CMYK color model? |
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Cyan, magenta, yellow, and key (black)CMYK (or sometimes YMCK) is a subtractive color model used in color printing. This color model is based on mixing pigments of the following colors in order to make other colors:
The mixture of ideal CMY colors is subtractive (Cyan, Magenta, and Yellow printed together on white result to black). CMYK works through light absorption. The colors that are seen are from the part of light that is not absorbed. In CMYK magenta plus yellow produces red, magenta plus cyan makes blue, cyan plus yellow generates green and the combination of cyan, magenta, and yellow form black.
For several reasons, the 'black' generated by mixing the subtractive primaries is not ideal and so four-color printing uses black ink in addition to the subtractive primaries yellow, magenta, and cyan. The reasons for using black ink include
- A mixture of practical cyan, magenta, and yellow pigments is not pure black, but a dark murky color.
- Mixing all three process color inks together merely to make black can make the paper rather wet, which is an issue in high speed printing where the paper must dry extremely rapidly to avoid marking the next sheet, and poor quality paper such as newsprint may break if it becomes too wet
- Text is typically printed in black and includes fine detail (such as serifs); so to reproduce text using three inks without slight blurring would require impractically accurate registration (i.e. all three images would need to be positioned extremely precisely)
- Using a unit amount of black ink rather than three unit amounts of the process color inks can lead to significant cost savings, especially because black ink can be less expensive than any of the color inks.
(The black is referred to as K for key – a shorthand for the printing term key plate. This plate impressed the artistic detail of an image, usually in black ink.)
The amount of black to use to replace amounts of the other ink is variable, and the choice depends on the technology, paper and ink in use. Processes called under color removal, under color addition, and grey component replacement are used to decide on the final mix, so that different CMYK recipes will be used depending on the printing task. Where black is mixed with the other colors, this is "blacker than black" and is referred to as rich black.
Use of four-color printing generates a good final printed result with greater contrast. However the color a person sees on a computer screen is often somewhat different from the color of the same object on a printout since CMYK and the RGB color model used in computer monitors have different gamuts. For example, dark blue reproduces poorly in CMYK, coming out rather murky (as it has to be approximated by darkening cyan with black and a little magenta). ** It should be noted that computer (and other) screens operate RGB modes as we require light to see the initial content being displayed. RGB (Red, Green and Blue) are the constituent colours of white light. In printed materials, we cannot directly reproduce this light combination, so rely on the CMYK equivalent in ink colours. **
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Conversions
Note that the conversions here are best described as "nominal". They will produce an invertible conversion between RGB and a subset of CMYK, that is, one can take a RGB color and convert to certain CMYK colors and from these CMYK colors, obtain the corresponding, original RGB equivalents. However, conversion of CMYK colors in general to RGB colors is not invertible, that is, given a CMYK color which is converted to RGB, performing the former conversion may not give the original CMYK color. In addition, CMYK colors may print wildly differently from how the RGB colors display on a monitor. There is no single "good" conversion rule between RGB and CMYK, because neither RGB nor CMYK is an absolute color space.
These conversions should never be used in commercial printing or any other application where color matching is important.
Converting between RGB and CMYK
To convert between RGB and CMYK, an intermediate CMY value is used. Color values are represented as a vector, with each color component varying from 0.0 (no color) to 1.0 (fully saturated color):
| tCMYK | = {C,M,Y,K} is the CMYK quadruple on  , |
| tCMY | = {C,M,Y} is the CMY triple on  , |
| tRGB | = {R,G,B} is the RGB triple on . |
Converting CMYK to RGB
To convert, we first convert CMYK to CMY, then convert the CMY value to RGB
Converting now
- tCMYK = {C,M,Y,K}
then
- tCMY = {C',M',Y'} = {C(1 − K) + K,M(1 − K) + K,Y(1 − K) + K}
and
- tRGB = {R,G,B} = {1 − C',1 − M',1 − Y'}
or substituting in
- tRGB = {1 − (C(1 − K) + K),1 − (M(1 − K) + K),1 − (Y(1 − K) + K)} = {1 − C(1 − K) − K,1 − M(1 − K) − K,1 − Y(1 − K) − K}
Mapping RGB to CMYK
This is a formula for mapping a given RGB colour to one of many possible semi-equivalent CMYK colours. In this case: the CMYK colour that uses the most black (K) and the least colour (CMY). E.g. #808080 (grey, exactly halfway between black and white) will map to (0,0,0,0.5), and not to (0.5,0.5,0.5,0).
Converting RGB → CMY, with the same color vectors as before:
Converting now
- tRGB = {R,G,B}
converting to CMY
- tCMY = {C',M',Y'} = {1 − R,1 − G,1 − B}
and then to CMYK:
- if min{C',M',Y'} = 1
- then
- tCMYK = {0,0,0,1}
- else
- K = min{C',M',Y'}
See also
- Color model
- RGB color model commonly used for color monitors
- HSV color space
- HLS color space
- RYB color model the traditional color model used by artists.
- YUV for PAL television
- YIQ for NTSC television
External links
- A song about YMCK, to the tune of 'YMCA'
- List CMYK values for hundreds of colors
- THE MOTHER OF ALL HTML COLO(U)R CHARTS — Sites with tables containing large number of colors, including their RGB, CMYK, and HLS values, names, sources, etc.