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Auto Level gives good results when an image with an average distribution of pixel
values needs a simple contrast adjustment or when an image has an overall color
cast. However, adjusting the Curves manually is more precise.
See Auto Contrast for another auto adjust command.
11.2.3 Auto Contrast
The Auto Contrast command automatically adjusts the overall contrast and mixture
of colors in an RGB image. Since it does not adjust channels individually, Auto
Contrast does not introduce or remove color casts. It maps the lightest and darkest
pixels in the image to white and black, which makes highlights appear lighter and
shadows appear darker.
When identifying the lightest and darkest pixels on an image, Auto Contrast clips the
white and black pixels by 0.5%--that is, it ignores the first 0.5% of either extreme.
Choose Options>Auto Correction••• menu to modify this default setting. This
ensures that white and black values are representative without being determined by
extreme pixel values. The Auto Correction••• dialog is shown below:
Auto Contrast can improve the appearance of many photographic or continuous-tone
images. It does not improve flat-color images.
See Auto Level for another auto operation.
11.2.4 Histogram Equalization