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Chapter 3 ■ Digital Morphology 131

Because a grey-level opening will decrease the level of an object, the image
was opened with circular structuring elements of gradually increasing radius.
At some point, when the radius of the structuring element exceeds that of the
coin, the coin will be removed from the image. The radii actually used were
those from 5 to 14; opening by a circular structuring element of radius 14
actually removes all coins, leaving a dark and empty image.
The first change is at radius 6.5 (diameter 13), where the dimes are reduced
in level sufficiently that thresholding can delete them. An opening using a
radius of 8 removes the pennies, allowing them to be counted. Finally, an
opening using a radius of 10 removes the quarters, leaving only the loon.
By counting the regions that vanish after each iteration, it should be possible
to accumulate the total value of the coins in the image. In many countries
the paper money also varies in size, allowing bills to be classified by size,
as well.

3.5 Color Morphology

Color can be used in two ways. As before, we can assume that the existence
of three color components (red, green, and blue) is an extension of the idea of
a grey level, or each color can be thought of as a separate domain containing
new information. In either case, morphology is not commonly applied to color
images, possibly because the construction of the structuring elements necessary
to perform a particular task is really quite complex. Color morphology will
only be touched on here through the use of a single example.
Figure 3.33a is a grey-level version of a color image, which shows an insect
sitting on a leaf. Both the insect and the background are basically green, so
automatically locating the insect could be a little tricky. On close examination
of each of the three color basis images (red, green, and blue) it is observed that
there are slight variations in each component: the insect seems to be brighter
in the red and blue images, whereas the background is brighter in the green
image.
Closing the red and blue images should brighten the insect further, and
opening the green image should suppress the background a little. A circular
structuring element with a radius of four was used in each case. Following the
closings and opening the three component images were recombined to form a
single color image. The insect is now a bright pinkish color, and can be seen in
sharp contrast to the darker green background. Figure 3.33e is a grey version
of this color image, but the insect is still clearly present. Using this image as a
mask of the original will give an isolated picture of the insect, or at least most
of it, as seen in Figure 3.33f.