Introduction - Feature Extraction and Image Processing - page 4

useful when developing computer vision techniques, whereas others are actually undesirable
in a computer vision system. But we shall see computer vision techniques which can to
some extent replicate, and in some cases even improve upon, the human vision system.
You might ponder this, so put one of the fingers from each of your hands in front of your
face and try to estimate the distance between them. This is difficult, and we are sure you
would agree that your measurement would not be very accurate. Now put your fingers very
close together. You can still tell that they are apart even when the distance between them
is tiny. So human vision can distinguish relative distance well, but is poor for absolute
distance. Computer vision is the other way around: it is good for estimating absolute
difference, but with relatively poor resolution for relative difference. The number of pixels
in the image imposes the accuracy of the computer vision system, but that does not come
until the next chapter. Let us start at the beginning, by seeing how the human vision system
works.
In human vision, the sensing element is the eye from which images are transmitted via
the optic nerve to the brain, for further processing. The optic nerve has insufficient capacity
to carry all the information sensed by the eye. Accordingly, there must be some pre-
processing before the image is transmitted down the optic nerve. The human vision system
can be modelled in three parts:

1. the eye − this is a physical model since much of its function can be determined by

pathology;

2. the neural system − this is an experimental model since the function can be modelled,

but not determined precisely;

3. processing by the brain − this is a psychological model since we cannot access or

model such processing directly, but only determine behaviour by experiment and
inference.

1.3.1 The eye

The function of the eye is to form an image; a cross-section of the eye is illustrated in
Figure 1.3 . Vision requires an ability to focus selectively on objects of interest. This is
achieved by the ciliary muscles that hold the lens . In old age, it is these muscles which
become slack and the eye loses its ability to focus at short distance. The iris , or pupil, is
like an aperture on a camera and controls the amount of light entering the eye. It is a
delicate system and needs protection, this is provided by the cornea (sclera). The choroid
has blood vessels that supply nutrition and is opaque to cut down the amount of light. The
retina is on the inside of the eye, which is where light falls to form an image. By this
system, muscles rotate the eye, and shape the lens, to form an image on the fovea (focal
point) where the majority of sensors are situated. The blind spot is where the optic nerve
starts; there are no sensors there.
Focusing involves shaping the lens, rather than positioning it as in a camera. The lens
is shaped to refract close images greatly, and distant objects little, essentially by 'stretching'
it. The distance of the focal centre of the lens varies from approximately 14 mm to around
17 mm depending on the lens shape. This implies that a world scene is translated into an
area of about 2 mm 2 . Good vision has high acuity (sharpness), which implies that there
must be very many sensors in the area where the image is formed.
There are actually nearly 100 million sensors dispersed around the retina. Light falls on