society—a nonprofit that "promotes scientific research into the neural correlates of perception and cognition."
"How we see the outside world―our perception―is generated indirectly by brain mechanisms, and so all perception is illusory to some extent," they explain. "The study of illusions is critical to how we understand sensory perception, and many ophthalmic and neurological diseases."
Above, the 2021 First Prize Winner, "The Phantom Queen" by Matt Pritchard:
The main illusion in this video shows a chess board and its reflection in a mirror. However, there's a phantom White Queen piece that only appears as a reflection, leaving a mysterious empty square in the foreground. The illusion is achieved by creating a camouflaged invisibility cloak that shields the Queen from one viewing angle. The shape and pattern of this shield also disguise its presence when viewed from a second angle that comes from the mirror's reflection. The video also shows other applications of this anamorphic camouflage to create a remarkable magical appearance and an invisible cube.
"The Changing Room Illusion" (Second Prize) by Michael A. Cohen:
The Changing Room Illusion is an example of "graduate change blindness," a phenomenon in which observers are unable to notice changes to the world around them when those changes occur gradually. In virtually all prior cases, gradual change blindness is studied by changing individual objects (e.g., a chimney disappearing or a facial expression shifting). While trying to prepare a novel example of this phenomenon for students, I realized that I could change dozens of items change without observers noticing. Overall, this illusion highlights how people may actually perceive and remember far more of the world around them than they intuitively realize.
"The Double Ring Illusion" (Third Prize) by Dawei Bai & Brent Strickland:
When two bistable rings are presented separately, they appear to move in unstable 360° rotations. However, if the same rings partially overlap, they now appear to move in stable 180° rotations, bouncing back and forth – as if they avoid passing through each other. Remarkably, when one ring is holed, in a way that the other ring can pass through the holes, the unstable 360° percept is restored. In all three cases, the rings undergo the exact same motion, but our visual system interprets the stimuli differently depending on whether the rings can traverse each other.