64 Advanced image processing
Histogram corrections
The histogram indicates the distribution of pixels with specific brightness or color values in the
image. Using the histogram can optimize the output of the image data. Changes made with the
histogram are also displayed on the tone curve.
Input shadow slider
Input gamma slider
Input highlight slider
Input shadow text box
Input gamma text box
Input highlight text box
Output shadow slider
Output highlight slider Output shadow text box
Output highlight text box
The color histograms can be displayed with the channel list box or with keyboard shortcuts.
While holding the control key (Windows) or command key (Macintosh), press 0 (zero) to display
the RGB channel, 1 to display the red channel, 2 to display the green channel, or 3 to display the
blue channel.
The histogram can be used to optimise the distribution of the pixels in the image. The highlight
level, shadow level, and gamma can be set manually with the sliders or text boxes.
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 correc-
tion described on page 62, the gamma slider allows the brightness of the image to be adjusted
without losing 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 imperfec-
tions 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.
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