Size constancy occurs because we know the sizes that characterize familiar objects. Cars are bigger than people, buildings are bigger than cars, for example. So when we see a car and it is far away, the image of that car takes up less space on our retinas. As the car comes closer, its image takes up more space on our retinas, and we say it is getting closer. As noted earlier, when we view new or unfamiliar objects, we may have some trouble perceiving their distance from us.
Shape constancy refers to our knowledge about the shapes of objects. Again, we know that most objects do not change their shape, at least not very quickly. Perhaps that is why we are so fascinated with the recent computer graphic technique of morphing. Through that technique, objects (on film or computer monitor) can be made to appear to change their shape quickly. The last "Terminator" movie made heavy use of morphing. However, in the real world, objects tend to retain their shapes, and we know that.
Color constancy is a product of the difference between the action of the cones cells and the rod cells. Remember that the cones are responsible for color vision, and the rods for black-and- white-vision. When we are in a dark enough setting, we cannot see color because our cone cells are not getting enough light to work. They are below their threshold. But, we perceive the colors anyway. Why? Because we know what the colors should be.
So the constancies provide firm evidence that perception and sensation are not the same. Perception is more than the simple reporting of sensations. Perception is profoundly altered by our experience.