Shape
Tessellation is a special case of the  face/vase illusion. It involves dividing a module into shapes that begin to have an ambiguous figure/ground relationship.  These modules are then assembled into a grid structure so that when the portions of the module that were originally perceived as negative space are arranged side by side, they complete a shape that can now be perceived as a positive shape. The artist M. C. Escher used tessellation to create illusions that reveal the contradictions between what makes visual sense and what makes logical sense.