Curve (tonality)

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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.[http://docs.gimp.org/en/gimp-tool-curves.html The gimp manual][http://www.adobe.com/designcenter/photoshop/articles/phscs2at_learncurves_02.html Adobe web site on curves in Photoshop]

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.Detailed [http://www.photoshopessentials.com/photo-editing/levels-curves/ instructions] on the usage of the 'curves' and 'levels' functionality of image editing software; PhotoshopEssentials.com

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,{{cite book|title=Photoshop LAB Color: The Canyon Conundrum and Other Adventures in the Most Powerful Colorspace|authorlink=Dan Margulis|first=Dan|last=Margulis|isbn=0-321-35678-0|year=2005|publisher=Peachpit Press }} but it can also be used in RGB, CMYK or whatever other colour models the software supports.