User Guide

GNU Image Manipulation Program

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• YUV

• YCbCr

CMY, CMYK

CMYK is a color model which has components for Cyan, Magenta, Yellow and Black. It is a subtractive color model, and

that fact is important when an image is printed. It is complementary to the RGB color model.

The values of the individual colors vary between 0% and 100%, where 0% corresponds to an unprinted color, and 100%

corresponds to a completely printed area of color. Colors are formed by mixing the three basic colors.

The last of these values, K (Black), doesn’t contribute to the color, but merely serves to darken the other colors. The letter

K is used for Black to prevent confusion, since B usually stands for Blue.

Figure 17.1: Subtractive color model

GIMP does not currently support the CMYK model. (An experimental plug-in providing rudimentary CMYK support can

be found [PLUGIN-SEPARATE].)

This is the mode used in printing. These are the colors in the ink cartridges in your printer. It is the mode used in painting

and in all the objects around us, where light is reﬂected, not emmitted. Objects absorb part of the light waves and we

see only the reﬂected part. Note that the cones in our eyes see this reﬂected light in RGB mode. An object appears Red

because Green and Blue have been absorbed. Since the combination of Green and Blue is Cyan, Cyan is absorbed when

you add Red. Conversely, if you add Cyan, its complementary color, Red, is absorbed. This system is subtractive. If you

add Yellow, you decrease Blue, and if you add Magenta, you decrease Green.

It would be logical to think that by mixing Cyan, Magenta and Yellow, you would subtract Red, Green and Blue, and the

eye would see no light at all, that is, Black. But the question is more complex. In fact, you would see a dark brown. That

is why this mode also has a Black value, and why your printer has a Black cartridge. It is less expensive that way. The

printer doesn’t have to mix the other three colors to create an imperfect Black, it just has to add Black.

Dithering

Dithering is a technique used in computer graphics to create the illusion of more colors when displaying an image which

has a low color depth. In a dithered image, the missing colors are reproduced by a certain arrangement of pixels in the

available colors. The human eye perceives this as a mixture of the individual colors.

The Gradient tool uses dithering. You may also choose to use dithering when you convert an image to Indexed format. If

you are working on an image with indexed colors, some tools (such as the pattern ﬁll tool) may also use dithering, if the

correct color is not available in the colormap.

The Newsprint ﬁlter uses dithering as well. You can use the NL Filter (Non Linear ﬁlter) to remove unwanted dithering

noise from your image.

Also note that although GIMP itself uses 24-bit colors, your system may not actually be able to display that many colors.

If it doesn’t, then the software in between GIMP and your system may also dither colors while displaying them.