In image editing, a curve is a remapping of image tonality, specified as a function from input level to output level, used as a way to emphasize colours or other elements in a picture.^[1][2]

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. Light parts of a picture can be easily made lighter and dark parts darker to increase contrast.^[3]

Applying a curve to individual channels can be used to stress a colour. This is particularly efficient in the Lab colour space due to the separation of luminance and chromaticity,^[4] but it can also be used in RGB, CMYK or whatever other colour models the software supports.