{{short description|Color space}} In [[colorimetry]], the '''CIE 1976''' '''''L''*, ''u''*, ''v''*''' '''color space''', commonly known by its abbreviation '''CIELUV''', is a [[color space]] adopted by the [[International Commission on Illumination]] (CIE) in 1976, as a simple-to-compute transformation of the 1931 [[CIE 1931 color space|CIE XYZ color space]], which attempted [[Color_difference#Tolerance|perceptual uniformity]]. It is extensively used for applications such as computer graphics which deal with colored lights. Although additive mixtures of different colored lights will fall on a line in CIELUV's uniform [[chromaticity|chromaticity diagram]] (called the ''CIE 1976 UCS''), such additive mixtures will not, contrary to popular belief, fall in the midpoint between the chromaticities of the two colors mixed, unless they both have the same [[lightness]].
==Historical background== {{multiple image | width = 150 | image1 = SRGB gamut within CIELUV color space mesh.webm | alt1 = SRGB gamut in CIEXYZ space | thumbtime1 = 0 | image2 = Visible gamut within CIELUV color space D65 whitepoint mesh.webm | alt2 = Visible gamut in CIEXYZ space | thumbtime2 = 0 | footer = The [[sRGB]] gamut (''left'') and [[Color solid#Optimal_color_solid|optimal color solid]] (theoretical gamut of surfaces) under D65 illumination (''right'') plotted within the CIELUV color space. ''u'' and ''v'' are the horizontal axes; ''L'' is the vertical axis. }} CIELUV is an [[Adams chromatic valence color space]] and is an update of the [[CIE 1964 color space|CIE 1964 (''U''*, ''V''*, ''W''*) color space]] (CIEUVW). The differences include a slightly modified [[Lightness (color)|lightness]] scale and a modified uniform chromaticity scale, in which one of the coordinates, ''v''′, is 1.5 times as large as ''v'' in its [[CIE 1960 color space|1960 predecessor]]. CIELUV and [[CIELAB]] were adopted simultaneously by the CIE when no clear consensus could be formed behind only one or the other of these two color spaces.
CIELUV uses Judd-type (translational) [[white point]] adaptation (in contrast with CIELAB, which uses a [[von Kries transform]]).<ref>{{cite journal |first=Deane B. |last=Judd |title=Hue saturation and lightness of surface colors with chromatic illumination |journal=[[JOSA]] |volume=30 |issue=1 |date=January 1940 |pages=2–32 |doi=10.1364/JOSA.30.000002}}</ref> This can produce useful results when working with a single illuminant, but can predict [[imaginary color]]s (i.e., outside the [[spectral locus]]) when attempting to use it as a [[chromatic adaptation transform]].<ref name="Fairchild1998">Mark D. Fairchild, ''Color Appearance Models''. Reading, MA: Addison-Wesley, 1998.</ref> The translational adaptation transform used in CIELUV has also been shown to perform poorly in predicting corresponding colors.<ref name="Alman1989">D. H. Alman, R. S. Berns, G. D. Snyder, and W. A. Larson, "Performance testing of color difference metrics using a color-tolerance dataset". ''Color Research and Application'', '''21''':174–188 (1989).</ref>
== XYZ → CIELUV and CIELUV → XYZ conversions == By definition, {{nobr|0 ≤ ''L''* ≤ 100 }}.
===The forward transformation=== CIELUV is based on CIEUVW and is another attempt to define an encoding with uniformity in the perceptibility of [[color difference|color differences]].<ref name=schanda/> The non-linear relations for ''L''*, ''u''*, and ''v''* are given below:<ref name=schanda>{{cite book|title=Colorimetry: Understanding the CIE System|first=János|last=Schanda|publisher=Wiley Interscience|year=2007|isbn=978-0-470-04904-4|quote=As 24/116 is not a simple ratio, in some publications the 6/29 ratio is used, in others the approximate value of 0.008856 (used in earlier editions of CIE 15). Similarly some authors prefer to use instead of 841/108 the expression (1/3)×(29/6)<sup>2</sup> or the approximate value of 7.787, or instead of 16/116 the ratio 4/29.|pages=61–64}}</ref>
:<math> \begin{align} L^* &= \begin{cases} \bigl(\tfrac{29}{3}\bigr)^3 Y / Y_n,& Y / Y_n \le \bigl(\tfrac{6}{29}\bigr)^3, \\[3mu] 116 \sqrt[3]{ Y / Y_n } - 16,& Y / Y_n > \bigl(\tfrac{6}{29}\bigr)^3, \end{cases}\\[5mu] u^* &= 13 L^*\cdot (u^\prime - u_n^\prime), \\[3mu] v^* &= 13 L^*\cdot (v^\prime - v_n^\prime). \end{align}</math>
The quantities ''u''′<sub>''n''</sub> and ''v''′<sub>''n''</sub> are the {{nobr|(''u''′, ''v''′)}} chromaticity coordinates of a "specified white object" – which may be termed the [[white point]] – and ''Y''<sub>''n''</sub> is its luminance. In reflection mode, this is often (but not always) taken as the {{nobr|(''u''′, ''v''′)}} of the [[Diffuser (optics)|perfect reflecting diffuser]] under that illuminant. (For example, for the [[standard colorimetric observer|2° observer]] and [[standard illuminant]] C, {{nobr|1=''u''′<sub>''n''</sub> = 0.2009}}, {{nobr|1=''v''′<sub>''n''</sub> = 0.4610}}.) Equations for ''u''′ and ''v''′ are given below:<ref name="CIE15point2">''Colorimetry,'' second edition: CIE publication 15.2. Vienna: Bureau Central CIE, 1986.</ref><ref name=poynton/>
:<math>\begin{alignat}{3} u^\prime &= \frac{4 X}{X + 15 Y + 3 Z} &&= \frac{4 x}{-2 x + 12 y + 3}, \\[5mu] v^\prime &= \frac{9 Y}{X + 15 Y + 3 Z} &&= \frac{9 y}{-2 x + 12 y + 3}. \end{alignat}</math>
===The reverse transformation=== [[Image:CIE 1976 UCS.png|right|thumb|300px|{{nobr|(''u''′, ''v''′)}} chromaticity diagram, also known as the CIE 1976 UCS (uniform chromaticity scale) diagram.]] The transformation from {{nobr|(''u''′, ''v''′)}} to {{nobr|(''x'', ''y'')}} is:<ref name=poynton/>
:<math>\begin{align} x &= \frac{9u^\prime}{6u^\prime - 16v^\prime + 12}\\[5mu] y &= \frac{4v^\prime}{6u^\prime - 16v^\prime + 12} \end{align}</math>
The transformation from CIELUV to XYZ is performed as follows:<ref name=poynton/>
:<math>\begin{align} u^\prime&= \tfrac1{13}(u^*/L^*) + u^\prime_n, \\[3mu] v^\prime&=\tfrac1{13}(v^*/L^*) + v^\prime_n, \\[5mu] Y &= \begin{cases} \bigl(\frac{3}{29}\bigr)^3 L^*~\! Y_n, & L^* \le 8, \\[3mu] \bigl(\tfrac1{116}(L^* + 16)\bigr)^3\, Y_n, & L^* > 8, \end{cases}\\[5mu] X &= \frac{9u^\prime}{4v^\prime} Y, \\[5mu] Z &= \frac{12 - 3u^\prime - 20v^\prime}{4v^\prime} Y. \end{align}</math>
==Cylindrical representation (CIELCh)== {{multiple image | width = 150 | image1 = SRGB gamut within CIELCHuv color space mesh.webm | alt1 = SRGB gamut in CIELCHuv space | thumbtime1 = 0 | image2 = Visible gamut within CIELCHuv color space D65 whitepoint mesh.webm | alt2 = Visible gamut in CIELCHuv space | thumbtime2 = 0 | footer = The [[sRGB]] gamut (''left'') and [[Color solid#Optimal_color_solid|optimal color solid]] (theoretical gamut of surfaces) under D65 illumination (''right'') plotted within the CIELCHuv color space. ''L'' is the vertical axis; ''C'' is the cylinder radius; ''h'' is the angle around the circumference. }}
CIELCh<sub>uv</sub>, or [[HCL color space]] (hue–chroma–luminance) is increasingly seen in the [[information visualization]] community as a way to help with presenting data without the bias implicit in using varying [[Colorfulness|saturation]].<ref name="Ihaka2003">{{cite conference | last1 = Ihaka | first1 = Ross | author1-link = Ross Ihaka | title = Colour for Presentation Graphics | book-title = Proceedings of the 3rd International Workshop on Distributed Statistical Computing, Vienna, Austria | editor1-last = Hornik | editor1-first = Kurt | editor2-last = Leisch | editor2-first = Friedrich | editor3-last = Zeileis | editor3-first = Achim | issn = 1609-395X | year = 2003 | url = http://www.ci.tuwien.ac.at/Conferences/DSC-2003/Proceedings/ | access-date = 2019-07-15 | archive-date = 2015-09-17 | archive-url = https://web.archive.org/web/20150917034643/http://www.ci.tuwien.ac.at/Conferences/DSC-2003/Proceedings/ | url-status = dead }}</ref><ref name=Zeileis2009>{{cite journal | last1 = Zeileis | first1 = Achim | last2 = Hornik | first2 = Kurt | last3 = Murrell | first3 = Paul | title = Escaping RGBland: Selecting Colors for Statistical Graphics | journal = Computational Statistics & Data Analysis | volume = 53 | issue = 9 | pages = 3259–3270 | year = 2009 | doi = 10.1016/j.csda.2008.11.033 | url = https://epub.wu.ac.at/1692/1/document.pdf }}</ref><ref>{{cite journal | last1 = Stauffer | first1 = Reto | last2 = Mayr | first2 = Georg J. | last3 = Dabernig | first3 = Markus | last4 = Zeileis | first4 = Achim | title = Somewhere over the Rainbow: How to Make Effective Use of Colors in Meteorological Visualizations | journal = Bulletin of the American Meteorological Society | volume = 96 | issue = 2 | pages = 203–216 | year = 2015 | doi = 10.1175/BAMS-D-13-00155.1 | bibcode = 2015BAMS...96..203S | hdl = 10419/101098 | hdl-access = free }}</ref>
The [[cylindrical coordinate system|cylindrical]] version of CIELUV is known as CIELCh<sub>uv</sub>, or CIELChuv, CIELCh(uv) or CIEHLC<sub>uv</sub>, where ''C''*<sub>''uv''</sub> is the [[colorfulness|chroma]] and ''h''<sub>''uv''</sub> is the [[hue]]:<ref name=poynton/> : <math>C_{uv}^* = \operatorname{hypot}(u^*, v^*) = \sqrt{(u^*)^2 + (v^*)^2},</math> : <math>h_{uv} = \operatorname{atan2}(v^*, u^*),</math> where [[atan2]] function, a "two-argument arctangent", computes the [[polar coordinates|polar]] angle from a Cartesian coordinate pair.
Furthermore, the saturation correlate can be defined as
: <math>s_{uv} = \frac{C^*}{L^*} = 13 \sqrt{(u' - u'_n)^2 + (v' - v'_n)^2}.</math>
Similar correlates of chroma and hue, but not saturation, exist for CIELAB. See [[Colorfulness#Saturation|Colorfulness]] for more discussion on saturation.
==Color and hue difference== The [[color difference]] can be calculated using the [[Euclidean distance]] of the {{nobr|(''L''*, ''u''*, ''v''*)}} coordinates.<ref name=poynton>{{cite book |first=Charles |last=Poynton |title=Digital Video and HDTV |publisher=Morgan-Kaufmann |isbn=1-55860-792-7 |year=2003 |pages=226}}</ref> It follows that a chromaticity distance of <math>\sqrt{(\Delta u')^2 + (\Delta v')^2} = 1/13</math> corresponds to the same Δ''E''*<sub>''uv''</sub> as a lightness difference of {{nobr|1=Δ''L''* = 1}}, in direct analogy to CIEUVW.
The Euclidean metric can also be used in CIELCh, with that component of Δ''E''*<sub>''uv''</sub> attributable to difference in hue as<ref name=schanda/> {{nobr|1=Δ''H''* = {{radic|''C''*<sub>1</sub>''C''*<sub>2</sub>}} 2 sin (Δ''h''/2)}}, where {{nobr|1=Δ''h'' = ''h''<sub>2</sub> − ''h''<sub>1</sub>}}.
== See also == *[[Y′UV]] *[[CIELAB color space]]
==References== <references/>
==External links== * [https://web.archive.org/web/20080416044228/http://www.efg2.com/Lab/Graphics/Colors/Chromaticity.htm Chromaticity diagrams, including the CIE 1931, CIE 1960, CIE 1976] * [http://isp.uv.es/code/visioncolor/colorlab.html Colorlab] MATLAB toolbox for color science computation and accurate color reproduction (by Jesus Malo and Maria Jose Luque, Universitat de Valencia). It includes CIE standard tristimulus colorimetry and transformations to a number of non-linear color appearance models (CIELAB, CIE CAM, etc.).
{{Color space}}
[[Category:Color space]] [[Category:1976 introductions]] [[Category:Articles containing video clips]]