Editing Hue

{{Short description|Property of a color}}
{{About|the color property|the city in Vietnam|Huế|other uses}}

[[File:Colors-i54-ring.png|thumb|right|All colors on this [[color wheel]] should appear to have the same lightness and the same saturation, differing only by hue.]]
{{wikt | hue}}

In [[color theory]], '''hue''' is one of the properties (called [[Color appearance model#Color appearance parameters|color appearance parameters]]) of a [[color]], defined in the [[CIECAM02]] model as "the degree to which a [[Stimulus (physiology)#vision|stimulus]] can be described as similar to or different from stimuli that are described as [[red]], [[Orange (colour)|orange]], [[yellow]], [[green]], [[blue]], [[violet (color)|violet]],"<ref>Mark Fairchild, "Color Appearance Models: CIECAM02 and Beyond". Tutorial slides for IS&T/SID 12th Color Imaging Conference.</ref> within certain theories of [[color vision]].

Hue can typically be represented quantitatively by a single number, often corresponding to an angular position around a central or neutral point or axis on a [[color space]] coordinate diagram (such as a [[chromaticity diagram]]) or [[color wheel]], or by its [[dominant wavelength]] or by that of its [[complementary color]]. The other color appearance parameters are [[colorfulness]], [[Saturation (color theory)| saturation]] (also known as intensity or chroma),<ref>{{Cite web|url= http://learn.leighcotnoir.com/artspeak/elements-color/hue-value-saturation/|title=Hue, Value, Saturation {{!}} learn.|archive-url= https://web.archive.org/web/20170630065346/http://learn.leighcotnoir.com/artspeak/elements-color/hue-value-saturation/|archive-date=2017-06-30|url-status=dead|access-date=October 27, 2017}}</ref> [[lightness]], and [[brightness]]. Usually, colors with the same hue are distinguished with adjectives referring to their lightness or colorfulness - for example: "light blue", "[[pastel]] blue", "vivid blue", and "cobalt blue". Exceptions include [[brown]], which is a dark [[Orange (colour) |orange]].<ref name="Bartleson1976">C J Bartleson, "Brown". ''Color Research and Application'', '''1''' : 4, pp. 181–191 (1976).</ref>

In [[painting]], a hue is a ''pure'' [[pigment]]—one without [[Tints and shades |tint or shade]] (added white or black pigment, respectively).<ref>{{cite web |url= http://creativecurio.com/2008/05/the-color-wheel-and-color-theory/ |title=The Color Wheel and Color Theory |publisher= Creative Curio |date= 2008-05-16 |access-date= 2011-06-09 |url-status= live |archive-url= https://web.archive.org/web/20110705211646/http://creativecurio.com/2008/05/the-color-wheel-and-color-theory/ |archive-date=2011-07-05 }}</ref>

The [[human brain]] first processes hues in areas in the extended [[Visual area V4 | V4]] called [[glob (visual system) |globs]].<ref name="Conway07">{{cite journal | doi = 10.1016/j.neuron.2007.10.008 | last1 = Conway | first1 = BR | last2 = Moeller | first2 = S | last3 = Tsao | first3 = DY. | year = 2007 | title = Specialized color modules in macaque extrastriate cortex | url = https://authors.library.caltech.edu/100800/2/1-s2.0-S0896627307007702-mmc1.pdf | journal = Neuron | volume = 56 | issue = 3 | pages = 560–73 | pmid = 17988638 | pmc = 8162777 | s2cid = 11724926 | access-date = 2020-09-10 | archive-date = 2021-03-02 | archive-url = https://web.archive.org/web/20210302120817/https://authors.library.caltech.edu/100800/2/1-s2.0-S0896627307007702-mmc1.pdf | url-status = dead }}</ref><ref name="Conway">
{{cite journal
 | last1 = Conway | first1 = BR | last2 = Tsao | first2 = DY | title = Color-tuned neurons are spatially clustered according to color preference within alert macaque posterior inferior temporal cortex | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 106 | issue = 42 | pages = 18034–9 | year = 2009 | pmid = 19805195 | pmc = 2764907 | doi = 10.1073/pnas.0810943106 | bibcode = 2009PNAS..10618034C | doi-access = free
}}
</ref>

== Deriving a hue ==

[[File:Munsell hue wheel equal value and chroma.png|thumb|Gradient Munsell hue wheel at value 5 and constant chroma (6.24)]]

The concept of a color system with a hue was explored as early as 1830 with [[Philipp Otto Runge]]'s color sphere. The [[Munsell color system]] from the 1930s was a great step forward, as it was realized that [[Color difference#Tolerance|perceptual uniformity]] means the color space can no longer be a sphere.

As a convention, the hue for [[red]] is set to 0° for most color spaces with a hue.

=== Opponent color spaces ===
In [[opponent process|opponent color spaces]] in which two of the axes are perceptually orthogonal to lightness, such as the CIE 1976 (''L''*, ''a''*, ''b''*) ([[CIELAB]]) and 1976 (''L''*, ''u''*, ''v''*) ([[CIELUV]]) color spaces, hue may be computed together with chroma by converting these coordinates from [[rectangular coordinates|rectangular]] form to [[polar coordinates|polar]] form. Hue is the angular component of the polar representation, while chroma is the radial component.

Specifically, in CIELAB<ref name="CIE1986">''Colorimetry,'' second edition: CIE Publication 15.2. Vienna: Bureau Central of the CIE, 1986.</ref>

:<math>h_{ab} = \mathrm{atan2}(b^*, a^*),</math>

while, analogously, in CIELUV<ref name = "CIE1986"/>

:<math>h_{uv} = \mathrm{atan2}(v^*, u^*) =  \mathrm{atan2}(v', u'),</math>

where, [[atan2]] is a two-argument inverse tangent.

=== Defining hue in terms of RGB ===

[[File:HSV cone.jpg|thumb|HSV color space as a conical object]]
[[File:HSV-RGB-comparison.svg|thumb|right|An illustration of the relationship between the "hue" of colors with maximal saturation in HSV and HSL with their corresponding RGB coordinates]]
[[File:RGB color wheel 24.svg|thumb|Hue circle in 24 colors (15°)]]

Preucil<ref name="Preucil1953">Frank Preucil, "Color Hue and Ink Transfer … Their Relation to Perfect Reproduction", ''TAGA Proceedings,'' p 102-110 (1953). [TAGA article #T530102, paid registration required from [https://www.printing.org/communities/interest-groups/taga TAGA]]</ref> describes a color hexagon, similar to a trilinear plot described by Evans, Hanson, and Brewer,<ref name = "Evans1953">Ralph Merrill Evans, W T Hanson, and W Lyle Brewer, ''Principles of Color Photography.'' New York: Wiley, 1953</ref> which may be used to compute hue from [[RGB color model|RGB]]. To place [[red]] at 0°, [[green]] at 120°, and [[blue]] at 240°,

:<math> h_{rgb} = \mathrm{atan2}\left( \sqrt{3} \cdot (G - B), 2 \cdot R - G - B \right). </math>

Equivalently, one may solve
:<math> \tan( h_{rgb}) = \frac{\sqrt{3}\cdot (G - B)}{2\cdot R - G - B}. </math>

Preucil used a polar plot, which he termed a color circle.<ref name = "Preucil1953"/> Using R, G, and B, one may compute hue angle using the following scheme: determine which of the six possible orderings of R, G, and B prevail, then apply the formula given in the table below.

{| class="wikitable"
|-
! Ordering
! Hue region
! <math>h_\text{Preucil circle}</math>
|-
| <math>R \ge G \ge B</math>
| Orange
| <math>60^\circ \cdot \frac{G - B}{R - B}</math>
|-
| <math>G > R \ge B</math>
| Chartreuse 
| <math>60^\circ \cdot \left( 2 - \frac{R - B}{G - B}\right)</math>
|-
| <math>G \ge B > R</math>
| Spring Green
| <math>60^\circ \cdot \left( 2 + \frac{B - R}{G - R}\right)</math>
|-
| <math>\ B > G > R\ </math>
| Azure
| <math>60^\circ \cdot \left( 4 - \frac{G - R}{B - R}\right)</math>
|-
| <math>B > R \ge G</math>
| Violet
| <math>60^\circ \cdot \left( 4 + \frac{R - G}{B - G}\right)</math>
|-
| <math>R \ge B > G</math>
| Rose
| <math>60^\circ \cdot \left( 6 - \frac{B - G}{R - G}\right)</math>
|}

In each case the formula contains the fraction <math>\frac{M - L}{H - L}</math>, where ''H'' is the highest of R, G, and B; ''L'' is the lowest, and ''M'' is the mid one between the other two. This is referred to as the "Preucil hue error" and was used in the computation of mask strength in photomechanical color reproduction.<ref name = "Southworth">Miles Southworth, ''Color Separation Techniques'', second edition. Livonia, New York: Graphic Arts Publishing, 1979.</ref>

Hue angles computed for the Preucil circle agree with the hue angle computed for the Preucil hexagon at integer multiples of 30° (red, yellow, green, cyan, blue, magenta, and the colors midway between contiguous pairs) and differ by approximately 1.2° at odd integer multiples of 15° (based on the circle formula), the maximal divergence between the two.

The process of converting an RGB color into an [[HSL and HSV|HSL or HSV color space]] is usually based on a 6-piece piecewise mapping, treating the HSV cone as a [[hexagonal pyramid|hexacone]], or the HSL double cone as a double hexacone.<ref>{{cite book | title = Computer Graphics and Geometric Modeling v. 1: Implementation and Algorithms | author = Max K. Agoston | isbn = 1-85233-818-0 | publisher = Springer | year = 2004 | url = https://books.google.com/books?id=fGX8yC-4vXUC&q=hsv++hue+rgb&pg=PA301 | pages = 301–304 | url-status = live | archive-url = https://web.archive.org/web/20170321080302/https://books.google.com/books?id=fGX8yC-4vXUC&pg=PA301&lpg=PA301&dq=hsv++hue+rgb#PPA304,M1 | archive-date = 2017-03-21 }}</ref>  The formulae used are those in the table above.

<gallery mode=packed caption="Additional images for hue in the HSL and HSV systems">
File:HueScale.svg|Hue in the [[HSL and HSV|HSL/HSV]] encodings of [[RGB color model|RGB]]
File:Hue shift six photoshop.jpg|An image with the hues cyclically shifted in HSL space
File:Hue.gif|The hues in this image of a [[painted bunting]] are cyclically rotated over time in HSL.
</gallery>

One might notice that the HSL/HSV hue "circle" does not appear to all be of the same [[lightness]]. This is a known issue of this RGB-based derivation of hue.<ref>{{cite conference |url=http://www.personal.psu.edu/cab38/ColorSch/ASApaper.html |title=Color Use Guidelines for Data Representation |first=Cynthia A. |last=Brewer |date=1999 |book-title=Proceedings of the Section on Statistical Graphics |location=Alexandria, VA |publisher=American Statistical Association |pages=55–60 |access-date=2024-01-03 |archive-date=2009-08-07 |archive-url=https://web.archive.org/web/20090807105257/http://www.personal.psu.edu/cab38/ColorSch/ASApaper.html |url-status=dead }}</ref>

== Usage in art ==

Manufacturers of pigments use the word hue, for example, "cadmium yellow (hue)" to indicate that the original pigmentation ingredient, often toxic, has been replaced by safer (or cheaper) alternatives whilst retaining the hue of the original.  Replacements are often used for [[chromium]], [[cadmium]] and [[alizarin]].

== Hue vs. dominant wavelength ==

[[Dominant wavelength]] (or sometimes equivalent wavelength) is a physical analog to the perceptual attribute hue. On a [[chromaticity diagram]], a line is drawn from a [[white point]] through the coordinates of the color in question, until it intersects the [[CIE 1931 color space#The CIE xy chromaticity diagram|spectral locus]]. The wavelength at which the line intersects the spectrum locus is identified as the color's [[dominant wavelength]] if the point is on the same side of the white point as the spectral locus, and as the color's [[complementary wavelength]] if the point is on the opposite side.<ref name = "Judd1976">Deane B Judd and Günter Wyszecki, ''Color in Business, Science, and Industry.'' New York: Wiley, 1976.</ref>

{{anchor|Hue difference: dh or dH*}}

== Hue difference notation ==

There are two main ways in which hue difference is quantified. The first is the simple difference between the two hue angles. The symbol for this expression of hue difference is <math>\Delta h_{ab}</math> in CIELAB and <math>\Delta h_{uv}</math> in CIELUV. The other is computed as the residual total color difference after Lightness and Chroma differences have been accounted for; its symbol is <math>\Delta H^*_{ab}</math> in CIELAB and <math>\Delta H^*_{uv}</math> in CIELUV.

== Names and other notations ==
There exists some correspondence, more or less precise, between hue values and [[color term]]s (names). One approach in color science is to use traditional color terms but try to give them more precise definitions. See [[spectral color#Spectral color terms]] for names of highly saturated colors with the hue from ≈ 0° (red) up to ≈ 275° (violet), and [[line of purples#Table of highly-saturated purple colors]] for color terms of the remaining part of the color wheel.

Alternative approach is to use a systematic notation. It can be a standard [[angle]] notation for certain color model such as HSL/HSV mentioned above, [[CIELUV]], or [[CIECAM02]]. Alphanumeric notations such as of [[Munsell color system]], [[Natural Color System|NCS]], and [[Pantone Matching System]] are also used.

== See also ==
* [[Lightness (color)]]
* [[Chromaticity]]
* [[Munsell color system]]
* [[Bezold–Brücke shift]]

== References ==
{{Reflist}}

== External links ==
* [http://gimps.de/en/tutorials/gimp/picture-photo-image/improve-colors/ Editing of hue in photography]

{{Color topics}}
{{Authority control}}

[[Category:Color]]