Binocular vision
Most people see naturally through two eyes. The eyes are placed an average
of 63.5mm apart - depending on genetic factors. Two similar but different
viewpoints generate a small amount of parallax error where the depth of
the scene viewed is not equivalent to the point the eyes are converged at.
The computational power of the brain then takes over and facilitates
depth perception. There is a subgroup of the population who can only
see out of one eye due to anatomical or physiological problems and cannot
see stereoscopically.
Depth cues
Binocular parallax is a powerful depth cue in binocular individuals, but
it is by no means the only one used by the mind. A lot of depth cues
are available to one eye and anything that can be seen as a depth cue in a
standard photograph can be included in this category.
Motion parallax is a depth cue which can be used by the monocular individual
or the cinematographer: this occurs due to a smaller amount of motion in a
scene when an object is far away - for example in scenes from a moving train
window. Perspective and texture gradient allow depth perception by
objects getting smaller as the distance from the user increases. Fog
and haze increasing with distance also contribute to depth perception. Occlusion
of objects by other objects which are nearer to the viewer are powerful depth
cues that are available in normal scenes but are not applicable to radiographs.
Known objects also help with depth perception - where a object of known
size (say, an elephant) appears in the scene, one can often make a guess as
to how far away it is and compare it to other objects in the same scene. Shading
and lighting also plays an important role, especially if one knows where
the light source is or if it is included in the scene.
Benefits of stereoscopy
A well presented stereoscopic image is pleasing to look at. The depth
of structures is readily apparent and the appreciation of details not observed
before becomes evident. The combination of two views can provide more
useful information in a scene than a single view, or two views that are taken
from widely disparate viewpoints. Relative depth can be easily gauged
and with proper measurement apparatus (as in aerial photography or radiostereogrammetrical
analysis) absolute depth can be measured in a pair of images.
Problems in stereoscopy
A poorly presented stereoscopic image leaves no impression; or one of headaches,
eyestrain and nausea with the unfortunate viewer.
Most of the bad stereoscopic experiences (of which there are many) are
due to poor presentation. It is not uncommon to see images that are
presented with rotation or vertical parallax errors, and also not uncommon
to see images with the views flipped (pseudostereo). Geometry is
also important and if this is changed (eg. shifting from small screen to
large projection screen), eyestrain may be expected.
Accommodation-convergence mismatch is the problem that arises when the
projected image plane is in focus but the eyes are converging at a different
depth plane. Humans have a tolerance for some mismatch but when this
becomes too large, either the image defocusses or stereo fusion is lost.
This link between accommodation and convergence is hard-wired into
our nervous systems but can be overcome to an extent by practice. It
is also easier to compensate for viewing close images (converging) as most
people are not able to make their eyes diverge.
Ghosting is the presence of a small proportion of the image intended
for the other eye being visible. It is a problem for most of the computer
based viewing systems where the image pathways are not kept physically
separated.
Where high contrast images are used (such as when looking at metallic
implants on a radiograph) ghosting becomes a significant problem.
The solution has yet to be devised for computer viewers, but using
a purely optical viewing system should not give any ghosting.