Second place winner of the Best Illusion of the Year Contest 2016, this is the Ambiguous Cylinder Illusion by mathematics professor Kokichi Sugihara of Meiji University in Tokyo, Japan.

The direct views of the objects and their mirror images generate quite different interpretations of the 3D shapes. They look like vertical cylinders, but their sections appear to be different; in one view they appear to be rectangles, while in the other view they appear to be circles. We cannot correct our interpretations although we logically know that they come from the same objects. Even if the object is rotated in front of a viewer, it is difficult to understand the true shape of the object, and thus the illusion does not disappear.

Update and spoiler alert: Via Reddit, Make Anything tries to figure out the illusion by reverse engineering with a 3D printed shape.

And from the 2015 competition, Sugihara’s Ambiguous Garage Roof:

Sugihara’s work is well known for seeming impossible. Previously: his Impossible Motion Illusions and his Impossible Rooftop & Folding Ladder Illusions.