shape  constancy definition
When it is closed and you stand in front of it the image on your retina is approximately rectangular. But when you open the door it will become trapezoidal on your retina. You can get an idea of how rectangular shapes are projected on your retina by looking at the shape constancy demo. This phenomenon of your perceiving the real shapes of objects regardless of their retinal projections is called shape constancy. It is one of the several perceptual constancy that one encounters.