[css-color-4] correct rec2020 transfer function

Author: svgeesus

css-color-4/conversions.js

@@ -216,28 +216,37 @@ 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;
 
+	let sign = val < 0? -1 : 1;
+	let abs = Math.abs(val);
+
 	return RGB.map(function (val) {
-		if (val < β * 4.5 ) {
+		if (abs < β * 4.5 ) {
 			return val / 4.5;
 		}
 
-		return Math.pow((val + α -1 ) / α, 2.4);
+		return sign * Math.pow((val + α -1 ) / α, 1/0.45);
 	});
 }
-//check with standard this really is 2.4 and 1/2.4, not 0.45 was wikipedia claims
 
 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;
 
+	let sign = val < 0? -1 : 1;
+	let abs = Math.abs(val);
+
 	return RGB.map(function (val) {
-		if (val > β ) {
-			return α * Math.pow(val, 1/2.4) - (α - 1);
+		if (abs > β ) {
+			return sign * α * Math.pow(val, 0.45) - (α - 1);
 		}
 
 		return 4.5 * val;