Introduction - Feature Extraction and Image Processing - page 6

images. These are illustrated in Figure 1.4 and are the darker bands that appear to be where
two stripes of constant shade join. By assigning values to the image brightness levels, the
cross-section of plotted brightness is shown in Figure 1.4 (a). This shows that the picture is
formed from stripes of constant brightness. Human vision perceives an image for which
the cross-section is as plotted in Figure 1.4 (c). These Mach bands do not really exist, but
are introduced by your eye. The bands arise from overshoot in the eyes' response at
boundaries of regions of different intensity (this aids us to differentiate between objects in
our field of view). The real cross-section is illustrated in Figure 1.4 (b). Note also that a
human eye can distinguish only relatively few grey levels. It actually has a capability to
discriminate between 32 levels (equivalent to five bits) whereas the image of Figure 1.4 (a)
could have many more brightness levels. This is why your perception finds it more difficult
to discriminate between the low intensity bands on the left of Figure 1.4 (a). (Note that that
Mach bands cannot be seen in the earlier image of circles, Figure 1.2 (a), due to the
arrangement of grey levels.) This is the limit of our studies of the first level of human
vision; for those who are interested, Cornsweet (1970) provides many more details concerning
visual perception.

(a) Image showing the Mach band effect

200

200

seen x 100

mach 0,x 100

0 50 100 0 50 100

x
(b) Cross-section through (a)

x
(c) Perceived cross-section through (a)

Figure 1.4 Illustrating the Mach band effect

So we have already identified two properties associated with the eye that it would be
difficult to include, and would often be unwanted, in a computer vision system: Mach