[css-color-hdr] Add ICtCp

Author: svgeesus

css-color-hdr/Overview.bs

@@ -380,6 +380,58 @@ Converting JzCzHz colors to Jzazbz  colors</h4>
 			<li>Jz is the same
 		</ol>
 
+<dfn export>ICtCp</dfn> {#ICtCp}
+-----------------
+
+	The ''ICtCp'' colorspace
+	is defined as Constant Intensity IC<sub>T</sub>C<sub>P</sub> signal format
+	in [[!Rec BT.2100]] and accepts three numeric parameters,
+	with I representing the Intensity
+	(similar to L in Lab, but covering a luminance range
+	up to 10,000 cd/m<sup>2</sup>)
+	while C<sub>T</sub> and C<sub>P</sub> represent the
+	yellowness-blueness (tritanope) and
+	redness-greenness (protanope)
+	opponent-color axes (similar to b and a, respectively, in Lab).
+
+	It is based on human visual system LMS cone primaries;
+	the Hunt-Pointer-Estevez (HPE) XYZ to LMS transform
+	is used,
+	normalized to a D65 white point.
+	A crosstalk matrix is then applied,
+	to reduce the gamut hull concavities of BT.2020 RGB,
+	thus reducing interpolation errors.
+	The crosstalk also provides improved lines of constant hue
+	and improved uniformity of Just Noticeable Difference (JND) MacAdam ellipses.
+
+	It has the following characteristics:
+
+	<table>
+		<thead><td></td><td>x</td><td>y</td></thead>
+		<tr><th>White chromaticity</th><td>0.3127</td><td>0.3290</td></tr>
+		<tr><th>Transfer function</th><td colspan="2">Perceptual Quantizer</td></tr>
+		<tr><th>White luminance</th><td colspan="2">203 cd/m²</td></tr>
+		<tr><th>Peak white luminance</th><td colspan="2">10,000 cd/m²</td></tr>
+		<tr><th>Black luminance</th><td colspan="2">0.001 cd/m²</td></tr>
+	</table>
+
+	Although [[Rec BT.2100]] defines IC<sub>T</sub>C<sub>P</sub>
+	by a conversion from linear-light BT.2100 RGB [[BT.2100]],
+	this conversion proceds via LMS
+	and thus, any other color space can also be represented
+	by applying an XYZ to LMS transform.
+
+	Note that unlike Lab, a D65 whitepoint is used.
+
+	Also unlike Lab, which has primarily been tested with
+	lower-intensity reflective colors,
+	IC<sub>T</sub>C<sub>P</sub> has been tested with
+	high-chroma,
+	self-luminous, and
+	high intensity (HDR) colors.
+	This makes it suitable for color difference measurement (deltaE ITP)
+	and for gamut mapping of SDR and HDR colors.
+
 Compositing SDR and HDR content {#Compositing-SDR-HDR}
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