Definitions

CMYK color model

CMYK color model

CMYK (short for cyan, magenta, yellow, and key (black), and often referred to as process color or four color) is a subtractive color model, used in color printing, also used to describe the printing process itself. Though it varies by print house, press operator, press manufacturer and press run, ink is typically applied in the order of the abbreviation.

The CMYK model works by partially or entirely masking certain colors on the typically white background (that is, absorbing particular wavelengths of light). Such a model is called subtractive because inks “subtract” brightness from white.

In additive color models such as RGB, white is the “additive” combination of all primary colored lights, while black is the absence of light. In the CMYK model, it is just the opposite: white is the natural color of the paper or other background, while black results from a full combination of colored inks. To save money on ink, and to produce deeper black tones, unsaturated and dark colors are produced by substituting black ink for the combination of cyan, magenta and yellow.

Halftoning

With CMYK printing, halftoning (also called screening) allows for less than full saturation of the primary colors; tiny dots of each primary color are printed in a pattern small enough that human beings perceive a single color. Magenta printed with a 20% halftone, for example, produces a pink color, because the eye perceives the tiny magenta dots and the white paper between the dots as lighter and less saturated than the color of pure magenta ink.

Without halftoning, the three primary process colors could be printed only as solid blocks of color, and therefore could produce only six colors: the three primaries themselves, plus three complementary colors produced by layering two of the primaries—cyan and yellow produce green; cyan and magenta produce a purplish blue; yellow and magenta produce red (these subtractive complementary colors correspond roughly to the additive primary colors). With halftoning, a full continuous range of colors can be produced.

Screen angle

To improve print quality and reduce moiré patterns, the screens for individual colors are set at unique angles. While the specific angles depend on how many colors are used and the preference of the press operator, typical CMYK process printing uses any of the following screen angles:

C 75° 15° 105°
M 15° 45° 75°
Y 90°
K 45° 75° 15°

Why black ink is used

The “black” generated by mixing Cyan, Magenta and Yellow primaries is unsatisfactory, and so four-color printing uses black ink in addition to the subtractive primaries. Common reasons for using black ink include:

  • Text is typically printed in black and includes fine detail (such as serifs), so to reproduce text or other finely detailed outlines using three inks without slight blurring would require impractically accurate registration (i.e. all three images would need to be aligned extremely precisely).
  • A combination of 100% cyan, magenta, and yellow inks soaks the paper with ink, making it slower to dry, and sometimes impractically so.
  • A combination of 100% cyan, magenta, and yellow inks often results in a muddy dark brown color that does not quite appear black. Adding black ink absorbs more light, and yields much darker blacks.
  • Using black ink is less expensive than using the corresponding amounts of colored inks.

When a very dark area is desirable, a colored or gray CMY “bedding” is applied first, then a full black layer is applied on top, making a rich, deep black; this is called rich black. A black made with just CMY inks is sometimes called a composite black.

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 gray component replacement are used to decide on the final mix; different CMYK recipes will be used depending on the printing task.

Other printer color models

CMYK or process color printing is contrasted with spot color printing, in which specific colored inks are used to produce the colors appearing on paper. Some printing presses are capable of printing with both four-color process inks and additional spot color inks at the same time. High-quality printed materials, such as marketing brochures and books, may include photographs requiring process-color printing, other graphic effects requiring spot colors (such as metallic inks), and finishes such as varnish, which enhances the glossy appearance of the printed piece.

CMYK process printers often have a relatively small color gamut. Processes such as Pantone's proprietary six-color (CMYKOG) Hexachrome can considerably expand the gamut. Additionally, light, saturated colors often cannot be created with CMYK, and light colors in general can make visible the halftone pattern. Using a CcMmYK process, with the addition of light cyan and magenta inks to CMYK, can solve these problems, and such a process is used by many inkjet printers, including desktop models.

Comparison with RGB

Comparisons between RGB displays and CMYK prints can be difficult, since the color reproduction technologies and properties are so different. A laser or ink-jet printer prints in dots per inch (dpi) which is very different from a computer screen, which displays graphics in pixels per inch (ppi). A computer screen mixes shades of red, green, and blue to create color pictures. A CMYK printer must compete with the many shades of RGB with only one shade of each of cyan, magenta and yellow, which it will mix using dithering, halftoning or some other optical technique; this dithering produces a lower level of detail than the printer's dpi suggests.

Conversion

Since RGB and CMYK spaces are both device-dependent spaces, there is no simple or general conversion formula that converts between them. Conversions are generally done through color management systems, using color profiles that describe the spaces being converted. Nevertheless, the conversions cannot be exact, since these spaces have very different gamuts.

The problem of computing a colorimetric estimate of the color that results from printing various combinations of ink has been addressed by many scientists. A general method that has emerged for the case of halftone printing is to treat each tiny overlap of color dots as one of 8 (combinations of CMY) or of 16 (combinations of CMYK) colors, which in this context are known as Neugebauer primaries. The resultant color would be an area-weighted colorimetric combination of these primary colors, except that the Yule–Nielsen effect ("dot gain") of scattered light between and within the areas complicates the physics and the analysis; empirical formulas for such analysis have been developed, in terms of detailed dye combination absorption spectra and empirical parameters.

function [r,g,b] = cmyk2rgb(c,m,y,k)

r = 1.0 - (c + k );
g = 1.0 - (m + k );
b = 1.0 - (y + k );

See also

Notes and references

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