Depth Perception, Continued
Parallax: As we move, we notice that the relative position of nearby objects changes more than far objects. In the photos below, as a virtual camera moves from left to right across a virtual three-dimensional scene, you can observe that objects that are closer appear to move (right to left) more than far objects.
Unlike other monocular depth and distance cues, parallax is only observed over time, in moving images. Of course, movies and video are moving images, and as a result, parallax will be observed.
Texture gradient: On surfaces that have a regular pattern, we can judge distance based on the spacing of the pattern. We notice that the spacing of the pattern is wider and the features are larger on the area that is closer. In the photo below, the pattern formed by the paving stones helps us determine the relative distance of the people and objects that we see. Both the density of the pattern and the perspective that the pattern provides help us sense distance.
Air quality: Far-away objects are sometimes obscured by haze or fog, while nearby objects are not.
Accommodation (Focus), and Convergence: When we look at objects that are close to us in the real world, our eyes do two things necessary to see the object clearly. First, our eyes converge inward, so that each eye is aimed at the spot that we want to focus on. Second, to adjust the focus of the lens in our eyes, our eye muscles adjust the shape of our eyes in a process called accommodation. The feedback that your eye muscles give your brain as you focus on different objects gives your brain some idea of the distance to each object that you see.
All of these cues provide depth information, even when we view a scene with only one eye. They also help us sense depth when we view standard two-dimensional images. Artists and filmmakers have understood these important visual cues, and have exploited them to add a feeling of realism and depth to paintings, photos, and movies for many years.
Of course, a 2D movie is a flat 2D rendering of a 3D scene. When you watch a 2D movie, your eyes focus on the screen, and they stay focused on the screen (which remains at the same distance) throughout the movie. You don’t need two eyes to perceive depth, but you do need two eyes to see 3D.
3D movies replicate the images that your eyes would see if you were standing where the 3D camera was when the movie was filmed. Objects and scenery appear to be at different distances, and if all goes according to plan, the audience perceives that they are “on location."