2.2.2 Depth Perception

depth perception

Depth perception is based on 10 cues. These cues contain information which, when added to the 2-D image projected onto the retina, allow us to relate the objects of the image to 3-D space.

There are four physiological and six psychological cues.

The four physiological cues are:

1. accommodation, the adjustment of the focal length of the lens;
2. convergence, the angle made by the two viewing axes of a pair of eyes;
3. binocular disparity, the disparity between images of the same object projected onto the retinas; and
4. motion parallax, the result of changing positions of an object in space due to either the motion of the object, or of the viewer's head.

Accommodation & Convergence

accomodation

Accommodation and convergence are associated with the eye muscles, and interact with each other in depth perception. Accommodation is considered a monocular depth cue since it is available even when we see with a single eye. This cue is effective only when combined with other binocular cues, and for a viewing distance of less than two metres (Okoshi, 1976). Accommodation and convergence are considered to be minor cues in depth perception.

convergence

Binocular Disparity

binocular disparity

Binocular disparity is considered the most important depth perception cue over medium viewing distances. Binocular parallax is the difference between the images of the same object projected onto each retina. The degree of disparity between the two images depends on the parallactic (convergence) angle. This is the angle formed by the optical axes of each eye converging on an object. The parallactic angle is related to the distance of an object from the eyes. At great distances the parallactic angle decreases and depth perception becomes increasingly difficult. The smallest parallactic angle the average person is able to discern, is three arc seconds.

Visual Motion Parallax

motion parallax

Visual motion parallax is a function of the rate at which the image of an object moves across the retina. Distant objects will appear slow in comparison with close objects even when the two are moving at the same speed. Motion parallax can also be caused by the movement of the viewer's head. Objects closest to the observer will appear to move faster than those further away. This is an important cue to those who only have the use of one eye.

The Six Psychological Cues

1. retinal image size, the larger an object image the closer it appears;

retinal image size

2. linear perspective, the gradual reduction of image size as distance from the object increases;

linear perspective

3. aerial perspective, the haziness of distant objects;

aerial perspective

4. overlapping, the effect where continuous outlines appear closer to the observer;

overlapping

5. shade and shadows, the impression of convexity or concavity based on the fact that most illumination is from above; and

shade and shadow

6. texture gradient, a kind of linear perspective describing levels of roughness of a uniform material as it recedes into the distance.

Texture gradient

Psychological cues are learned cues, therefore, they are assisted by experience. When combined, these cues enhance depth perception greatly.

six psychological cues combined

In remote sensing, several cues are used to represent depths of terrain. Perspective views of remotely sensed data, created with digital elevation models (DEM) and orthorectified images, provide an important source of 3-D visualization. When stereo viewing aerial photographs and other visible light imagery, such as that supplied by the Landsat, SPOT (Système pour l'observation de la Terre) or other satellites, binocular disparity is the most important cue.

Stereo viewing of radar imagery combines binocular disparity and shade and shadow cues for effective depth perception.