[css-color-hdr] Added sample code for ICtCp

Author: svgeesus

css-color-hdr-1/ICtCp.js

@@ -0,0 +1,79 @@
+const c1 = 3424 / 4096;
+const c2 = 2413 / 128;
+const c3 = 2392 / 128;
+const m1 = 2610 / 16384;
+const m2 = 2523 / 32;
+const im1 = 16384 / 2610;
+const im2 = 32 / 2523;
+
+// The matrix below includes the 4% crosstalk components
+// and is from the procedure in the Dolby "What is ICtCp" paper"
+const XYZtoICtCp_LMS_M = [
+	[  0.3592832590121217,  0.6976051147779502, -0.0358915932320290 ],
+	[ -0.1920808463704993,  1.1004767970374321,  0.0753748658519118 ],
+	[  0.0070797844607479,  0.0748396662186362,  0.8433265453898765 ],
+];
+
+// This matrix includes the Ebner LMS coefficients,
+// the rotation, and the scaling to [-0.5,0.5] range
+// rational terms are from Fröhlich p.97
+// and ITU-R BT.2124-0 pp.2-3
+const ICtCp_LMStoIPT_M = [
+	[  2048 / 4096,   2048 / 4096,       0      ],
+	[  6610 / 4096, -13613 / 4096,  7003 / 4096 ],
+	[ 17933 / 4096, -17390 / 4096,  -543 / 4096 ],
+];
+
+// inverted matrices, calculated from the above
+const IPTtoICtCp_LMS_M = [
+	[ 0.9999999999999998,  0.0086090370379328,  0.1110296250030260 ],
+	[ 0.9999999999999998, -0.0086090370379328, -0.1110296250030259 ],
+	[ 0.9999999999999998,  0.5600313357106791, -0.3206271749873188 ],
+];
+
+const ICtCp_LMStoXYZ_M = [
+	[  2.0701522183894223, -1.3263473389671563,  0.2066510476294053 ],
+	[  0.3647385209748072,  0.6805660249472273, -0.0453045459220347 ],
+	[ -0.0497472075358123, -0.0492609666966131,  1.1880659249923042 ],
+];
+
+function XYZ_to_ICtCp (XYZ) {
+	// convert an array of absolute, D65 XYZ to the ICtCp form of LMS
+
+    let LMS = multiplyMatrices(XYZtoICtCp_LMS_M, XYZ);
+    return LMStoICtCp(LMS);
+}
+
+function LMStoICtCp (LMS) {
+	// apply the PQ EOTF
+	// we can't ever be dividing by zero because of the "1 +" in the denominator
+	let PQLMS = LMS.map (function (val) {
+		let num = c1 + (c2 * ((val / 10000) ** m1));
+		let denom = 1 + (c3 * ((val / 10000) ** m1));
+
+		return (num / denom)  ** m2;
+	});
+
+	// LMS to IPT, with rotation for Y'C'bC'r compatibility
+	return multiplyMatrices(LMStoIPT_M, PQLMS);
+}
+
+function ICtCp_to_XYZ (ICtCp) {
+    // convert ICtCp to an array of absolute, D65 XYZ
+
+    let LMS = ICtCptoLMS(ICtCp);
+	return multiplyMatrices(ICtCp_LMStoXYZ_M, LMS);
+}
+
+function ICtCptoLMS (ICtCp) {
+	let PQLMS = multiplyMatrices(IPTtoICtCp_LMS_M, ICtCp);
+
+	// Undo PQ encoding, From BT.2124-0 Annex 2 Conversion 3
+	let LMS = PQLMS.map (function (val) {
+		let num  = Math.max((val ** im2) - c1, 0);
+		let denom = (c2 - (c3 * (val ** im2)));
+		return 10000 * ((num / denom) ** im1);
+	});
+
+	return LMS;
+}

css-color-hdr-1/Overview.bs

@@ -1070,17 +1070,23 @@ Serializing values of the ''color()'' function</h3>
 	This code also assumes all of the conversion code from
 	[[css-color-4#color-conversion-code]] is available.
 
+	The LMS used in ''Jzazbz'' are not the same
+	as the ones used in ''ICtCp'',
+	take care to use the right ones!
+
 <h3 id="ictcp_code">
 	Sample code for ''ICtCp''
 </h3>
 
-Rather than first requiring conversion to 
-''rec2100-linear'', as defined in [[!Rec_BT.2100]],
+Rather than first requiring 
+conversion to ''rec2100-linear'',
+which is how the conversion is defined in [[!Rec_BT.2100]],
 this sample code proceeds directly from absolute CIE XYZ
 for compatibility with the other color conversion code.
 
-The 4% crosstalk matrix is also built into the XYZ to LMS step,
-rather than being applied separately.
+The 4% crosstalk matrix, and the rotation ,
+are also built into the XYZ to LMS step,
+rather than being applied as three separate steps.
 
 The end result is identical, this just takes fewer steps.