“Gestalt Theory can be used to explain the illusory contours in the Kanizsa Triangle (in this case square). A floating white triangle (square/ diamond), which does not exist, is seen. The brain has a need to see familiar simple objects and has a tendency to create a "whole" image from individual elements.” Brain can derive conclusion because of earlier experience.
“In the Ponzo illusion the converging parallel lines tell the brain that the image higher in the visual field is farther away therefore the brain perceives the image to be larger, although the two images hitting the retina are the same size. The Optical illusion seen in a diorama/falls also exploits assumptions based on monocular cues of depth perception.”
