Figure-Ground Perception

We have a tendency to simplify a scene into a main object (figure) and then, everything else is used to form the background.
For example, you might see a maple leaf on this flag.. If you take another look you'll see two faces with pointy noses arguing. We simplify the flag to see a red leaf and therefore, the white is just a background for the image.

http://en.wikipedia.org/wiki/Figure%E2%80%93ground_%28perception%29
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Shape Constancy

Shape constancy is our ability to perceive an object as its original shape, even when it is positioned at different angles.

Even though these tables are tilted at different angles, we constantly perceive them to be the same original shapes.

opticalillusion.net

Brightness Constancy

Color remains the same no matter how it looks under different colors of light. If we didn't have this sense of color constancy, we would think that everything was always changing shades of color.

The image below shows the same flower shown in different brightness levels. Observe how the color appears to change. 

colorconstancy.com

Size Constancy

The perceptions of the size of objects is constant even though the size of the object on the retina varies with distance.
Everyone in the left photograph seems to be the same size right?

Take a look at the photograph on the right. The man in the white shirt was copied and pasted next to the woman on the left to show how the size varies with distance.

http://psych.hanover.edu/Krantz/sizeconstancy/page2.html

proximity

We tend to group nearby objects together. In this sweater, the holes are close together, so we group them together into lines that form shapes.

similarity

We group figures together that are similar. In this picture, we see columns of x's and columns of o's instead of rows of x's and o's because we group similar letters together.

X   O   X   O   X   O

X   O   X   O   X   O

X   O   X   O   X   O

continuity

We see smooth and continuous patterns instead of discontinuous parts. An example of this is a recycling symbol made of disconnected arrows because our minds see it as a complete circle.