Editing Colorfulness

{{short description|Perceived intensity of a specific color}}
{{Use American English|date=March 2021}}
{{Use mdy dates|date=March 2021}}
[[File:Brightness and colorfulness.png|thumb|The red stripe exhibits higher brightness and colorfulness in the light than in the shadow, but is seen as having the same object color, including the same chroma, in both areas. Because the brightness increases proportionately to the colorfulness, the stripe also exhibits similar saturation in both areas.|300x300px]]
[[File:Saturation of digital colors.png|thumb|right|300x300px|7.5PB and 10BG Munsell hue pages of RGB colors, showing lines of uniform saturation (chroma in proportion to lightness) in red. Lines of uniform saturation radiate from near the black point, while lines of uniform chroma are vertical. Also compared to the 10BG colors, the 7.5PB colors attain higher saturation as well as higher chroma.]]
{{multiple image
| direction = vertical
| width = 220
| image1 = Surfing in Hawaii unmodified.jpg|caption1 = Original image, with relatively muted colors
| image2 = Surfing in Hawaii+50 LCh chroma.jpg|caption2 = L*C*h ([[CIELAB]]) chroma increased 50%
| image3 = Surfing in Hawaii+50 saturation.jpg|caption3 = [[HSL and HSV|HSL]] saturation increased 50%; changing HSL saturation also affects the perceived lightness of a color
| image4 = Surfing in Hawaii L* channel.jpg|caption4 = CIELAB lightness preserved, with ''a''* and ''b''* stripped, to make a [[grayscale]] image
}}
[[File:Red saturations.svg|thumb|right|Saturation scale (0% at left, corresponding to black and white)]]
[[File:Saturation change photoshop.jpg|alt=Examples of saturation.Top left = original image.|thumb|Examples of saturation. Original image at top left.]]

'''Colorfulness''', '''chroma''' and '''saturation''' are attributes of perceived color relating to chromatic intensity. As defined formally by the [[International Commission on Illumination]] (CIE) they respectively describe three different aspects of chromatic intensity, but the terms are often used loosely and interchangeably in contexts where these aspects are not clearly distinguished. The precise meanings of the terms vary by what other functions they are dependent on.

* '''Colorfulness''' is the "attribute of a visual perception according to which the perceived color of an area appears to be more or less chromatic (Any color that is absent of white, grey, or black){{clarify|date=July 2019}}".<ref>{{Cite web|url=http://eilv.cie.co.at/term/233|title=colourfulness {{!}} eilv|website=eilv|language=en|access-date=2017-12-20|url-status=live|archive-url=https://web.archive.org/web/20170806140347/http://eilv.cie.co.at/term/233|archive-date=2017-08-06}}</ref><ref name=":0">{{Cite book|title=Color Appearance Models|last=Fairchild|first=Mark|publisher=[[John Wiley & Sons]]|year=2013}}, page 87.</ref> The colorfulness evoked by an object depends not only on its spectral [[reflectance]] but also on the strength of the illumination, and increases with the latter unless the [[brightness]] is very high ([[Hunt effect (color)|Hunt effect]]).
* '''Chroma''' is the "colorfulness of an area judged as a proportion of the brightness of a similarly illuminated area that appears white or highly transmitting".<ref>{{Cite web|url=http://eilv.cie.co.at/term/139|title=CIE e-ILV 17-139|archive-url=https://web.archive.org/web/20170410214923/http://eilv.cie.co.at/term/139|archive-date=2017-04-10|url-status=live}}</ref><ref name=":0" /> As a result, chroma is mostly only dependent on the spectral properties, and as such is seen to describe the '''object color'''.<ref>{{Cite web|url=http://eilv.cie.co.at/term/831|title=CIE e-ILV 17-831|archive-url=https://web.archive.org/web/20170410215355/http://eilv.cie.co.at/term/831|archive-date=2017-04-10|url-status=live}}</ref> It is how different from a grey ''of the same lightness'' such an object color appears to be.<ref>{{Cite web|url=http://www.huevaluechroma.com/015.php|title=The Dimensions of Colour|website=www.huevaluechroma.com|access-date=2017-04-10|url-status=live|archive-url=https://web.archive.org/web/20170330104913/http://www.huevaluechroma.com/015.php|archive-date=2017-03-30}}</ref>
* '''Saturation''' is the "colorfulness of an area judged in proportion to its [[brightness]]",<ref>{{Cite web|url=http://eilv.cie.co.at/term/1136|title=CIE e-ILV 17-1136|archive-url=https://web.archive.org/web/20170410214912/http://eilv.cie.co.at/term/1136|archive-date=2017-04-10|url-status=live}}</ref><ref name=":0" /> which in effect is the perceived freedom from whitishness of the light coming from the area. An object with a given spectral reflectance exhibits approximately constant saturation for all levels of illumination, unless the brightness is very high.<ref name=":1">{{Cite web|url=http://www.huevaluechroma.com/017.php|title=The Dimensions of Colour|website=www.huevaluechroma.com|access-date=2017-04-10|url-status=live|archive-url=https://web.archive.org/web/20170330104936/http://www.huevaluechroma.com/017.php|archive-date=2017-03-30}}</ref>

As colorfulness, chroma, and saturation are defined as attributes of perception, they can not be physically measured as such, but they can be quantified in relation to psychometric scales intended to be perceptually even—for example, the chroma scales of the [[Munsell color system|Munsell system]]. While the chroma and lightness of an object are its colorfulness and brightness judged in proportion to the same thing ("the brightness of a similarly illuminated area that appears white or highly transmitting"), the saturation of the light coming from that object is in effect the chroma of the object judged in proportion to its lightness. On a [[Munsell color system|Munsell]] hue page, lines of uniform saturation thus tend to radiate from near the black point, while lines of uniform chroma are vertical.<ref name=":1" />

== Chroma ==
{{See also|Psychophysics}}

The naïve definition of saturation does not specify its response function. In the CIE XYZ and RGB color spaces, the saturation is defined in terms of additive color mixing, and has the property of being proportional to any scaling centered at white or the white point illuminant. However, both color spaces are non-linear in terms of psychovisually perceived [[color difference]]s. It is also possible — and sometimes desirable — to define a saturation-like quantity that is linearized in term of the psychovisual perception.

In the [[CIELAB color space|CIE 1976 LAB and LUV color spaces]], the unnormalized '''chroma''' is the radial component of the cylindrical coordinate [[HCL color space|CIE LCh]] (lightness, chroma, hue) representation of the LAB and LUV color spaces, also denoted as CIE LCh(ab) or CIE LCh for short, and CIE LCh(uv). The transformation of <math>(a, b)</math> to <math>\left(C_{ab}, h_{ab}\right)</math> is given by:
<math display=block>C_{ab}^* = \sqrt{a^{*2} + b^{*2}}</math>
<math display=block>h_{ab} = \operatorname{atan2}\left({b^\star},{a^\star}\right)</math>
and analogously for CIE LCh(uv).

The chroma in the CIE LCh(ab) and CIE LCh(uv) coordinates has the advantage of being more psychovisually linear, yet they are non-linear in terms of linear component color mixing. And therefore, chroma in CIE 1976 Lab and LUV color spaces is very much different from the traditional sense of "saturation".

===In color appearance models===

Another, psychovisually even more accurate, but also more complex method to obtain or specify the saturation is to use a [[color appearance model]] like CIECAM02. Here, the '''chroma''' color appearance parameter might (depending on the color appearance model) be intertwined with e.g. the physical brightness of the illumination or the characteristics of the emitting/reflecting surface, which is more sensible psychovisually.

The CIECAM02 chroma <math>C,</math> for example, is computed from a lightness <math>J</math> in addition to a naively evaluated color magnitude <math>t.</math> In addition, a colorfulness <math>M</math> parameter exists alongside the chroma <math>C.</math> It is defined as <math>M = CF_B^{0.25},</math> where <math>F_L</math> is dependent on the viewing condition.<ref name=ciecam02/>

== Saturation ==
The saturation of a color is determined by a combination of light intensity and how much it is distributed across the spectrum of different wavelengths. The purest (most saturated) color is achieved by using just one wavelength at a high intensity, such as in laser light. If the intensity drops, then as a result the saturation drops.  To desaturate a color of given intensity in a [[Subtractive color|subtractive]] system (such as [[watercolor]]), one can add white, black, [[Grey|gray]], or the hue's [[Complementary color|complement]].

Various correlates of saturation follow.

=== CIELUV and CIELAB ===

In [[CIELUV]], saturation is equal to the ''chroma'' normalized by the ''lightness'':
<math display=block>s_{uv} = \frac{C^*_{uv}}{L^*} = 13 \sqrt{(u' - u'_n)^2 + (v' - v'_n)^2}</math>
where <math>\left(u_n, v_n\right)</math> is the chromaticity of the white point, and chroma is defined below.<ref>{{Cite book|last=Schanda|first=János|title=Colorimetry: Understanding the CIE System|publisher=[[Wiley Interscience]]|year=2007|isbn=978-0-470-04904-4|url=https://books.google.com/books?id=g8VDAgAACAAJ|url-status=live|archive-url=https://web.archive.org/web/20170117120237/https://books.google.com/books?id=g8VDAgAACAAJ&dq=intitle:Colorimetry+intitle:Understanding+intitle:the+intitle:CIE+intitle:System|archive-date=2017-01-17}}, page 88.</ref>

By analogy, in [[CIELAB]] this would yield:
<math display=block>s_{ab} = \frac{C^*_{ab}}{L^*} = \frac{\sqrt{{a^*}^2 + {b^*}^2}}{L^*}</math>

The CIE has not formally recommended this equation since CIELAB has no chromaticity diagram, and this definition therefore lacks direct connection with older concepts of saturation.<ref>{{cite book|url=https://archive.org/details/focalencyclopedi00lesl|url-access=registration|title=The Focal Encyclopedia of Photography|publisher=Focal Press|editor=Leslie D. Stroebel, Richard D. Zakia|isbn=0-240-51417-3|year=1993|page=[https://archive.org/details/focalencyclopedi00lesl/page/124 124]|first=Robert William Gainer|last=Hunt}}</ref> Nevertheless, this equation provides a reasonable predictor of saturation, and demonstrates that adjusting the lightness in CIELAB while holding {{nobr|(''a''*, ''b''*)}} fixed does affect the saturation.

But the following verbal definition of Manfred Richter and the corresponding formula proposed by Eva Lübbe are in agreement with the human perception of saturation:
Saturation is the proportion of pure chromatic color in the total color sensation.<ref>{{Cite book|last=Lübbe|first=Eva|title=Colours in the Mind - Colour Systems in Reality- A formula for colour saturation|publisher=[Book on Demand]|year=2010|isbn=978-3-7881-4057-1 }}</ref>
<math display=block>S_{ab} = \frac{C^*_{ab}}{\sqrt{{C^*_{ab} }^2 + {L^*}^2}} 100\%</math>
where <math>S_{ab}</math> is the saturation, <math>L^*</math> the  lightness and <math>C^*_{ab}</math> is the chroma of the color.

=== CIECAM02 ===

In [[CIECAM02]], saturation equals the square root of the ''colorfulness'' divided by the ''brightness'':
<math display=block>s = \sqrt\frac{M}{Q}</math>

This definition is inspired by experimental work done with the intention of remedying [[CIECAM97s]]'s poor performance.<ref name=ciecam02>{{cite conference|conference=The CIECAM02 Color Appearance Model|title=IS&T/SID Tenth Color Imaging Conference|last=Moroney|first=Nathan|author2=Fairchild, Mark D. |author3=Hunt, Robert W.G. |author4=Li, Changjun |author5=Luo, M. Ronnier |author6= Newman, Todd |url=http://www.polybytes.com/misc/Meet_CIECAM02.pdf|location=[[Scottsdale, Arizona]]|isbn=0-89208-241-0|date=November 12, 2002|publisher=The [[Society for Imaging Science and Technology]]|url-status=dead|archive-url=https://web.archive.org/web/20111002195211/http://www.polybytes.com/misc/Meet_CIECAM02.pdf|archive-date=Oct 2, 2011}}
</ref><ref>{{cite conference|title=Magnitude estimation for scaling saturation|first=Lu-Yin G.|last=Juan|author2=Luo, Ming R.|conference-url=http://spiedl.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PSISDG004421000001000575000001&idtype=cvips&gifs=yes| work=[[Proceedings of SPIE]]|volume=4421|conference=9th Congress of the International Colour Association|editor=Robert Chung |editor2=Allan Rodrigues|date=June 2002|pages=575–578|doi=10.1117/12.464511}}</ref> <math>M</math> is proportional to the chroma <math>C,</math> thus the CIECAM02 definition bears some similarity to the CIELUV definition.<ref name=ciecam02/>

=== HSL and HSV===

Saturation is also one of three coordinates in the [[HSL and HSV]] [[color space]]s. However, in the HSL color space saturation exists independently of lightness. That is, both a very light color {{em|and}} a very dark color can be heavily saturated in HSL; whereas in the previous definitions&mdash;as well as in the HSV color space&mdash;colors approaching white all feature low saturation.

== Excitation purity ==

[[Image:Excitation Purity.svg|300px|thumb|right|'''Excitation purity''' is the relative distance from the white point. Contours of constant purity can be found by shrinking the spectral locus about the white point. The points along the line segment have the same hue, with p<sub>e</sub> increasing from 0 to 1 between the white point and position on the [[Spectral color|spectral locus]] (position of the color on the horseshoe shape in the diagram) or (as at the saturated end of the line shown in the diagram) position on the [[line of purples]].]]

The '''excitation purity''' (purity for short) of a stimulus is the difference from the illuminant's [[white point]] to the furthest point on the chromaticity diagram with the same [[dominant wavelength]]; using the [[CIE 1931 color space]]:<ref>{{cite book|isbn=0-240-51417-3|url=https://archive.org/details/focalencyclopedi00lesl|url-access=registration|quote=excitation purity.|title=The Focal Encyclopedia of Photography|publisher=[[Focal Press]]|first=Leslie D.|last=Stroebel|author2=Zakia, Richard D.|page=[https://archive.org/details/focalencyclopedi00lesl/page/121 121]|year=1993|edition=3E}}</ref>
<math display=block>p_e = \sqrt{\frac{\left(x - x_n\right)^2 + \left(y - y_n\right)^2}{\left(x_I - x_n\right)^2 + \left(y_I - y_n\right)^2}}</math>
where <math>\left(x_n, y_n\right)</math> is the chromaticity of the white point and <math>\left(x_I, y_I\right)</math> is the point on the perimeter whose line segment to the white point contains the chromaticity of the stimulus. Different color spaces, such as CIELAB or CIELUV may be used, and will yield different results.

== References ==

{{reflist|30em}}

{{Color topics}}

[[Category:Color]]