[css-color-hdr] Added ΔEITP color difference metric from BT.2124

Author: svgeesus

css-color-hdr-1/Overview.bs

@@ -58,6 +58,12 @@ WPT Display: open
 			"publisher": "ITU",
 			"rawDate": "2018-07-00"
 		},
+		"Rec_BT.2124": {
+			"title": "ITU-R BT.2124-0 Objective metric for the assessment of the potential visibility of colour differences in television",
+			"href": "https://www.itu.int/dms_pubrec/itu-r/rec/bt/R-REC-BT.2124-0-201901-I!!PDF-E.pdf",
+			"publisher": "ITU",
+			"rawDate": "2019-01"
+		},
 		"Rec_BT.2390": {
 			"href": "https://www.itu.int/dms_pub/itu-r/opb/rep/R-REP-BT.2390-8-2020-PDF-E.pdf",
 			"title": "ITU-R BT.2390-8 High dynamic range television for production and international programme exchange",
@@ -1023,6 +1029,38 @@ Serializing values of the ''color()'' function</h3>
 	Values must be <a href="https://drafts.csswg.org/css-values-4/#combine-integers">rounded towards +∞</a>, not truncated.
 
 
+<!--
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+
+<h2 id="deltaE">
+	Sample code for ΔEITP Color Differences
+</h2>
+
+<em>This section is not normative.</em>
+<wpt title="This section is not normative, it does not need tests."></wpt>
+
+The ΔEITP [[Rec_BT.2124]] color difference metric 
+may be used to measure the perceptual color difference between 
+two display-referred colors
+in an HDR or mixed SDR/HDR context,
+for example in display calibration
+or gamut and tone mapping.
+
+It is a Euclideam distance in ''ICtCp'' color space,
+scaled such that a ΔEITP of 1.0
+represents one just-noticeable difference.
+
+<pre class="include-code lang-javascript">
+	path: deltaEITP.js
+	highlight: js
+	</pre>
 
 <!-- Sample section {#sample-topic}
 ==============================

css-color-hdr-1/deltaEITP.js

@@ -0,0 +1,16 @@
+// Calculate deltaE ITP
+// scaled root sum of squares
+// ITU-R BT.2124-0 Annex 1
+/**
+ * @param {number[]} reference - Array of ICtCp: I as 0..1, Ct and Cp as -1..1
+ * @param {number[]} sample -    Array of ICtCp: I as 0..1, Ct and Cp as -1..1
+ * @return {number} How different a color sample is from reference
+ */
+function deltaEITP (reference, sample) {
+    let [I1, Ct1, Cp1] = reference;
+	let [I2, Ct2, Cp2] = sample;
+	let ΔI = I1 - I2;
+	let ΔT = 0.5 * (Ct1 - Ct2);
+	let ΔP = Cp1 - Cp2;
+	return Math.sqrt(ΔI ** 2 + ΔT ** 2 + ΔP ** 2);
+}