[css-color-hdr] forward and reverse for HLG

Author: svgeesus

css-color-hdr/Overview.bs

@@ -217,22 +217,44 @@ Predefined colorspaces for HDR: {#predefined-HDR}
 		</pre>
 	</div>
 
-	Linear-light RGB signals are converted to HLG as follows:
+	Linear-light RGB signals are converted to HLG encoded as follows [[Rec.2390]]:
+
+	<!-- From BT.2390 p.26
+	-->
 
 	<pre class="lang-javascript">
-		var E;			// the red, green or blue component, normalized to reference white
-		var r = 0.5; 	// reference white level
+		var E;			// the red, green or blue component, [0, 1]]
 		const a = 0.17883277;
-		const b = 0.28466892;
-		const c = 0.55991073;
-		if (E <= 1) {
-			Edash = r * Math.sqrt(E);
+		const b = 0.28466892;   // 1 - (4 * a)
+		const c = 0.55991073;   // 0.5 - a * Math.log(4 *a)
+		if (E <= 1/12) {
+			Edash = Math.sqrt( 3 * E);
 		}
 		else {
-			Edash = a * Math.log(E - b) + c;
+			Edash = a * Math.log(12 * E - b) + c;
 		}
 	</pre>
-		Edash is the "gamma-corrected" signal.
+		Edash is the "gamma-corrected" (OETF-adjusted) signal.
+
+	The reverse conversion (HLG encoded to linear light) is as follows:
+
+	<!-- From BT.2390 p.33
+	-->
+
+	<pre class="lang-javascript">
+		var Edash;			// the red, green or blue encoded component, [0, 1]]
+		const a = 0.17883277;
+		const b = 0.28466892;   // 1 - (4 * a)
+		const c = 0.55991073;   // 0.5 - a * Math.log(4 *a)
+		if (Edash <= 0.5) {
+			E = (Edash ** 2) / 3;
+		}
+		else {
+			E = Math.exp(((Edash - c) / a) + b) / 12;
+		}
+	</pre>
+
+	<!-- add black level lift as defined on same page?-->
 
 <dfn export>Jzazbz</dfn> {#Jzazbz}
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