HCL (Hue-Chroma-Luminance) or Lch refers to any of the many cylindrical color space models that are designed to accord with human perception of color with the three parameters. Lch has been adopted by information visualization practitioners to present data without the bias implicit in using varying saturation.[1][2][3] They are, in general, designed to have characteristics of both cylindrical translations of the RGB color space, such as HSL and HSV, and the L*a*b* color space. Some conflicting definitions of the terms are:
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- A name for a cylindrical transformation of CIELuv (CIE Lch(uv)) employed by Ihaka (2003)[1] and adopted by Zeileis et al. (2009).[2] This name appears to be the one most commonly used in information visualization. Ihaka, Zeileis, and co-authors also provide software implementations and web pages to promote its use.[4]
- A name for cylindrical CIELab (CIE Lch(ab)), employed by chroma.js.
- 'HCL' designed in 2005 by Sarifuddin and Missaou, which is a transformation of whatever type of RGB color space is in use.[5]
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Derivation[edit]
Color-making attributes[edit]
HCL concerns the following attributes of color appearance:[A]
- Hue
- The 'attribute of a visual sensation according to which an area appears to be similar to one of the perceived colors: red, yellow, green, and blue, or to a combination of two of them'.[6]
- Lightness, value
- The 'brightness relative to the brightness of a similarly illuminated white'.[6]
- Luminance (Y or Lv,Ω)
- The radiance weighted by the effect of each wavelength on a typical human observer, measured in SI units in candela per square meter (cd/m2). Often the term luminance is used for the relative luminance, Y/Yn, where Yn is the luminance of the reference white point.
- Colorfulness
- The 'attribute of a visual sensation according to which the perceived color of an area appears to be more or less chromatic'.[6]
The HSL and HSV color spaces are more intuitive translations of the RGB color space, because they provide a single hue number. However, their luminance variation does not match the way humans perceive color. Perceptually uniform color spaces outperform RGB in cases such as high noise environments.[7]
CIE color spaces[edit]
CIE-based Lch color spaces are transformations of the two chroma values (ab or uv) into the polor coordinate. The source color spaces are still very well-regarded for their uniformity, and the transformation does not cause degradation in this aspect. See the respective articles for how the underlying coordinates are derived.
Sarifuddin 2005[edit]
Sarifuddin, noting the lack of blue hue consistency of CIELAB—a common complaint among its users—[8]decided to make their own color space by mashing up some of the features.[5]
According to the Stack Overflow user Tatarize, what Sarifuddin proposes as 'HCL' is algorithmically similar to HSL. While pointing out advantages in computational efficiency, they argue that Sarifuddin's work does not represent a significant improvement over the CIELAB color space.[9][10] They also propose what they consider to be an improved version of Sarifuddin's algorithm.[11][original research]
Other color appearance models[edit]
In general, any color appearance model with a lightness and two chroma components can also be transformed into a HCL-type color space by turning the chroma components into polar coordinates.
Implementations[edit]
CIE Lch has been implemented in a wide range of ways: as programmatic code for generating color swatches in statistics tools, as standalone tools for designing and testing swatches, or as libraries that allow other programs to use the color space. Some implementations include:
- Statistical tools:
- d3.js: Data Driven Documents JavaScript library (CIE Lch[ab])
- Swatch designs:
- The colorspace package, for the R statistical programming language (CRAN) and for the Python language (Documentation). Also comes with pre-made sets of swatches in hclwizard.
- The scientific color maps, a set of pre-made swatches.
- Library:
- The aforementioned colorspace library (CIE Lch[uv]).
- ac-colors JavaScript library (CIE Lch[ab] and CIE Lch[uv]).
- chroma.js JavaScript library (CIE Lch[ab]).
- colorio for Python.
- Most other color space libraries handle at least one of CIE LUV or CIE LAB.
References[edit]
- ^'Clearly, if color appearance is to be described in a systematic, mathematical way, definitions of the phenomena being described need to be precise and universally agreed upon.'[6]
- ^ abIhaka, Ross (2003). 'Colour for Presentation Graphics'. In Hornik, Kurt; Leisch, Friedrich; Zeileis, Achim (eds.). Proceedings of the 3rd International Workshop on Distributed Statistical Computing, Vienna, Austria. ISSN1609-395X.
- ^ abZeileis, Achim; Hornik, Kurt; Murrell, Paul (2009). 'Escaping RGBland: Selecting Colors for Statistical Graphics'. Computational Statistics & Data Analysis. 53 (9): 3259–3270. doi:10.1016/j.csda.2008.11.033.
- ^Stauffer, Reto; Mayr, Georg J.; Dabernig, Markus; Zeileis, Achim (2015). 'Somewhere over the Rainbow: How to Make Effective Use of Colors in Meteorological Visualizations'. Bulletin of the American Meteorological Society. 96 (2): 203–216. Bibcode:2015BAMS...96..203S. doi:10.1175/BAMS-D-13-00155.1. hdl:10419/101098.
- ^Zeileis, Achim; Fisher, Jason C.; Hornik, Kurt; Ihaka, Ross; McWhite, Claire D.; Murrell, Paul; Stauffer, Reto; Wilke, Claus O. (2019). 'Colorspace: A Toolbox for Manipulating and Assessing Colors and Palettes'. arXiv:1903.06490 [stat.CO].
- ^ abSarifuddin, M. & Missaoui, Rokia (2005). A New Perceptually Uniform Color Space with Associated Color Similarity Measure for Content-Based Image and Video Retrieval(PDF). Multimedia Information Retrieval Workshop, 28th Annual ACM SIGIR Conference.. Abstract/long-form corrected report
- ^ abcdFairchild (2005), pp. 83–93
- ^Paschos, G. (2001). 'Perceptually Uniform Color Spaces for Color Texture Analysis: An Empirical Evaluation'. IEEE Transactions on Image Processing. 10 (6): 932–937. Bibcode:2001ITIP...10..932P. doi:10.1109/83.923289.
- ^McLellan, M. R.; Lind, L. R.; Kime, R. W. (1995). 'Hue Angle Determinations and Statistical Analysis for Multiquadrant Hunter L,a,b Data'. Journal of Food Quality. 18 (3): 235–240. doi:10.1111/j.1745-4557.1995.tb00377.x.
- ^tatarize. 'HCL color to RGB and backward'. Stack Overflow.
- ^Tatarize (4 September 2012). 'HCL: a new Color Space for a pack of lies'. Ssnot!. Retrieved 22 May 2019.
- ^'algorithm - HCL color to RGB and backward'. Stack Overflow. Retrieved 2020-12-08.
External links[edit]
In this section we have a look at the terminology of color properties and their meaning in different contexts. Color properties allow us to distinguish and define colors. The more we know about color properties, the better we can adjust colors to our needs.
Hue
Hue defines pure color in terms of 'green', 'red' or 'magenta'. Hue also defines mixtures of two pure colors like 'red-yellow' (~ 'orange'), or 'yellow-green' (limitations to this statement will be addressed later).
Hue is usually one property of three when used to determine a certain color.
Hue is a more technical definition of our color perception which can be used to communicate color ideas.
Hue ranges from 0° to 359° when measured in degrees.
Hues are basic colors we learn to connect with words as children.
Hues can refer to the set of 'pure' colors within a color space.
Tint
Tint is a color term commonly used by painters.
A tint is a mixing result of an original color to which has been added white.
If you tinted a color, you've been adding white to the original color.
A tint is lighter than the original color.
When used as a dimension of a color space, tint can be the amount of white added to an original color. In such a color space a pure color would be non-tinted.
Other usage / meanings of tint:
* A soft touch or shimmer of a different hue!
* Hair color which doesn't fully cover natural hair color. It adds a touch of color which is supposed to wash out within five to eight weeks.
* Car Window Tint: means of changing the color/transparency of car windows.
Shade
Shade is a color term commonly used by painters.
A shade is a mixing result of an original color to which has been added black.
If you shaded a color, you've been adding black to the original color.
A shade is darker than the original color.
When used as a dimension of a color space, shade can be the amount of black added to an original color. In such a color space a pure color would be non-shaded.
Tone
Tone is a color term commonly used by painters.
There is a broader and a narrower definition of tone.
The broader definition defines tone as a result of mixing a pure color with any neutral/grayscale color including the two extremes white and black. By this definition all tints and shades are also considered to be tones.
The narrower definition defines tone as a result of mixing a pure color with any grayscale color excluding white and black. By this definition a certain amount of white and black must have been added to the original color. Furthermore the following is true: If you changed the tonal value of a color, you've been adding gray (any ratio of mixture) to the original color.
A tone is softer than the original color.
Tone is not used as a dimension of a color space. Instead, the tonal difference consists of the amounts of white and/or black used to determine a certain color.
Exception:
Tone as a result of mixing an original color with a hue-scale color (e.g. brownscale / sepia).
Saturation
Saturation is a color term commonly used by (digital / analog) imaging experts.
Saturation is usually one property of three when used to determine a certain color and measured as percentage value.
Saturation defines a range from pure color (100%) to gray (0%) at a constant lightness level. A pure color is fully saturated.
From a perceptional point of view saturation influences the grade of purity or vividness of a color/image. A desaturated image is said to be dull, less colorful or washed out but can also make the impression of being softer.
We will clear up the term saturation from a color mixing point of view in the color spaces section.
Lightness
Lightness is a color term commonly used by (digital / analog) imaging experts.
Lightness is usually one property of three when used to determine a certain color and measured as percentage value.
Lightness defines a range from dark (0%) to fully illuminated (100%). Any original hue has the average lightness level of 50%.
A painter might say lightness is the range from fully shaded to fully tinted.
You can lighten or darken a color by changing its lightness value.
Chromatic Signal / Chromaticity / Chroma
This family of color terms is commonly used by (digital / analog) imaging and video experts.
In the previous section we learned that color perception is a result of achromatic and chromatic signals.
We can therefore define a chromatic signal as the component of color perception that is not achromatic, i.e. any deviation from neutral-color perception (dark, grayscale, illuminated).
The chromatic intensity or chromaticity is the intensity of the chromatic signal contributing to color perception. Chromaticity is similar to saturation since
Chroma is a component of a color model. There's a blue-yellow and a red-green chroma component.
Intensity / Luminosity / Luma
In general, intensity is a synonym for magnitude, degree or strength. It can therefore be used in conjunction with any color property. Nevertheless, it carries special meaning in certain contexts.
For painters the meaning of intensity is equivalent to the meaning of saturation.
For physicists intensity refers to different aspects of radiation.
When speaking of light, the intensity can mean the number of photons a light source emits.
The following sources provide a deeper insight:
- Luminosity
- Intensity
- Luminosity Function
- Lumen
Luma (%) is the intensity of the achromatic signal contributing to our color perception.
Chroma For Hue Color
Brightness / (relative) Luminance
Brightness is an attribute of our perception which is mainly influenced by a color's lightness. This is probably why brightness and lightness are often mixed up. Brightness is not a color property, if used 'correctly'.
For one color of specific hue the perception of brightness is also more intense, if we increase saturation. A higher level of saturation makes a color look brighter.
In relation to other colors the brightness intensity of a color is also influenced by its hue. We can then speak of (relative) luminance to refer to brightness.
It's very important to know more about luminance.
Grayscale
A grayscale is a series of neutral colors, ranging from black to white, or the other way around. Each step's color value is usually shifted by constant amounts.
A grayscale color can be determined by a value of a one-dimensional color space:
On a white surface (e.g. paper) the grayscale color's value equals to the relative intensity of black (ink) applied to the medium.
On a black surface (e.g. monitor) the grayscale color's value equals to the relative intensity of white (light) applied to the medium.
Chroma For Hue Mac
Chroma Hue Scale