Curves (tonality)
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A Curve in Digital image editing is a way to emphasize colours or other elements in a picture.
Curves can usually be applied to all channels together in an image, or to each channel individually.
Applying a curve to all channels typically changes the brightness in part of the spectrum. The software user may for example make light parts of a picture lighter and dark parts darker to increase contrast.
Applying a curve to individual channels can be used to stress a colour. This is particularly efficient in the Lab color space but it can also be used in RGB, CMYK or whatever other color models the software supports.

Photo and curve dialog in the GIMP