The problem of estimating the true gray levels of a picture when noise is
present will be discussed in Section 7.5. Such estimates require some degree
of knowledge about the statistics of the picture and of the noise. If we know
that the nonnoisy picture contains a large region of constant gray level, we
can get some insight into the noise statistics by analyzing the gray level
fluctuations in the corresponding region of the noisy picture. For example, if
we assume the noise to have a Gaussian distribution with zero mean, its
standard deviation is just the standard deviation of these fluctuations. We
can also estimate the
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