The Stretch Contrast command automatically stretches the histogram values in the active layer. For each channel of the active layer, it finds the minimum and maximum values and uses them to stretch the Red, Green and Blue histograms to the full contrast range. The bright colors become brighter and the dark colors become darker, which increases the contrast. “Stretch Contrast” works on layers of RGB, Grayscale and Indexed images. Use “Stretch Contrast” only if you want to remove an undesirable color tint from an image which should contain pure white and pure black.

This command is also similar to the Color Balance command, but it does not reject any of the very dark or very bright pixels, so the white might be impure.