Chapter 2 Principles of Stereoscopic Depth Perception and ...

2.2. Human depth perception
provide the same information to the visual system as spatially separated
views (Rogers & Graham 1982).
2.2.2 Accommodation and vergence
In normal vision, the oculomotor mechanisms (accommodation and vergence)
work in concert with stereopsis. Thus, when viewing an object the
eyes converge on it, so that the disparities providing depth information
about the object and its environment fall within Panum’s fusional area.
The eye’s lens automatically focuses (accommodation) on the currently
fixated object, making it stand out against its surroundings. Thus, double
images in front or behind the plane of fixation tend to be out of focus
and will ’disappear’ in increasing optical blur. Accommodation and vergence
operate in a closely coupled fashion, which implies that under natural
viewing conditions all objects which are in focus are also fixated upon,
thus bringing them within the limits of fusion. However, most of the currently
available stereoscopic display techniques do not support the linkage
between accommodation and vergence, forcing the observer to focus at a
fixed distance (i.e., on the screen plane where the image is sharpest) irrespective
of the fixation distance (i.e., where the object is located in depth
according to the disparity information). This de-coupling of accommodation
and vergence has been suggested as a potential cause of visual strain.
This topic will be addressed in more detail in Chapter 4.
2.2.3 Individual differences
The mechanism that underlies stereopsis is extremely precise, but also
somewhat fragile. Visual disorders in early childhood, even if only temporary,
may result in stereoblindness, which is estimated to affect 5-10% of the
population. Richards (1970) did a survey of 150 members of the student
community at MIT and found that 4 % of the population was unable to
use the depth cue offered by disparity and 10% had great difficulty deciding
the direction of a hidden Julesz figure (i.e., a random-dot stereogram)
relative to the background. More recent studies have shown that the performance
of observers on tests of stereoanomaly depends to a large extent
on the duration the observer is allowed to look at the test figures. The proportion
of the population unable to perform a depth discrimination task
decreases with increasing display duration (Patterson & Fox 1984, Tam &
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