The images projected onto the retina are flat and two-dimensional. Your brain uses binocular cues (i.e., two eyes) to create the perception of a three-dimensional world.
Our two eyes provide slightly different views of the world, by virtue of the distance between them. This is referred to as retinal disparity. Your brain uses the extent of retinal disparity to predict the distance of objects.
Retinal disparity is greatest when objects are nearby, and decreases as objects move farther away. Thus our brain is able to predict the distance of objects.
Your brain also uses monocular depth cues (i.e., one eye) to estimate the distance of objects.
Motion parallax is the change in the position of an object on the retina due to movement of your body.
Your brain also examines the relative size of objects on the retina. If two objects are identical, the object with the smaller retinal size is thought to be farther away.
Your brain also uses texture gradients because details are more clearly visible on closer objects.
Linear perspective causes objects within converging lines to seem more distant.
