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Oklab color space

(Redirected from OKLab)

The Oklab color space is a uniform color space for device independent color designed to improve perceptual uniformity, hue and lightness prediction, color blending,[a] and usability while ensuring numerical stability and ease of implementation.[1] Introduced by Björn Ottosson in December 2020, Oklab and its cylindrical counterpart, Oklch, have been included in the CSS Color Level 4 and Level 5 drafts for device-independent web colors since December 2021.[2][3] They are supported by recent versions of major web browsers[6] and allow the specification of wide-gamut P3 colors.[7]

Oklab and Oklch
Irregular perceptual lightness in a gradient generated with constant value in HSV space, with light yellows and dark blues, compared to one generated with constant lightness in Oklch.
StatusRecommendation draft
First published23 December 2020 (2020-12-23)[1]
Latest version13 February 2024 (2024-02-13)[2]
Preview version29 February 2024 (2024-02-29)[3]
OrganizationW3C
CommitteeCSS Working Group
SeriesCSS Color
PredecessorCIELAB color space
DomainColor model
LicensePublic domain
Websitewww.w3.org

Oklab's model is fitted with improved color appearance data: CAM16 data for lightness and chroma, and IPT data for hue. The new fit addresses issues such as unexpected hue and lightness changes in blue colors present in the CIELAB color space, simplifying the creation of color schemes and smoother color gradients.[1][7][4]

Coordinates

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Oklch color wheel

Oklab uses the same spatial structure as CIELAB, representing color using three components:

  • L for perceptual lightness, ranging from 0 (pure black) to 1 (reference white, if achromatic), often denoted as a percentage
  • a and b for opponent channels of the four unique hues, unbounded but in practice ranging from −0.5 to +0.5
    • a for green (negative) to red (positive)
    • b for blue (negative) to yellow (positive)

Like CIELCh, Oklch represents colors using:

  • L for perceptual lightness
  • C for chroma representing chromatic intensity, with values from 0 (achromatic) with no upper limit, but in practice not exceeding +0.5
  • h for hue angle in a color wheel, typically denoted in decimal degrees

Achromatic colors

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Neutral greys, pure black and the reference white are achromatic, that is,  ,  ,  , and h is undefined. Assigning any real value to their hue component has no effect on conversions between color spaces.[2]

While pure black is a coordinate singularity in Oklab and Oklch spaces, high-dynamic-range displays can display many chromatic colors ( ) at   (the lightness of the reference white) due to the extended gamut.

Color differences

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The perceptual color difference in Oklab is calculated as the Euclidean distance between the (L, a, b) coordinates.[8][2]

Conversions between color spaces

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Conversion to and from Oklch

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Like CIELCh, the Cartesian coordinates a and b are converted to the polar coordinates C and h as follows:

 

And the polar coordinates are converted to the Cartesian coordinates as follows:

 

Conversion from CIE XYZ

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Converting from CIE XYZ with a Standard Illuminant D65 involves:[1]

  1. Converting to an LMS color space with a linear map:
     
  2. Applying a cube root non-linearity:
     
  3. Converting to Oklab with another linear map:
     

Given:

 

Conversion from sRGB

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Converting from sRGB requires first converting from sRGB to CIE XYZ with a Standard Illuminant D65. As the last step of this conversion is a linear map from linear RGB to CIE XYZ, the reference implementation directly employs the multiplied matrix representing the composition of the two linear maps:[1]

 

Conversion to CIE XYZ and sRGB

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Converting to CIE XYZ and sRGB simply involves applying the respective inverse functions in reverse order:[1]

 

Notes

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  1. ^ However, it is not suitable for image blending or processing, for which the gamma-expanded linear RGB color space is more appropriate.[4][5]

References

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  1. ^ a b c d e f Ottosson, Björn (23 Dec 2020). "A perceptual color space for image processing". GitHub Pages. Retrieved 24 July 2024.
  2. ^ a b c d CSS Color Module Level 4 (Candidate recommendation draft). W3C. Retrieved 24 July 2024.
  3. ^ a b CSS Color Module Level 5 (Working draft). W3C. Retrieved 25 July 2024.
  4. ^ a b Levien, Raph (18 January 2021). "An interactive review of Oklab". GitHub Pages. Retrieved 24 July 2024.
  5. ^ Novak, John (21 September 2016). "What every coder should know about gamma". Retrieved 26 July 2024.
  6. ^ "Oklab color model". Can I Use. Retrieved 24 July 2024.
  7. ^ a b Sitnik, Andrey; Turner, Travis (25 January 2023). "OKLCH in CSS: why we moved from RGB and HSL". Evil Martians. Retrieved 24 July 2024.
  8. ^ Lilley, Chris (July 2021). "Better than Lab? Gamut reduction CIE Lab & OKLab". W3C. Retrieved 24 July 2024.