When a fuzzy fax or faded photocopy is received, it is usually possible to figure out what it says although the text may be badly damaged or corrupted by noise. This is because the human brain has knowledge of the characters and fonts of the alphabet and is able to fill in the gaps (or ignore noise) using experience. Over time, the brain has learned roughly what to expect and can correct it. The image in Fig. 2.1 is a text document that has been corrupted with 10% additive noise. This type of corruption is called salt-and-pepper noise.
It is clear that the document could be typed out again to reproduce the original version precisely. Therefore it is possible to restore the document fully, using intelligent human intervention. It might be more difficult to do this for unfamiliar alphabets such as Chinese or Arabic if the person had no previous knowledge of the shapes of the characters.
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