At this stage, the reader might be asking the obvious question of why do we need to restore an image if the ideal original is available? In practice, a filter is used that has been designed on a representative training set. This means that examples similar to the image to be restored must be produced in some way. In the case of a fax machine, this is easy - a test image would simply be passed through the same process. The same is true for resolution changing and OCR examples discussed in the previous chapter. For old film restoration and other processes, it can be more difficult to recreate an ideal image for training. A section at the end of this chapter deals with this subject in more detail.
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