Xerox WorkCentre 480cx User Manual page 210

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The shape and slant of the tonal map represent brightness level ratios
between the input of the original image and the output image data. By
clicking on the tonal map line, a hand-shaped cursor will appear which can
be dragged to change the shape of the tonal map line.
The horizontal axis represents the original input brightness and the vertical
axis represents the enhanced brightness. Values on either scale range from 0
to 255.
As shown in the figure below, if the curve is changed into a convex shape
above the original diagonal, the enhanced image will be brighter than the
original.
Normal
Image
Lightened
Image
6-32 Scanning