What is round and square and confusing all over? An "ambiguous cylinder"! The Ambiguous Cylinder Illusion, seen in this video, took second prize in the 2016 Illusion of the Year contest. The illusionist places several groups of cylinders in front of a mirror -- but are they round or square?
The answer depends on whether you're looking at the cylinders or at their reflection: one will appear to be square-shaped and the other will appear to be round. The illusionist, from Meiki University in Japan, then turns each group of cylinders around, reversing the appearance of the shape.
The key to the illusion is the way that the top of the cylinder is cut; instead of flat, the top is an uneven wave. This leads the eye into perceiving the entire cylinder (which is actually a rounded square shape, or "squircle") as either square or round, depending upon the angle your line of sight is taking. That's why the real cylinders appear to be one shape and their reflections appear to be another. Make Anything has posted a quick and dirty explanation video to their YouTube channel, as well as files for creating your own cylinders with a 3-D printer.
The annual Illusion of the Year contest, hosted by the Neural Correlate Society, is designed to encourage the creation of illusions that assist ophthalmologists and neurologists in learning how the brain perceives images. This knowledge is helpful in understanding and treating certain cognitive and perceptual disorders.
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