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CHAPTER
7

Image Restoration

7.1 Image Degradations—The Real World

Anyone who has ever taken a photograph will understand that capturing an
image exactly as it appears in the real world is very difficult, if not impossible.
There is noise to contend with, which in the case of photography is caused by
the graininess of the emulsion, or the resolution and quantization of the image
sensor motion blur, focus problems, depth-of-field issues, and the imperfect
nature of even the best lens system. The result of all these degradations is that
the image (photograph) is an approximation of the scene.
Often the image is good enough for the purpose for which it was produced.
On the other hand, there are some instances where the correction of an image
by computer is the only way to obtain a usable picture. The original problems
with the Hubble Space Telescope are a case in point; the optics produced
images that did not approach the potential of the telescope, and a repair
mission was not immediately possible. Computers were used to repair some
of the distortion caused by the optics and give images that were of high quality.
Image restoration is the art and science of improving the quality of an image
based on some absolute measure. It usually involves some means of undoing
a distortion that has been introduced, such as motion blur or film graininess.
This can’t be done in any perfect way, but vast improvements are possible in
some circumstances.
The techniques of image restoration are very mathematical in nature, and
this may distress some people who are interested in the subject. The purpose
of this section is to provide insight, and so a very practical approach is taken.

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