Hue
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2008-07-07T22:42:38Z
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[[Image:HueScale.svg|thumb|right|360px|Hue spectrum]]
[[Image:Hue shift six photoshop.jpg|thumb|An image with the hues cyclically shifted in HSL space.]]
[[Image:Hue.gif|thumb|The hues in the image of this [[Painted Bunting]] are cyclically rotated with time.]]
Hue is one of the main property of a [[color]] described with names such as "[[red]]", "[[yellow]]", etc. The two other main properites are [[lightness (color)|lightness]] and [[colorfulness]]. Hue is also one of the three dimensions in some [[colorspaces]] along with [[saturation (color theory)|saturation]], and lightness.
Usually, colors with the same hue are distinguished with adjectives referring to their lightness and/or [[chroma]], such as with "light blue", "pastel blue", "vivid blue". Exceptions include [[brown]], which is a dark [[Orange (colour)|orange]],<ref name="Bartleson1976">C J Bartleson, "Brown". ''Color Research and Application,'' '''1''' : 4, p 181-191 (1976).</ref> and [[pink]], a light red with reduced chroma.
In [[painting]] [[color theory]], a '''hue''' refers to a ''pure'' color—one without [[Tints and shades|tint or shade]] (added white or black [[pigment]], respectively).{{Fact|date=August 2007}} A hue is an element of the [[color wheel]].
== Computing hue ==
In [[opponent process|opponent color spaces]], such as [[CIELAB|CIE ''L*a*b*'']] (CIELAB) and [[CIELUV color space|CIE ''L*u*v*'']] (CIELUV), hue may be computed together with saturation (or chroma) by converting a color's [[chromaticity]] coordinates from [[rectangular coordinates]] into [[polar coordinates]]. 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>\tan h_{ab} = \frac{b^*}{a^*}</math>
while, analogously, in CIELUV:<ref name = "CIE1986"/>
:<math>\tan h_{uv} = \frac{v^*}{u^*} = \frac{v'}{u'}</math>
In practice, a four-quadrant [[Arctangent#Two argument variant of arctangent|arctangent]] may be used if available to invert these formulae.
=== Computing hue from RGB ===
Preucil<ref name = "Preucil1953">Frank Preucil, "Color Hue and Ink Transfer … Their Relation to Perfect Reproduction, ''TAGA Proceedings,'' p 102-110 (1953).</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]]. To place red at 0°, [[green]] at 120°, and [[blue]] at 240°, one may solve:
:<math>\tan h_{Preucil\ hexagon} = \frac{\sqrt{3}\cdot (G - B)}{2\cdot R - G - B}</math>
He also used a polar plot, which he termed a color circle.<ref name = "Preucil1953"/> Using R, G, and B, rather than the R, G, and B densities Preucil used, 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 appropriate formula; see table below.
[[Image:HSV cone.jpg|thumb|HSV color space as a conical object]]
[[Image: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.]]
{| class="wikitable"
|-
! Ordering
! Hue Region
! Formula
|-
| <math>R \ge G \ge B</math>
| Red-Yellow
| <math>h_{Preucil\ circle} = 60^{\circ} \cdot \frac{G - B}{R - B}</math>
|-
| <math>G > R \ge B</math>
| Yellow-Green
| <math>h_{Preucil\ circle} = 60^{\circ} \cdot \left( 1 + \frac{R - B}{G - B}\right)</math>
|-
| <math>G \ge B > R</math>
| Green-Cyan
| <math>h_{Preucil\ circle} = 60^{\circ} \cdot \left( 2 + \frac{B - R}{G - R}\right)</math>
|-
| <math>\ B > G > R\ </math>
| Cyan-Blue
| <math>h_{Preucil\ circle} = 60^{\circ} \cdot \left( 3 + \frac{G - R}{B - R}\right)</math>
|-
| <math>B > R \ge G</math>
| Blue-Magenta
| <math>h_{Preucil\ circle} = 60^{\circ} \cdot \left( 4 + \frac{R - G}{B - G}\right)</math>
|-
| <math>R \ge B > G</math>
| Magenta-Red
| <math>h_{Preucil\ circle} = 60^{\circ} \cdot \left( 5 + \frac{B - G}{R - G}\right)</math>
|}
Note that 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 degrees (red, yellow, green, cyan, blue, magenta, and the colors mid-way between contiguous pairs), and differ by approximately 1.2 degrees at odd integer multiples of 15 degrees (based on the circle formula), the maximum divergence between the two.
The process of converting an RGB color into an [[HSL color space]] or [[HSV color space]] is usually based on a 6-piece piecewise mapping, treating the HSV cone as a hexacone, or the HSL double cone as a double hexacone.<ref>{{cite book | title = Computer Graphics and Geometric Modelling v. 1: Implementation and Algorithms | author = Max K. Agoston | isbn = 1852338180 | publisher = Springer | year = 2004 | url = http://books.google.com/books?vid=ISBN1852338180&id=fGX8yC-4vXUC&pg=PA301&lpg=PA301&ots=w8cDX3NhWs&dq=hsv++hue+rgb&sig=apoei-VxRxFVMLXedUukOW5OZYE#PPA304,M1 | pages = 301–304}}</ref> The formulae used are those in the table above.
== Specialized hues ==
The hues exhibited by [[caramel|caramel colorings]] and [[beer]]s are fairly limited in range. The [[Linner hue index]] is used to quantify the hue of such products.
== Hue as a qualification in the names of artist's colors ==
Manufacturers of pigments use the word hue e.g. '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>
== References ==
{{reflist}}
==See also==
*[[Lightness (color)]]
*[[Colorfulness]]
*[[Chromaticity]]
*[[Munsell color system]]
*[[Bezold-Brücke shift]]
==External links==
*[http://gimps.de/en/tutorials/gimp/picture-photo-image/improve-colors/ Editing of hue in photography]
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