Feature Extraction & Image Processing, 2nd Edition

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 I am 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 bandwidth to carry all the

information sensed by the eye. Accordingly, there must be some preprocessing before the image is

transmitted down the optic nerve. The human vision system can be modelled in three parts:

•

the eye: this is a physical model since much of its function can be determined by pathology

•

a processing system: this is an experimental model since the function can be modelled, but

not determined precisely

•

analysis 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,

which is provided by the cornea (sclera). This is outside the choroid, which has blood vessels

Lens

Ciliary muscle

Choroid/sclera

Optic nerve

Fovea

Blind spot

Retina

Figure 1.3 Human eye

4 Feature Extraction and Image Processing