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Auto contrast
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The Auto Contrast filter adjusts the overall contrast and mixture of colors in an image automatically. Because 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 in an image, Auto Contrast clips the white and black pixels by 0.5%--that is, it ignores the first 0.5% of either extreme. This ensures that white and black values are based on representative rather than extreme pixel values.
Auto Contrast can improve the appearance of many photographic or continuous-tone images. It does not improve flat-color images.

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