Editing CMYK color model

{{Short description|Subtractive color model, used in color printing}}
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| caption1 = Color printing typically uses ink of four colors: cyan, magenta, yellow, and black.
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| caption2 = When subtractive CMY inks are combined at full strength, pairwise combinations are red, green, and blue. Combining all three gives an imperfect black color.
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|caption2 = What appears as [[cerulean]] ({{Color sample|#489490}}) in the top image is actually a blend of cyan, magenta, yellow and black, as magnification under a [[microscope]] demonstrates.
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The '''CMYK color model''' (also known as '''process color''', or '''four color''') is a [[Subtractive color|subtractive]] [[color model]], based on the [[CMY color model]], used in [[color printing]], and is also used to describe the printing process itself. The abbreviation ''CMYK'' refers to the four ink plates used: [[cyan|'''c'''yan]], [[magenta|'''m'''agenta]], [[yellow|'''y'''ellow]], and [[key plate|'''k'''ey]] (most often black).

The CMYK model works by partially or entirely masking colors on a lighter, usually white, background. The ink reduces the light that would otherwise be reflected. Such a model is called ''subtractive'', as inks ''subtract'' some colors from white light; in the CMY model, white light minus red leaves cyan, white light minus green leaves magenta, and white light minus blue leaves yellow.

In [[additive color]] models, such as [[RGB color model|RGB]], white is the ''additive'' combination of all [[primary color|primary]] colored lights, and black is the absence of light. In the CMYK model, it is the opposite: white is the natural color of the paper or other background, and black results from a full combination of colored inks. To save cost on ink, and to produce deeper black tones, [[saturation (color theory)|unsaturated]] and dark colors are produced by using black ink instead of or in addition to combinations of cyan, magenta, and yellow.

<!-- In [[color printing]], the shade of cyan called '''process cyan''' or '''pigment cyan''' is one of the three primary pigment colors which, along with [[yellow]] and [[magenta]], constitute the three [[Primary colors#Color space primaries|subtractive primary colors]] of pigment. (The secondary colors of pigment are blue, green and red.) As such, -->The CMYK printing process was invented in the 1890s, when newspapers began to publish color [[comic strip]]s.

== Halftoning ==
{{Main|Halftone}}
[[File:Halftoningcolor.svg|thumb|This diagram shows three examples of color halftoning with CMYK separations, as well as the combined halftone pattern and how the human eye would observe the combined halftone pattern from a sufficient distance.]]

With CMYK printing, ''[[halftone|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 humans [[color vision|perceive]] a solid color.<ref>{{cite book|url=https://books.google.com/books?id=nRoMo3V5AEwC&pg=PA1|title=Digital Color Halftoning|page=1|last=Kang|first=Henry R.|publisher=SPIE Press|year=1999|isbn=0-8194-3318-7}}</ref> Magenta printed with a 20% halftone, for example, produces a pink color, because the eye perceives the tiny magenta dots on the large white paper as lighter and less saturated than the color of pure magenta ink.{{citation needed|date=October 2020}} Halftoning allows for a continuous variability of each color, which enables continuous color mixing of the primaries. Without halftoning, each primary would be binary, i.e. on/off, which only allows for the [[color reproduction|reproduction]] of eight colors: white, the three primaries (cyan, magenta, yellow), the three secondaries (red, green, blue), and black.

== Comparison to CMY <span class="anchor" id="Why black ink is used"></span><span class="anchor" id="CMY"></span>==
[[File:Barns grand tetons.jpg|thumb]]
{{multiple image|width=197|image1=CMY separation – no black.jpg|image2=CMYK separation – maximum black.jpg|197|footer=The image above, (left) separated for printing with process cyan, magenta, and yellow (CMY) inks, and (right) separated for CMY and black (K)}}
[[File:CMYK offset on paper2.jpg|thumb|Inspection [[CMYK]] colors of offset printing on a paper]]

The CMYK color model is based on the CMY color model, which omits the black ink. Four-color printing uses black ink in addition to subtractive primaries for several reasons:<ref>{{cite book |author=Roger Pring |url=https://archive.org/details/wwwcolor00prin |title=WWW.Color |publisher=Watson–Guptill |year=2000 |isbn=0-8230-5857-3 |page=178 |url-access=registration}}</ref>

* In traditional preparation of color separations, a red [[keyline]] on the black line art marked the outline of solid or tint color areas. In some cases a black keyline was used when it served as both a color indicator and an outline to be printed in black because usually the black plate contained the keyline. The ''K'' in ''CMYK'' represents the keyline, or black, plate, also sometimes called the ''key plate''.<ref>{{cite web|url=https://gizmodo.com/the-difference-between-rgb-and-cmyk-explained-1777830600|title=The Difference Between RGB and CMYK, Explained|last=Menegus|first=Bryan|website=[[Gizmodo]]|date=May 20, 2016|access-date=October 4, 2020}}</ref>
* Text is typically printed in black and includes fine detail (such as [[serif]]s). To avoid even slight blurring when reproducing text (or other finely detailed outlines) using three inks would require impractically accurate [[printing registration|registration]].
* A combination of 100% cyan, magenta, and yellow inks soaks the paper with ink, making it slower to dry, causing bleeding, or (especially on low-quality paper such as [[newsprint]]) weakening the paper so much that it tears.{{citation needed|date=October 2020}}
* Although a combination of 100% cyan, magenta, and yellow inks would, in theory, completely absorb the entire [[visible spectrum]] of light and produce a perfect black, practical inks fall short of their ideal characteristics, and the result is a dark, muddy color that is not quite black. Black ink absorbs more light and yields much better blacks. 
* Black ink is less expensive than the combination of colored inks that makes black.

A black made with just CMY inks is sometimes called a ''composite black''.<ref>{{cite book|url=https://books.google.com/books?id=3iuwCgAAQBAJ&pg=PA331|page=331|title=Forensic Chemistry: Fundamentals and Applications|last=Sigel|first=Jay A.|publisher=John Wiley & Sons|year=2015|isbn=978-1-118-89772-0}}</ref>

When a very dark area is wanted, 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]]''.<ref>{{cite book |title=The Guild Handbook of Scientific Illustration |author=R. S. Hodges|url=https://books.google.com/books?id=YWaOBSjevD0C&pg=PA242|page=242|year=2003|publisher=John Wiley and Sons |isbn=0-471-36011-2 }}</ref>

The amount of black to use to replace amounts of the other inks 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.<ref>{{cite book|url=https://books.google.com/books?id=VrdqBRgSKasC&pg=PA87|page=87|title=Handbook of Print Media: Technologies and Production Methods|editor-last=Kipphan|editor-first=Helmut|publisher=Springer|year=2001|isbn=3-540-67326-1}}</ref>

== Other printer color models ==
CMYK, as well as all other process color printing, is contrasted with [[spot color]] printing, in which specific colored inks are used to generate the colors seen. 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, often 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.<ref>{{cite web |last=Davies |first=Helen |date=August 3, 2020 |title=Top 8 Large Format Printing Tips To Achieve High-End Projects |url=https://www.frontsigns.com/blog/top-large-format-printing-tips-to-achieve-the-highest-quality/ |url-status=live |archive-url=https://web.archive.org/web/20200929015633/https://www.frontsigns.com/blog/top-large-format-printing-tips-to-achieve-the-highest-quality/ |archive-date=September 29, 2020 |access-date=October 4, 2020 |website=Front Signs}}</ref>

CMYK are the process printers which often have a relatively small color [[gamut]]. Processes such as [[Pantone]]'s proprietary six-color (CMYKOG) [[Hexachrome]] considerably expand the gamut.<ref>{{cite journal|url=https://www.spiedigitallibrary.org/conference-proceedings-of-spie/5008/1/3D-color-separation-maximizing-the-printer-gamut/10.1117/12.472012.short?SSO=1|title=3-D Color Separation Maximizing the Printer Gamut|last=Zeng|first=Huanzhao|editor1-first=Reiner|editor1-last=Eschbach|editor2-first=Gabriel G|editor2-last=Marcu|journal=[[Proceedings of SPIE]]|series=Color Imaging VIII: Processing, Hardcopy, and Applications|volume=5008|year=2003|page=260|doi=10.1117/12.472012|bibcode=2003SPIE.5008..260Z|s2cid=20555157}}</ref> Light, saturated colors often cannot be created with CMYK, and light colors in general may make visible the halftone pattern. Using a [[CcMmYK color model|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|inkjet printers]], including desktop models.<ref>{{cite book |title = Sams Teach Yourself Adobe Photoshop Elements 2 in 24 Hours | author = Carla Rose | url = https://archive.org/details/samsteachyoursel00rose_4| url-access = registration | page = 108|publisher = Sams Publishing | year = 2003 | isbn = 0-672-32430-X }}</ref>

== Comparison with RGB displays ==
[[File:CIE1931xy gamut comparison.svg|thumb|Comparison of some RGB and CMYK color gamuts on a [[CIE 1931 color space|CIE 1931]] ''xy'' chromaticity diagram]]

Comparisons between [[RGB]] displays and CMYK prints can be difficult, since the color reproduction technologies and properties are very different. A computer monitor mixes shades of red, green, and blue light to create color images. A CMYK printer instead uses light-absorbing cyan, magenta, and yellow inks, whose colors are mixed using [[dithering]], halftoning, or some other optical technique.<ref>{{cite web |title=Overview of color in Illustrator |url=https://helpx.adobe.com/illustrator/using/color.html |url-status=live |archive-url=https://archive.today/20230207193742/https://helpx.adobe.com/illustrator/using/color.html |archive-date=February 7, 2023 |access-date=October 4, 2020 |publisher=[[Adobe Inc.]]}}</ref>

Similar to electronic displays, the inks used in printing produce color gamuts that are only a subsets of the visible gamut, and the two color modes have their own specific ranges, each being capable of producing colors the other is not. As a result of this, an image rendered on an electronic display and rendered in print can vary in appearance.<ref>Damien van Holten, Printinternational, "[http://www.printernational.org/rgb-versus-cmyk.php RGB Vs CMYK]"</ref> When designing images to be printed, designers work in RGB color spaces (electronic displays) capable of rendering colors a CMYK process cannot, and it is often difficult to accurately visualize a printed result that must fit into a different color space that both lacks some colors an electronic display can produce and includes colors it cannot.

=== Spectrum of printed paper ===
To reproduce color, the CMYK color model codes for absorbing light rather than emitting it (as is assumed by RGB). The ''K'' component ideally absorbs all wavelengths and is therefore achromatic. The cyan, magenta, and yellow components are used for color reproduction and they may be viewed as the inverse of RGB: Cyan absorbs red, magenta absorbs green, and yellow absorbs blue (−R,−G,−B).<ref>{{cite web|url=https://www.physics.wisc.edu/ingersollmuseum/exhibits/opticscolor/subcolormix/|title=Subtractive Color Mixing|publisher=[[L. R. Ingersoll Physics Museum]]|access-date=October 4, 2020}}</ref>

[[File:CMYK Spectrum printed paper.pdf|thumb|center|800px|Spectrum of the visible wavelengths on printed paper (SCA Graphosilk). Shown is the transition from red to yellow. White, red, blue, and green are shown for reference. Readings from a white orchid flower, a rose (red and yellow petals), and a red cyclamen flower are shown for comparison. The units of spectral power are simply raw sensor values (with a linear response at specific wavelengths).]]

== 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 profile]]s that describe the spaces being converted. An ICC profile defines the bidirectional conversion between a neutral "profile connection" color space (CIE XYZ or Lab) and a selected [[colorspace]], in this case both RGB and CMYK. The precision of the conversion depends on the profile itself, the exact methodology, and because the gamuts do not generally match, the [[rendering intent]] and constraints such as ink limit.

ICC profiles, internally built out of lookup tables and other transformation functions, are capable of handling many effects of ink blending. One example is the [[dot gain]], which show up as non-linear components in the color-to-density mapping. More complex interactions such as Neugebauer blending can be modelled in higher-dimension lookup tables.

The problem of computing a colorimetric estimate of the color that results from printing various combinations of ink has been addressed by many scientists.<ref name = Sharma>{{cite book | title = Digital Color Imaging Handbook | author = Gaurav Sharma | year = 2003 | publisher = CRC Press | isbn = 0-8493-0900-X | url = https://books.google.com/books?id=AkByHKRGTsQC&pg=PA68|page=68}}</ref> 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 primary|Neugebauer primaries]]. The resultant color would be an area-weighted colorimetric combination of these primary colors, except that the [[Dot gain#Yule–Nielsen effect and "optical dot gain"|Yule–Nielsen effect]] 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.<ref name = Sharma/>

Standardization of printing practices allow for some profiles to be predefined. One of them is the US [[Specifications for Web Offset Publications]], which has its ICC color profile built into some software including [[Microsoft Office]] (as [[Agfa-Gevaert|Agfa]] RSWOP.icm).<ref>{{cite web |title=KB933845 How to obtain and install the Microsoft Standard CMYK Profile (RSWOP.icm) in Windows Vista |url=https://www.betaarchive.com/wiki/index.php/Microsoft_KB_Archive/933845 |website=Beta Archive > Microsoft KB Archive |access-date=1 September 2023 |quote=The RSWOP.icm cyan-magenta-yellow-black (CMYK) color profile targets the "Specifications for Web Offset Publications" (SWOP) printing standard. This color profile is installed when you install Microsoft Office. However, by default, this color profile is not installed in Windows Vista. Therefore, you may experience unexpected results when you use certain programs that manage color.|date=March 15, 2007}}</ref>

The device-dependency of RGB and CMYK also means that a set of RGB or CMYK values cannot uniquely represent a color, so long as no device or paper–ink–process combination is specified. With RGB the [[sRGB]] standard is widespread enough to be the implied default, but there is not a single form of CMYK which is widespread enough to be the default.

<gallery widths=255 heights=400 class="center" >
File:NIEdot367.jpg|Early representation of the three-color process (1902)
File:NIEdot367-modern.jpg|Approximation of the image using CMY colors.
</gallery>

== See also ==
* [[CMY color model]]
* [[CcMmYK color model]]
* [[Cycolor]]
* [[RGB color model]]
* [[Grey component replacement|Gray component replacement]]
* [[Jacob Christoph Le Blon]]
* [[Specifications for Web Offset Publications|SWOP]] CMYK standard
* [[Color management]]
* [[Technicolor]], the three-strip version of which is based on the CMYK model

== References ==
{{reflist}}

== External links ==
{{Commons category|CMYK}}
* [http://www.codeproject.com/KB/applications/xcmyk.aspx XCmyk]{{nbsp}}– A Windows software with source code for converting CMYK to RGB.
* [https://www.edusters.com/tool/rgb-to-cmyk-color-converter-online.php RGB to CMYK converter]{{nbsp}}– Tool for RGB to CMYK color converter online.
* [https://web.archive.org/web/20110716053923/http://dx.sheridan.com/advisor/cmyk_color.html  Color Space Fundamentals]{{nbsp}}– animated illustration of RGB vs. CMYK
* [https://www.color.org/registry/index.xalter ICC profile registry], which lists some standard CMYK profiles, their paper types, and color separation limits

{{Color space}}
{{Photography}}

[[Category:Color space]]
[[Category:Printing]]
[[Category:Printing terminology]]