From 368c91efc44ab7b3ace595e649d8659514ff4dbe Mon Sep 17 00:00:00 2001
From: Chris Lilley <chris@w3.org>
Date: Mon, 16 Jan 2023 09:30:36 +0200
Subject: [PATCH] Correct rec2100-hlg, #190

---
 src/spaces/rec2020-linear.js |  5 ++++-
 src/spaces/rec2100-hlg.js    | 25 ++++++++++++++++++-------
 2 files changed, 22 insertions(+), 8 deletions(-)

diff --git a/src/spaces/rec2020-linear.js b/src/spaces/rec2020-linear.js
index f1069d422..fc2b20de9 100644
--- a/src/spaces/rec2020-linear.js
+++ b/src/spaces/rec2020-linear.js
@@ -22,5 +22,8 @@ export default new RGBColorSpace({
 	name: "Linear REC.2020",
 	white: "D65",
 	toXYZ_M,
-	fromXYZ_M
+	fromXYZ_M,
+	formats: {
+		color: {},
+	}
 });
diff --git a/src/spaces/rec2100-hlg.js b/src/spaces/rec2100-hlg.js
index 57eda65dd..22a3f9f5e 100644
--- a/src/spaces/rec2100-hlg.js
+++ b/src/spaces/rec2100-hlg.js
@@ -7,6 +7,8 @@ const a = 0.17883277;
 const b = 0.28466892; // 1 - (4 * a)
 const c = 0.55991073; // 0.5 - a * Math.log(4 *a)
 
+const scale = 3.7743;	// Place 18% grey at HLG 0.38, so media white at 0.75
+
 export default new RGBColorSpace({
 	id: "rec2100hlg",
 	cssid: "rec2100-hlg",
@@ -18,21 +20,30 @@ export default new RGBColorSpace({
 		// given HLG encoded component in range [0, 1]
 		// return media-white relative linear-light
 		return RGB.map(function (val) {
-			if (val <= 1/12) {
-				return Math.sqrt( 3 * val);
+			// first the HLG EOTF
+			// ITU-R BT.2390-10 p.30 section
+			// 6.3 The hybrid log-gamma electro-optical transfer function (EOTF)
+			// Then scale by 3 so media white is 1.0
+			if (val <= 0.5) {
+				return (val ** 2) / 3 * scale;
 			}
-			return a * Math.log(12 * val - b) + c;
+			return Math.exp(((val - c) / a) + b) / 12 * scale;
 		});
 	},
 	fromBase (RGB) {
 		// given media-white relative linear-light
+		// where diffuse white is 1.0,
 		// return HLG encoded component in range [0, 1]
-		// per ITU Rec BT.2390
 		return RGB.map(function (val) {
-			if (val <= 0.5) {
-				return (val ** 2) / 3;
+			// first scale to put linear-light media white at 1/3
+			val /= scale;
+			// now the HLG OETF
+			// ITU-R BT.2390-10 p.23
+			// 6.1 The hybrid log-gamma opto-electronic transfer function (OETF)
+			if (val <= 1/12) {
+				return Math.sqrt( 3 * val);
 			}
-			return Math.exp(((val - c) / a) + b) / 12;
+			return a * Math.log(12 * val - b) + c;
 		});
 	},
 	formats: {