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[css-color-hdr] Added section introducing Jzazbz and JzCzhz, moved definitions from color(). #11713 
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‎css-color-hdr-1/Overview.bs‎

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@@ -649,6 +649,32 @@ Mixing Dynamic Range Limits: the ''dynamic-range-limit-mix()'' function {#dynami
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<a href="#deltaE">(deltaE ITP)</a>
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and for gamut mapping of mixed SDR and HDR colors.
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<h3 id="jaz-intro" class="informative">
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Introducing Jzazbz and JzCzhz
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</h3>
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<em>This section is not normative.</em>
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The J<sub>z</sub>a<sub>z</sub>b<sub>z</sub> color space [[!Safdar-PUCS]],
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and its polar form J<sub>z</sub>C<sub>z</sub>h<sub>z</sub>,
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were designed to be more perceptually uniform
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for wide gamut colors such as BT.2020,
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to accurately predict a wide-range of lightness,
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including HDR colors,
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and to have minimal inter-dependence
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between the perceptual attributes (lightness, chroma, and hue).
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In particular it has superior hue uniformity,
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compared to CAM16-UCS [[!Safdar-PUCS]].
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CIE Lab uses a lightness scale with a [0%, 100%] range,
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relative to HDR reference white;
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similarly, Oklab uses a lightness scale with a [0, 1.0] range,
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again with 1.0 referring to HDR reference white.
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In contrast, the J<sub>z</sub> axis in ''Jzazbz'',
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which has a [0, 1.0] range,
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uses an encoding <em>similar to</em> Perceptual Quantizer,
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and can represent a dynamic range of more than 13 stops.
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<dfn export>ICtCp</dfn> {#ICtCp}
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-----------------
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@@ -709,6 +735,126 @@ Mixing Dynamic Range Limits: the ''dynamic-range-limit-mix()'' function {#dynami
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</pre>
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</div>
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<dfn export>Jzazbz</dfn> {#Jzazbz}
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-----------------
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The ''Jzazbz'' color space accepts three numeric parameters,
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with J<sub>z</sub> representing the Lightness (similar to L in Lab)
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while a<sub>z</sub> and b<sub>z</sub> represent the
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redness-greenness and yellowness-blueness
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opponent-color axes (similar to a and b in Lab).
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''Jzazbz'' has the following characteristics:
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<table>
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<thead><td></td><td>x</td><td>y</td></thead>
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<tr><th>White chromaticity</th><td colspan="2">[=D65=]</td></tr>
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<tr><th>Transfer function</th><td colspan="2">Perceptual Quantizer</td></tr>
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<tr><th>Peak white luminance</th><td colspan="2">10,000 cd/m²</td></tr>
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<tr><th>Black luminance</th><td colspan="2">0.001 cd/m²</td></tr>
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<tr><th>Image state</th><td colspan="2">display-referred</td></tr>
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<tr>
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<th>Percentages</th>
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<td colspan="2">Allowed for Jz, az and bz</td>
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</tr>
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<tr>
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<th>Percent reference range</th>
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<td>for Jz: 0% = 0.0, 100% = 1.0<br>
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for az and bz: -100% = -0.21, 100% = 0.21</td>
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</tr>
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</table>
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Note that unlike Lab, a [=D65=] whitepoint is used.
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Thus, for most RGB spaces
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(which also use a D65 whitepoint)
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no chromatic adaptation step need be performed.
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<a href="#jzazbz_code">Sample code</a>
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for converting between XYZ-D65 and ''Jzazbz''
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is provided.
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<div class="example">
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This color represents a comfortable "diffuse white"
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This color can be displayed for long periods,
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or used for subtitles,
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on any sized area of the screen.
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It is the same color as sRGB ''white''.
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Diffuse white is 203 cd/m² [[!Rec_BT.2100]].
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<pre>
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<span class="swatch" style="--color: white"></span> color(jzazbz 0.22207 -0.00016 -0.00012)
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</pre>
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</div>
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<!--
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unclear why az and bz are not zero here. Is this the crosstalk term?
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No, it is due to incomplete adaptation. See
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https://github.com/color-js/color.js/issues/613#issuecomment-2666872133
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-->
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<dfn export>JzCzhz</dfn> {#JzCzHz}
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-----------------
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Similar to LCH, which is the polar representation of Lab,
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''JzCzhz'' is the polar form of ''Jzazbz''.
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J<sub>z</sub> is identical to the value in ''Jzazbz'' and represents the Lightness,
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while C<sub>z</sub> represents the Chroma or colorfulness
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and h<sub>z</sub> represents a hue angle,
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measured from the positive a<sub>z</sub> axis,
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towards the positive b<sub>z</sub> axis.
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It has the following characteristics:
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<table>
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<thead><td></td><td>x</td><td>y</td></thead>
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<tr><th>White chromaticity</th><td colspan="2">[=D65=]</td></tr>
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<tr><th>Transfer function</th><td colspan="2">Perceptual Quantizer</td></tr>
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<tr><th>Peak white luminance</th><td colspan="2">10,000 cd/m²</td></tr>
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<tr><th>Black luminance</th><td colspan="2">0.001 cd/m²</td></tr>
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<tr><th>Image state</th><td colspan="2">display-referred</td></tr>
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<tr>
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<th>Percentages</th>
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<td colspan="2">Allowed for Jz and Cz</td>
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</tr>
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<tr>
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<th>Percent reference range</th>
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<td>for Jz: 0% = 0.0, 100% = 1.0<br>
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for Cz: -100% = -0.26, 100% = 0.26</td>
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</tr>
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</table>
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<div class="example">
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This color represents the sRGB color "lime" in ''Jzazbz''
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<pre>
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<span class="swatch" style="--color: lime"></span> color(jzazbz 0.17542 -0.1179 0.1092)
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</pre>
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and here is the same color in the polar form
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<pre>
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<span class="swatch" style="--color: lime"></span> color(jzczhz 0.17542 0.1614 132.50)
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</pre>
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</div>
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<h4 id='Jzazbz-to-JzCzHz'>
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Converting ''Jzazbz'' colors to ''JzCzHz'' colors</h4>
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Conversion to ''JzCzHz'' is trivial:
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<ol>
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<li>Hz = atan2(bz, az) // but convert to degrees!
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<li>Cz = sqrt(az^2 + bz^2)
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<li>Jz is the same
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</ol>
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<h4 id='JzCzHz-to-Jzazbz'>
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Converting JzCzHz colors to Jzazbz colors</h4>
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Conversion to ''Jzazbz'' is trivial:
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<ol>
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<li>az = Cz cos(H) // convert to radians first!
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<li>bz = Cz sin(H) // convert to radians first!
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<li>Jz is the same
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</ol>
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<!-- Big Text: color()
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███▌ ███▌ █▌ ███▌ ████▌ ██ ██
@@ -1052,128 +1198,7 @@ Predefined color spaces for HDR: {#predefined-HDR}
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</tr>
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</table>
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<dfn export>Jzazbz</dfn> {#Jzazbz}
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-----------------
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The ''Jzazbz'' color space [[!Safdar-PUCS]] accepts three numeric parameters,
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with J<sub>z</sub> representing the Lightness (similar to L in Lab)
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while a<sub>z</sub> and b<sub>z</sub> represent the
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redness-greenness and yellowness-blueness
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opponent-color axes (similar to a and b in Lab).
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CIE Lab uses a lightness scale with a [0%, 100%] range,
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relative to HDR reference white,
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and has been experimentally extended to L=400%.
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In contrast, the J<sub>z</sub> axis in ''Jzazbz'',
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which has a [0, 1.0] range,
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uses an encoding similar to Perceptual Quantizer,
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and can represent a dynamic range of more than 13 stops.
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It has the following characteristics:
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<table>
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<thead><td></td><td>x</td><td>y</td></thead>
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<tr><th>White chromaticity</th><td colspan="2">[=D65=]</td></tr>
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<tr><th>Transfer function</th><td colspan="2">Perceptual Quantizer</td></tr>
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<tr><th>Peak white luminance</th><td colspan="2">10,000 cd/m²</td></tr>
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<tr><th>Black luminance</th><td colspan="2">0.001 cd/m²</td></tr>
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<tr><th>Image state</th><td colspan="2">display-referred</td></tr>
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<tr>
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<th>Percentages</th>
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<td colspan="2">Allowed for Jz, az and bz</td>
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</tr>
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<tr>
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<th>Percent reference range</th>
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<td>for Jz: 0% = 0.0, 100% = 1.0<br>
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for az and bz: -100% = -0.21, 100% = 0.21</td>
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</tr>
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</table>
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Note that unlike Lab, a [=D65=] whitepoint is used.
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Thus, for most RGB spaces
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(which also use a D65 whitepoint)
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no chromatic adaptation step need be performed.
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<div class="example">
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This color represents a comfortable "diffuse white"
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This color can be displayed for long periods,
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or used for subtitles,
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on any sized area of the screen.
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It is the same color as sRGB ''white''.
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Diffuse white is 203 cd/m² [[!Rec_BT.2100]].
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<pre>
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<span class="swatch" style="--color: white"></span> color(jzazbz 0.22207 -0.00016 -0.00012)
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</pre>
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</div>
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<!--
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unclear why az and bz are not zero here. Is this the crosstalk term?
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No, it is due to incomplete adaptation. See
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https://github.com/color-js/color.js/issues/613#issuecomment-2666872133
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-->
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<dfn export>JzCzHz</dfn> {#JzCzHz}
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-----------------
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Similar to LCH, which is the polar representation of Lab,
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''JzCzHz'' is the polar form of ''Jzazbz''.
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J<sub>z</sub> is identical to the value in ''JzCzHz'' and represents the Lightness,
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while C<sub>z</sub> represents the Chroma or colorfulness
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and H<sub>z</sub> represents a hue angle,
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measured from the positive a<sub>z</sub> axis,
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towards the positive b<sub>z</sub> axis.
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It has the following characteristics:
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<table>
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<thead><td></td><td>x</td><td>y</td></thead>
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<tr><th>White chromaticity</th><td colspan="2">[=D65=]</td></tr>
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<tr><th>Transfer function</th><td colspan="2">Perceptual Quantizer</td></tr>
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<tr><th>Peak white luminance</th><td colspan="2">10,000 cd/m²</td></tr>
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<tr><th>Black luminance</th><td colspan="2">0.001 cd/m²</td></tr>
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<tr><th>Image state</th><td colspan="2">display-referred</td></tr>
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<tr>
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<th>Percentages</th>
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<td colspan="2">Allowed for Jz and Cz</td>
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</tr>
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<tr>
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<th>Percent reference range</th>
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<td>for Jz: 0% = 0.0, 100% = 1.0<br>
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for Cz: -100% = -0.26, 100% = 0.26</td>
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</tr>
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</table>
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<div class="example">
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This color represents the sRGB color "lime" in ''Jzazbz''
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<pre>
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<span class="swatch" style="--color: lime"></span> color(jzazbz 0.17542 -0.1179 0.1092)
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</pre>
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and here is the same color in the polar form
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<pre>
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<span class="swatch" style="--color: lime"></span> color(jzczhz 0.17542 0.1614 132.50)
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</pre>
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</div>
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<h4 id='Jzazbz-to-JzCzHz'>
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Converting ''Jzazbz'' colors to ''JzCzHz'' colors</h4>
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Conversion to ''JzCzHz'' is trivial:
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<ol>
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<li>Hz = atan2(bz, az) // but convert to degrees!
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<li>Cz = sqrt(az^2 + bz^2)
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<li>Jz is the same
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</ol>
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<h4 id='JzCzHz-to-Jzazbz'>
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Converting JzCzHz colors to Jzazbz colors</h4>
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Conversion to ''Jzazbz'' is trivial:
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<ol>
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<li>az = Cz cos(H) // convert to radians first!
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<li>bz = Cz sin(H) // convert to radians first!
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<li>Jz is the same
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</ol>
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