Parallax

Parallax is the apparent shift in the position of an object when viewed from two different lines of sight.

Try the following: hold your thumb up in front of your face and close one eye. Now open that eye and close the other one. Your thumb will appear to "jump" back and forth against the background. This perceived movement is parallax in action. The closer an object is, the more it appears to move against distant objects when your perspective changes.

Parallax in a 3D Context

In both real life and in created 3D worlds (like video games, CGI in movies, or VR), parallax is a fundamental principle for creating a sense of depth and dimension.

There are two main types of parallax used for 3D:

Motion Parallax

Motion Parallax is one of the most important monoscopic (only requires one eye to perceive it) depth cues (what helps your brain judge distance and dimensionality). Objects at different distances move at different apparent speeds: while close objects appear to move very fast across your field of vision, distant objects (like mountains or the moon) appear to move very slowly or not at all.

The brain interprets this difference in relative speed as depth. So even with one eye, you are able to differentiate which objects are closer and which are farther.

Binocular Parallax (Stereopsis)

This is the effect created by having two eyes. Because your eyes are spaced apart (about 2.5 inches or 6.5 cm), each one sees a slightly different view. For a detailed explanation see our entry in binocular vision and stereopsis.