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57
The highlight level, shadow level, and gamma can be set manually. The histogram can be
used to maximize the distribution of the pixels in the image. All the levels on the histogram
are displayed numerically to the right of the sliders. These numbers can be changed with
the keyboard.
The gamma slider defines the mid-tones of the image. Dragging the gamma slider to the
right will darken the image, and dragging it to the left will brighten it. Similar to the tone-
curve correction described on page 50, the gamma slider allows the brightness of the
image to be adjusted without loosing image information.
The input highlight slider sets the white level. As the slider is moved to the left, an
apparent increase in contrast can be seen in the displayed image. All pixels to the right
of the slider are set to 255 and any image detail they may contain will be lost. This can
be an important tool for improving copy images of text on a white background. Uneven
illumination, or faded or stained paper can be distracting when copying text or line art. By
adjusting the white level, the imperfections of the white background can be eliminated
leaving only the darker text visible.
The input shadow slider sets the black level. As the slider is moved to the right, an
apparent increase in contrast can be seen in the displayed image. All pixels to the left of
the slider are set to 0 and any image detail they may contain will be lost.
The black and white output levels can be adjusted. By moving the output highlight and
shadow sliders, the contrast of the image can be reduced.
For examples of histogram corrections, see the following section.
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