Xerox WorkCentre 480cx User Manual page 205

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The Midtone point represents the Gamma setting for your particular screen.
On the histogram, the Shadow and Highlight scales are divided into 255
units.
Therefore, all pixels in the original that are darker than the current shadow
point are now completely black in the enhanced image. Conversely, all
pixels in the original that are brighter than the current highlight point are
now completely white in the processed image.
Setting the highlight point to 200 renders all pixels greater than or equal to
200, on the original, completely white in the processed image. Similarly
setting the Shadow point to 200 renders all points less than or equal to 200,
on the original, completely black in the enhanced image. Photographers
often refer to these effects as "high-key" or "low-key". High-key is an
image pushed to the white end, and low-key is an image pushed to the black
end of the tone curve.
The Midtone is a relative measure of the gamma of your monitor. The scale
for Midtone ranges from 0 to 255. The gamma curve is on a geometric
series. This means that at a value of 255 it tends to infinity.
The Midtone is bound by the Shadow and Highlight terms and cannot
exceed them at any time. So, if the Highlight is set to 255 and the Shadow
is set to 0, a gamma of 1 is equal to a Midtone of 128. Similarly, if the
Shadow is set at 200 (lower boundary) and the Highlight is set at 220 (upper
boundary), a gamma of 1 is equal to a Midtone of 210.
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