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6 Enhancement
High-emphasis filtering in conjunction with a logarithmic transformation
of the gray scale can sometimes be used to reduce shading effects, due to
varying illumination, in a picture without degrading the detail in the picture.
As indicated at the end of Section 6.2.2, a simplified model for the picture
gray level is /(x, y) = I(x, y)R(x, y\ where / and R are illumination and
reflectivity, and where / is primarily composed of low spatial frequencies.
We cannot reduce the effects of / on / by simply boosting the high frequencies,
since / and R are combined multiplicatively, rather than additively. How-
ever, if we take logs o ...