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[css-color-hdr] Added an informative section introducing ICtCp, moved definition out from color(), w3c#11713 
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‎css-color-hdr-1/Overview.bs‎

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@@ -46,6 +46,15 @@ WPT Display: open
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"publisher": "VESA",
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"date": "17 May 2024"
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},
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"Development_ITP": {
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"authors": [
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"F.Ebner",
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"M.D.Fairchild"
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],
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"title": "Development and testing of a color space (IPT) with improved hue uniformity. In: Proceedings of The Sixth Color Imaging Conference, 8-13, 1998",
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"date": "January 1998",
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"href": "https://www.researchgate.net/publication/221677980_Development_and_Testing_of_a_Color_Space_IPT_with_Improved_Hue_Uniformity"
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},
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"ISO_21496-1": {
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"title": "Gain map metadata for image conversion: Part 1: Dynamic Range Conversion",
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"href": "https://www.iso.org/standard/86775.html",
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</pre>
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</div>
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<!-- Big Text: ICTCP Jaz -->
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<h2 id="DI-HDR">
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Device-Independent HDR Colors
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</h2>
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In the same way that [[css-color-4#lab-colors]]
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allows specifying device-independent SDR colors,
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this specification allows specifying device-independent HDR colors.
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<h3 id="ictcp-intro" class="informative">
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Introducing ICtCp
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</h3>
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<em>This section is not normative.</em>
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The IC<sub>T</sub>C<sub>P</sub> color space
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has better perceptual uniformity than CIE Lab,
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and is used as a part of the color image pipeline
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in video and digital photography systems
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for high dynamic range (HDR) and wide color gamut (WCG) imagery.
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The T and P subscripts
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refer to the Tritanope (yellowness-blueness)
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and Protanope (redness-greenness)
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opponent color axes in the human visual system.
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To avoid pesky subscripts,
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the simplification ICtCp is frequently used.
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IC<sub>T</sub>C<sub>P</sub> was
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originaly developed by Dolby Laboratories [[!What_is_ICtCp]]
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from the IPT color space by Ebner and Fairchild
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[[!Development_ITP]],
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and is defined in [[!Rec_BT.2100]] as the
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Constant Intensity IC<sub>T</sub>C<sub>P</sub> signal format.
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It was intended to replace YCbCr [[Perrin]].
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It is based on the human visual system LMS cone primaries;
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the Hunt-Pointer-Estevez (HPE) XYZ to LMS transform
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is used,
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normalized to a [=D65=] whitepoint.
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A 4% crosstalk matrix is then applied [[What_is_ICtCp]],
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to reduce the gamut hull concavities of BT.2020 RGB,
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thus reducing interpolation errors.
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The crosstalk also provides improved lines of constant hue
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and improved uniformity of Just Noticeable Difference (JND) MacAdam ellipses.
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Unlike ''Lab'', which has primarily been tested with
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lower-intensity <em>reflective</em> colors,
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IC<sub>T</sub>C<sub>P</sub> has been tested with
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high-chroma (wide color gamut),
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self-luminous, and
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high intensity (HDR) colors.
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This makes it suitable for HDR color difference measurement
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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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<dfn export>ICtCp</dfn> {#ICtCp}
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-----------------
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The ''ICtCp'' color space accepts three numeric parameters,
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with I representing the Intensity
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(similar to L in Lab, but covering a luminance range
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up to 10,000 cd/m<sup>2</sup>)
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while C<sub>T</sub> and C<sub>P</sub> represent the
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yellowness-blueness (tritanope) and
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redness-greenness (protanope)
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opponent-color axes (similar to b and a, respectively, in Lab).
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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>White luminance</th><td colspan="2">203 cd/m²</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 I, Ct and Cp</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 I: 0% = 0.0, 100% = 1.0<br>
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for Ct and Cp: -100% = -0.5, 100% = 0.5</td>
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</tr>
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</table>
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Although [[Rec_BT.2100]] defines IC<sub>T</sub>C<sub>P</sub>
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by a conversion from linear-light BT.2100 RGB [[Rec_BT.2100]],
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this conversion proceeds via LMS
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and thus, any other color space can also be represented
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by applying an XYZ to LMS transform
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as shown in [[What_is_ICtCp]].
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The <a href="#ictcp_code">sample code</a> uses this approach.
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<div class="example">
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This color represents the sRGB color "lime" in ''ICtCp''
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<pre>
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<span class="swatch" style="--color: lime"></span> color(ictcp 0.5393 -0.2643 -0.0625)
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</pre>
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</div>
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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(ictcp 0.58069 0 0)
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</pre>
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</div>
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<!-- Big Text: color()
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███▌ ███▌ █▌ ███▌ ████▌ ██ ██
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<li>Jz is the same
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</ol>
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<dfn export>ICtCp</dfn> {#ICtCp}
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-----------------
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The ''ICtCp'' color space [[!Perrin]]
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is defined as Constant Intensity IC<sub>T</sub>C<sub>P</sub> signal format
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in [[!Rec_BT.2100]] and accepts three numeric parameters,
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with I representing the Intensity
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(similar to L in Lab, but covering a luminance range
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up to 10,000 cd/m<sup>2</sup>)
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while C<sub>T</sub> and C<sub>P</sub> represent the
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yellowness-blueness (tritanope) and
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redness-greenness (protanope)
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opponent-color axes (similar to b and a, respectively, in Lab).
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It is based on human visual system LMS cone primaries;
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the Hunt-Pointer-Estevez (HPE) XYZ to LMS transform
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is used,
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normalized to a [=D65=] whitepoint.
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A 4% crosstalk matrix is then applied [[What_is_ICtCp]],
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to reduce the gamut hull concavities of BT.2020 RGB,
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thus reducing interpolation errors.
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The crosstalk also provides improved lines of constant hue
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and improved uniformity of Just Noticeable Difference (JND) MacAdam ellipses.
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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>White luminance</th><td colspan="2">203 cd/m²</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 I, Ct and Cp</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 I: 0% = 0.0, 100% = 1.0<br>
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for Ct and Cp: -100% = -0.5, 100% = 0.5</td>
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</tr>
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</table>
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Although [[Rec_BT.2100]] defines IC<sub>T</sub>C<sub>P</sub>
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by a conversion from linear-light BT.2100 RGB [[Rec_BT.2100]],
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this conversion proceeds via LMS
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and thus, any other color space can also be represented
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by applying an XYZ to LMS transform
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as shown in [[What_is_ICtCp]].
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Note that unlike CIE ''Lab'', a [=D65=] whitepoint is used.
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Also unlike ''Lab'', which has primarily been tested with
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lower-intensity reflective colors,
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IC<sub>T</sub>C<sub>P</sub> has been tested with
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high-chroma,
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self-luminous, and
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high intensity (HDR) colors.
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This makes it suitable for color difference measurement (deltaE ITP)
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and for gamut mapping of SDR and HDR colors.
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<div class="example">
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This color represents the sRGB color "lime" in ''ICtCp''
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<pre>
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<span class="swatch" style="--color: lime"></span> color(ictcp 0.5393 -0.2643 -0.0625)
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</pre>
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</div>
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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(ictcp 0.58069 0 0)
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</pre>
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</div>
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<!-- Big Text: composite
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