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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 distract-
ing when copying text or line art. By adjusting the white level, the imperfections of the white back-
ground 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 slid-
ers, the contrast of the image can be reduced.
For examples of histogram corrections, see the following section.