Have you ever noticed, for example, when you approach the dinner table that the shapes of the plates do not change? When you look at them from some distance away from the table, the shapes of the round plates are elliptical on your retina. The only time that the image of a round plate is approximately round on your retina is when you look at it straight on. If you are not sure what I mean, look at the dinner plate demo.
The same situation occurs with a rectangular door. 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.
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