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[css-color-4] Update gamut mapping algorithm, fixes w3c#9715 
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‎css-color-4/Overview.bs

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@@ -5467,63 +5467,80 @@ CSS Gamut Mapping to an RGB Destination</h3>
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<h4 id="binsearch">
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Sample Pseudocode for the Binary Search Gamut Mapping Algorithm with Local MINDE</h4>
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<div algorithm="to CSS gamut map a color">
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To <dfn export>CSS gamut map</dfn> a color |origin|
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in color space |origin color space|
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to be in gamut of a destination color space |destination|:
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<div algorithm="to CSS gamut map a color">
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To <dfn export>CSS gamut map</dfn> a color |origin|
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in color space |origin color space|
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to be in gamut of a destination color space |destination|:
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<ol>
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<!-- check if we need gamut mapping at all -->
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<li>if |destination| has no gamut limits (XYZ-D65, XYZ-D50, Lab, LCH, Oklab, Oklch) convert |origin| to |destination| and return it as the gamut mapped color
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<!-- we do, so convert to Oklch -->
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<li>let |origin_Oklch| be |origin| converted
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from |origin color space| to the Oklch color space</li>
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<!-- constrain to SDR lightness range, which gamut maps to black or white -->
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<li>if the Lightness of |origin_Oklch| is greater than or equal to 100%,
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convert `oklab(1 0 0 / origin.alpha)` to |destination| and return it as the gamut mapped color</li>
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<li>if the Lightness of |origin_Oklch| is less than than or equal to 0%,
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convert `oklab(0 0 0 / origin.alpha)` to |destination| and return it as the gamut mapped color</li>
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<li>let inGamut(|color|) be a function which returns true if, when passed a color,
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that color is inside the gamut of |destination|.
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For HSL and HWB, it returns true if the color is inside the gamut of sRGB.
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</li>
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<!-- are we already in gamut? -->
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<li>if inGamut(|origin_Oklch|) is true, convert |origin_Oklch| to |destination| and return it as the gamut mapped color</li>
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<!-- now start to gamut map -->
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<li>otherwise, let delta(|one|, |two|) be a function which returns the deltaEOK of color |one| compared to color |two|</li>
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<li>let |JND| be 0.02</li>
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<li>let |epsilon| be 0.0001</li>
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<!-- we already excluded spaces with no gamut limits in the first step, so this is fine -->
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<li>let clip(|color|) be a function which converts |color| to |destination|,
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clamps each component to the bounds of the reference range for that component
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and returns the result</li>
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<!-- is clipped already indistinguishable from origin, and in gamut? -->
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<li>set |current| to |origin_Oklch|</li>
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<li>set |clipped| to clip(|current|)</li>
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<li>set |E| to delta(|clipped|, |current|)</li>
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<li>if |E| < |JND|
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<ol>
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<li>if |destination| has no gamut limits (XYZ-D65, XYZ-D50, Lab, LCH, Oklab, Oklch) convert |origin| to |destination| and return it as the gamut mapped color
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<li>let |origin_Oklch| be |origin| converted
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from |origin color space| to the Oklch color space</li>
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<li>if the Lightness of |origin_Oklch| is greater than or equal to 100%,
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return { 1 1 1 origin.alpha } in |destination|</li>
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<li>if the Lightness of |origin_Oklch| is less than than or equal to 0%,
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return { 0 0 0 origin.alpha } in |destination|</li>
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<li>let inGamut(|color|) be a function which returns true if, when passed a color,
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that color is inside the gamut of |destination|.
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For HSL and HWB, it returns true if the color is inside the gamut of sRGB.
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</li>
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<li>if inGamut(|origin_Oklch|) is true, convert |origin_Oklch| to |destination| and return it as the gamut mapped color</li>
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<li>otherwise, let delta(|one|, |two|) be a function which returns the deltaEOK of color |one| compared to color |two|</li>
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<li>let |JND| be 0.02</li>
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<li>let |epsilon| be 0.0001</li>
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<!-- we already excluded spaces with no gamut limits in the first step, so this is fine -->
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<li>let clip(|color|) be a function which converts |color| to |destination|,
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converts all negative components to zero,
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converts all components greater that one to one,
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and returns the result
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</li>
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<li>set |min| to zero</li>
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<li>set |max| to the Oklch chroma of |origin_Oklch|</li>
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<li> let |min_inGamut| be a boolean that represents when |min| is still in gamut, and set it to true
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<li>while (|max| - |min| is greater than |epsilon|) repeat the following steps
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<li>return |clipped| as the gamut mapped color</li>
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</ol>
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</li>
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<!-- reduce chroma -->
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<li>set |min| to zero</li>
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<li>set |max| to the Oklch chroma of |origin_Oklch|</li>
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<li> let |min_inGamut| be a boolean that represents when |min| is still in gamut, and set it to true
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<li>while (|max| - |min| is greater than |epsilon|) repeat the following steps
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<ol>
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<li>set |chroma| to (|min| + |max|) /2</li>
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<li>set the chroma component of |current| to |chroma|</li>
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<li>if |min_inGamut| is true and also if inGamut(|current|) is true, set |min| to |chroma| and continue to repeat these steps</li>
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<li>otherwise, if inGamut(|current|) is false carry out these steps:
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<ol>
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<li>set |chroma| to (|min| + |max|) /2</li>
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<li>set |current| to |origin_Oklch| and then set the chroma component to |chroma|</li>
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<li>if |min_inGamut| is true and also if inGamut(|current|) is true, set |min| to |chroma| and continue to repeat these steps</li>
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<li>otherwise, if inGamut(|current|) is false carry out these steps:
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<li>set |clipped| to clip(|current|)</li>
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<li>set |E| to delta(|clipped|, |current|)</li>
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<li>if |E| < |JND|
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<ol>
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<li>if (|JND| - |E| < |epsilon|) return |clipped| as the gamut mapped color</li>
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<li>otherwise,
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<ol>
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<li>set |clipped| to clip(|current|)</li>
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<li>set |E| to delta(|clipped|, |current|)</li>
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<li>if |E| < |JND|
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<ol>
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<li>if (|JND| - |E| < |epsilon|) return |clipped| as the gamut mapped color</li>
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<li>otherwise,
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<ol>
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<li>set |min_inGamut| to false</li>
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<li>set |min| to |chroma|</li>
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</ol>
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</li>
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</ol>
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</li>
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<li>otherwise, set |max| to |chroma| and continue to repeat these steps</li>
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<li>set |min_inGamut| to false</li>
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<li>set |min| to |chroma|</li>
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</ol>
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</li>
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</ol>
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</li>
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<li>otherwise, set |max| to |chroma| and continue to repeat these steps</li>
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</ol>
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</li>
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<li>return |clipped| as the gamut mapped color</li>
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</ol>
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</li>
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<li>return |clipped| as the gamut mapped color</li>
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</ol>
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</div>
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<!-- unbounded hsl
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<wpt ignore>

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