Stereoscopic Vision
Stereoscopic Vision
Stereoscopic Vision
Persons with normal vision are said to have binocular vision. They can view with both eyes simultaneously. These
persons are said to have stereoscopic viewing
The term monocular vision is applied to viewing with one eye only.
Depth perception
Monoscopic viewing provides only rough depth impression which is based on the following clues:
o
Hidden objects
Shadows
The human eye functions in a similar manner to a camera. The lens of the eye is biconvex in shape and is composed of
refractive transparent medium. The separation between eyes is fixed (called eye base). Therefore, in order to satisfy the
lens formula for varying object distance, the focal length of eye lens changes. For example, when a distant object is
seen, the lens muscle relax, causing the spherical surface of the lens to become flatter. This increases the focal length
to satisfy the lens formula and accommodate the long object distance. When close objects are viewed, a reverse
phenomenon happens. The eye's ability to focus varying object distance is called accommodation.
The figure shows two situations where eyes separated by eye base (b) are focussing on two objects A and B located at
two distances dA and dB respectively.
In binocular vision, there is convergence of axes of eyes when focusing at point A and B. The corresponding angles are
1 and 2. Angle 1 tells the mind that object A is distance d A. Similarly for point B. These angle are
called parallacticangles for points. The nearer the object, the greater the parallactic angle and vice versa. Difference
( 1 - 2 ) tells the mind that the distance (depth) between two points is e = d B - d A.
For average separation of eye and distinct vision of about 10 inch, the limiting upper value of 16. Lower limiting
value of ranges from 10 to 20 seconds and represents a distance of about 1700 to 1500 ft for average eye separation.
It is called stereoscopic acuity of the person.
Stereoscope
If two photographs taken from two exposure stations L1 and L2 are laid on table so that left photograph is
seen by left eye and the right photo is seen with right eye, a 3D-model is obtained. However, viewing in this
arrangement is quite difficult due to following reasons:
There is disparity in viewing. The eyes are focused at short range on photos lying on table where
as the brain perceives parallactic angles which tends to form the stereoscopic model at some depth
below the table.
By using stereoscope, these problems can be alleviated. Different types of stereoscopes are available for
different purposes - from pocket (inexpensive) for viewing small area to mirror (expensive) for viewing larger
area.
1.
The two images must be obtained from two different positions of the camera.
2.
The left photo and right photo should be presented to the left and right eye in the same order.
3.
Photographs of the object should be obtained from nearly the same distance, i.e. scales of the two adjacent
photographs should be nearly same. Normal eye can accommodate 5 to 15% variations in scale of
photograph.
4.
During stereovision, keep the area under vision in near vertical epipolar planes as far as possible. The plane
containing optical centers for eyes (S1 S2), object point A and corresponding images a and a' on left and
right photographs respectively is called the epipolar plane. Thus camera axes should be approximately in
one plane, though the eyes can accommodate the differences to a limited degree. The term epipolar planes
is used if it refers to eyes; the term basal planes is used if it refers to cameras.
5.
For a good stereovision, the pair of photos should be in the same orientation with each other as they were at
the time of photography.
6.
7.
The ratio B/H should be appropriate. If this ratio is too small say smaller than 0.02, we can obtain the fusion
of two pictures, but the depth impression will not be stronger than if only one photograph was used. The
ideal value of B/H is not known, but is probably not far from 0.25. In photogrammetry values up to 2.0 are
used.