The problem is, of course, not well defined, as there is no objective measure for image quality. Apart from geometrical transformations some preliminary greylevel adjustments may be indicated, to take into account imperfections in the acquisition system. This can be done pixel by pixel, calibrating with the output of an image with constant brightness. Frequently space-invariant greyvalue transformations are also done for contrast stretching, range compression, etc. The critical distribution is the relative frequency of each greyvalue, the greyvalue histogram .
All examples assume point processing , viz.
Therefore, a design goal for image enhancement often is to smooth images in more uniform regions, but to preserve edges. On the other hand, it has also been shown that somehow degraded images with enhancement of certain features, e.g. edges, can simplify image interpretation both for a human observer and for machine recognition. A second design goal, therefore, is image sharpening. All these operations need neighbourhood processing , viz. Smoothing and Sharpening .