Abstract
This paper presents a design principle for triply ambiguous objects. An orthographically projected picture of a rectangular object has exactly three groups of parallel edges. When this picture is placed on a horizontal plane and is seen in a slanted direction such that one group of parallel lines becomes vertical on the retina, we perceive a vertically compressed 3D object. This process gives three different interpretations of the picture, and thus creates triple ambiguity. We can strengthen the illusion by inserting real 3D objects that show the direction of gravity. An example of this object won the first prize at the Best Illusion of the Year Contest 2018.
