[spaces/rec2020] Use gamma 2.40 for display-referred rec2020 #669

data/modules.json

@@ -76,6 +76,13 @@
 			"url": "https://en.wikipedia.org/wiki/Rec._2020",
 			"description": "A very wide gamut RGB color space used in 4k and 8k UHDTV. It is currently difficult to make a screen which displays the entire REC.2020 gamut; current movies broadcast in this colorspace do not use the full gamut, although they do use a little more than P3."
 		},
+		{
+			"name": "REC.2020 Scene Referred",
+			"id": "--rec2020-oetf",
+			"dependencies": ["srgb"],
+			"url": "https://en.wikipedia.org/wiki/Rec._2020",
+			"description": "Scene-referred version of REC.2020"
+		},
 		{
 			"name": "REC.2100-PQ",
 			"id": "rec2100pq",

src/spaces/index-fn.js

@@ -21,6 +21,7 @@ export { default as ProPhoto_Linear } from "./prophoto-linear.js";
 export { default as ProPhoto } from "./prophoto.js";
 export { default as REC_2020_Linear } from "./rec2020-linear.js";
 export { default as REC_2020 } from "./rec2020.js";
+export { default as REC_2020_Scene_Referred } from "./rec2020-oetf.js";
 export { default as OKLab } from "./oklab.js";
 export { default as OKLCH } from "./oklch.js";
 export { default as OKLrab } from "./oklrab.js";

src/spaces/rec2020-oetf.js

@@ -0,0 +1,38 @@
+import RGBColorSpace from "../RGBColorSpace.js";
+import REC2020Linear from "./rec2020-linear.js";
+// import sRGB from "./srgb.js";
+
+const α = 1.09929682680944;
+const β = 0.018053968510807;
+
+export default new RGBColorSpace({
+	id: "--rec2020-oetf",
+	name: "REC.2020_Scene_Referred",
+	base: REC2020Linear,
+	referred: "scene",
+	// Non-linear transfer function from Rec. ITU-R BT.2020-2 table 4
+	toBase (RGB) {
+		return RGB.map(function (val) {
+			let sign = val < 0 ? -1 : 1;
+			let abs = val * sign;
+
+			if (abs < β * 4.5) {
+				return val / 4.5;
+			}
+
+			return sign * Math.pow((abs + α - 1) / α, 1 / 0.45);
+		});
+	},
+	fromBase (RGB) {
+		return RGB.map(function (val) {
+			let sign = val < 0 ? -1 : 1;
+			let abs = val * sign;
+
+			if (abs >= β) {
+				return sign * (α * Math.pow(abs, 0.45) - (α - 1));
+			}
+
+			return 4.5 * val;
+		});
+	},
+});

src/spaces/rec2020.js

@@ -2,36 +2,25 @@ import RGBColorSpace from "../RGBColorSpace.js";
 import REC2020Linear from "./rec2020-linear.js";
 // import sRGB from "./srgb.js";
 
-const α = 1.09929682680944;
-const β = 0.018053968510807;
-
 export default new RGBColorSpace({
 	id: "rec2020",
 	name: "REC.2020",
 	base: REC2020Linear,
-	// Non-linear transfer function from Rec. ITU-R BT.2020-2 table 4
+	//  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
 	toBase (RGB) {
 		return RGB.map(function (val) {
 			let sign = val < 0 ? -1 : 1;
 			let abs = val * sign;
-
-			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, 2.40);
 		});
 	},
 	fromBase (RGB) {
 		return RGB.map(function (val) {
 			let sign = val < 0 ? -1 : 1;
 			let abs = val * sign;
-
-			if (abs >= β) {
-				return sign * (α * Math.pow(abs, 0.45) - (α - 1));
-			}
-
-			return 4.5 * val;
+			return sign * Math.pow(abs, 1/2.40);
 		});
 	},
 });

test/contrast.js

@@ -136,7 +136,7 @@ export default {
 				},
 				{
 					args: ["#000", "color(rec2020 0.6301706 0.6301706 0.6301706)"],
-					expect: 58.146262578561334,
+					expect: 50.684920919,
 				},
 				{
 					args: ["color(xyz-d65 0.3820622 0.4019778 0.437777)", "#000"],

test/conversions.js

@@ -1127,7 +1127,7 @@ const tests = {
 				{
 					name: "Negative values",
 					args: "color(rec2020 -0.07 -0.5 0.2)",
-					expect: [-0.01556, -0.25972, 0.05552],
+					expect: [-0.001691357, -0.1894645708, 0.021012222],
 				},
 			],
 		},
@@ -1140,7 +1140,7 @@ const tests = {
 				{
 					name: "Negative values",
 					args: "color(--rec2020-linear -0.017 -0.5 0.2)",
-					expect: [-0.0765, -0.70544, 0.43352],
+					expect: [-0.183099455, -0.749153538, 0.511402090],
 				},
 			],
 		},