[css-color-4] Define rec2020 color space to use 2.4 gamma w3c#12574

Author: svgeesus

css-color-4/Overview.bs

@@ -106,6 +106,12 @@ spec: css-backgrounds-3; type: property; text: border-right-color
 			"title": "What is CCT? A Guide to Choosing Correlated Color Temperature for Your Lighting",
 			"date": "2024-08-14",
 			"href": "https://litomatic.com/blog/what-is-cct-in-lighting/"
+		},
+		"Rec_BT.1886": {
+			"href": "https://www.itu.int/rec/R-REC-BT.1886/en",
+			"title": "ITU-R BT.1886 Reference electro-optical transfer function for flat panel displays used in HDTV studio production",
+			"publisher": "ITU",
+			"date": "2011-03"
 		}
 	}
 </pre>
@@ -276,7 +282,7 @@ Value Definitions</h3>
 <span class="swatch" style="--color: rgb(41.587%, 50.3670%, 36.664%)"></span> color(display-p3 0.43313 0.50108 0.37950);
 <span class="swatch" style="--color: rgb(41.587%, 50.3670%, 36.664%)"></span> color(a98-rgb 0.44091 0.49971 0.37408);
 <span class="swatch" style="--color: rgb(41.587%, 50.3670%, 36.664%)"></span> color(prophoto-rgb 0.36589 0.41717 0.31333);
-<span class="swatch" style="--color: rgb(41.587%, 50.3670%, 36.664%)"></span> color(rec2020 0.42210 0.47580 0.35605);
+<span class="swatch" style="--color: rgb(41.587%, 50.3670%, 36.664%)"></span> color(rec2020 0.6295 0.9657 0.3633);
 		</pre>
 
 	</div>
@@ -3871,9 +3877,9 @@ Specifying Predefined Colors: the ''color()'' function</h3>
 		This very intense lime color is in-gamut for rec.2020:
 		<pre class="lang-css">color(rec2020 0.42053 0.979780 0.00579);</pre>
 		in LCH, that color is
-		<pre class="lang-css">lch(86.6146% 160.0000 136.0088);</pre>
+		<pre class="lang-css">lch(85.9017% 166.116 138.207);</pre>
 		in display-p3, that color is
-		<pre class="lang-css">color(display-p3 -0.6112 1.0079 -0.2192);</pre>
+		<pre class="lang-css">color(display-p3 -0.350289 1.00707 -0.144209);</pre>
 		and is out of gamut for display-p3
 		(red and blue are negative, green is greater than 1).
 		If you have a display-p3 screen, that color is:
@@ -4363,7 +4369,7 @@ The Predefined ITU-R BT.2020-2 Color Space: the ''rec2020'' keyword</h3>
 				<tr><th>Green chromaticity</th><td>0.170</td><td>0.797</td></tr>
 				<tr><th>Blue chromaticity</th><td>0.131</td><td>0.046</td></tr>
 				<tr><th>White chromaticity</th> <td  colspan="2">[=D65=]</td></tr>
-				<tr><th>Transfer function</th><td colspan="2">see below, from [[!Rec.2020]] table 4</td></tr>
+				<tr><th>Transfer function</th><td colspan="2">gamma 2.40, from [[REC_BT.1886]]</td></tr>
 				<tr><th>Image state</th><td colspan="2">display-referred</td></tr>
 				<tr>
 					<th>Percentages</th>
@@ -4375,23 +4381,6 @@ The Predefined ITU-R BT.2020-2 Color Space: the ''rec2020'' keyword</h3>
 				</tr>
 			</table>
 
-			<pre class="lang-javascript">
-				const α = 1.09929682680944 ;
-				const β = 0.018053968510807;
-
-				let sign = c < 0? -1 : 1;
-				let abs = Math.abs(c);
-
-				if (abs < β * 4.5 ) {
-					cl = c / 4.5;
-				}
-				else {
-					cl = sign * (Math.pow((abs + α -1 ) / α, 1/0.45));
-				}
-			</pre>
-			c is the gamma-encoded red, green or blue component.
-			cl is the corresponding linear-light component.
-
 			<figure id="fig-rec2020-lch">
 				<img src="images/2020-prim-sec.svg" width=670 height=580
 				alt="diagram of rec2020 primaries and secondaries in LCH">

css-color-4/conversions.js

@@ -234,48 +234,34 @@ function XYZ_to_lin_a98rgb(XYZ) {
 function lin_2020(RGB) {
 	// convert an array of rec2020 RGB values in the range 0.0 - 1.0
 	// to linear light (un-companded) form.
-	// ITU-R BT.2020-2 p.4
-
-	const α = 1.09929682680944 ;
-	const β = 0.018053968510807;
+	//  Reference electro-optical transfer function from Rec. ITU-R BT.1886 Annex 1
+	//  with b (black lift) = 0 and a (user gain) = 1
+	//  defined over the extended range, not clamped
 
 	return RGB.map(function (val) {
 		let sign = val < 0? -1 : 1;
 		let abs = Math.abs(val);
-
-		if (abs < β * 4.5 ) {
-			return val / 4.5;
-		}
-
-		return sign * (Math.pow((abs + α -1 ) / α, 1/0.45));
+		return sign * Math.pow(abs, 2.4);
 	});
 }
 
 function gam_2020(RGB) {
 	// convert an array of linear-light rec2020 RGB  in the range 0.0-1.0
 	// to gamma corrected form
-	// ITU-R BT.2020-2 p.4
-
-	const α = 1.09929682680944 ;
-	const β = 0.018053968510807;
-
+	//  Reference electro-optical transfer function from Rec. ITU-R BT.1886 Annex 1
+	//  with b (black lift) = 0 and a (user gain) = 1
+	//  defined over the extended range, not clamped
 
 	return RGB.map(function (val) {
 		let sign = val < 0? -1 : 1;
 		let abs = Math.abs(val);
-
-		if (abs > β ) {
-			return sign * (α * Math.pow(abs, 0.45) - (α - 1));
-		}
-
-		return 4.5 * val;
+		return sign * Math.pow(abs, 1 / 2.4);
 	});
 }
 
 function lin_2020_to_XYZ(rgb) {
 	// convert an array of linear-light rec2020 values to CIE XYZ
 	// using  D65 (no chromatic adaptation)
-	// http://www.brucelindbloom.com/index.html?Eqn_RGB_XYZ_Matrix.html
 	var M = [
 		[ 63426534 / 99577255,  20160776 / 139408157,  47086771 / 278816314 ],
 		[ 26158966 / 99577255, 472592308 / 697040785,   8267143 / 139408157 ],