defined as physiological sense of sight by which the form, color, size,
movements, and distance of objects are perceived.
*"During the process of seeing the eye has to fulfill
two main tasks. First the eye has to recognize details of a scene, which
means it has to perceive the spacial resolution of the picture. The
second task is to recognize changes in a scene, in other words, to
perceive a temporal resolution of a scene.
The term 'seeing,' as such, actually only describes
the idea that light reflected by the objects surrounding us enters our
eyes. The eye itself contains several parts that process reflected light
and generate the image that our brain understands. When light has
entered our eye, it passes through the cornea, the iris, the pupil and
finally the lens. All these parts work together to put a focused image
onto the back of the eye which is called the retina. Once on the retina,
the image can be recognized and processed by the brain. To process the
image information in the brain the retina is equipped with
photoreceptors, which are stimulated differently.
There are two different kinds of photoreceptors: rods
and cones. (These names are based on their actual shapes). It was found
that with the rods, we are able to see black and white; while the cones
give us the ability to distinguish between different colors. There are
different kind of cones, which are especially sensitive for red, green
and blue color. If light is reflected on a high number of cones, the
cones then enable us to get a high spacial resolution of the image since
small changes in the color can be recognized. Rods are more sensitive to
the intensity of light itself. An important aspect of the rods and cones
in the context of digital video is their number and their distribution
on the retina. If we look for an example on the center of the retina we
will only find cones. Areas further away from the center have a much
higher distribution of rods. This is the reason why we have to look
directly at some image to get all the details. (... )
In total, we have about one hundred and twenty
million rods and only around eight million cones on the retina. The
latter, as stated, are distributed close to the center of the retina.
This leads to the fact that the eye is, in general, relatively less
sensitive to color especially to color changes. Video compression
techniques, like the one used in MPEG-2 Video, therefore utilize this
low-color sensitivity by reducing the color information per image.
MPEG-2 uses Discrete Cosine Transformation to identify and subsequently
remove high frequency changes in color."
you can find more or less information
from *"ATM & MPEG-2, INTEGRATING DIGITAL VIDEO INTO BROADBAND NETWORKS"
by Michael Orzessek and Peter Sommer