[css-color-hdr] more transfer function stuff

Author: svgeesus

css-color-hdr/Overview.bs

@@ -108,6 +108,11 @@ Predefined colorspaces for HDR: {#predefined-HDR}
 	encoded values are mixed up.
 	Add examples showing conversion from binary values to the [0, 1] range.
 
+	Issue: should this specification recommend that the CSS Object Model
+	store these values in the 16-bit half-floating point representation
+	(IEEE standard 754-2008.)
+	from BT.2100 pp. 9-10? (or any wider representation, like float or double)?
+
 	Issue: should the range be in absolute nits, ie [0.0, 10,000]?
 
 	The Perceptual Quantizer (PQ) electro-optical transfer function is used
@@ -118,7 +123,7 @@ Predefined colorspaces for HDR: {#predefined-HDR}
 	In contrast to SDR colorspaces and some HDR colorspaces,
 	the values are absolute rather than relative.
 
-	Issue: add a diagrram showing SDR and HDR dynamic ranges on a log scale
+	Issue: add a diagram showing SDR and HDR dynamic ranges on a log scale
 
 	It has the following characteristics:
 	(The display primaries are the same as [[!Rec.2020]]):
@@ -134,6 +139,8 @@ Predefined colorspaces for HDR: {#predefined-HDR}
 		<tr><th>Black luminance</th><td colspan="2">0.001 cd/m²</td></tr>
 	</table>
 
+	Issue: from BT.2100 p.3 "Note 3d – For PQ in a non-reference viewing environment, or for HLG (in any viewing environment), the black level should be adjusted using the PLUGE test signal and procedure specified in Recommendation ITU-R BT.814."
+
 	<div class="example">
 		This color represents a blinding white
 		with a luminance of 10,000 cd/m².
@@ -166,6 +173,43 @@ Predefined colorspaces for HDR: {#predefined-HDR}
 
 	Issue: add other examples, including encoding of sRGB red, green, blue and P3 red, green, blue.
 
+	Linear-light RGB signals are converted to PQ encoded as follows. An absolute luminance scale is used, so the luminance of diffuse (media) white is required to scale relative luminances. The maximum encodable value (peak, small-area white) in PQ is 10,000 cd/m²:
+
+	<!-- From Froehlich p.42
+		and BT.2100 Table 4 p.4 Reference PQ OETF
+	-->
+
+	<pre class="lang-javascript">
+		var Er;			// the red, green or blue component, [0, 1] for SDR, [0, 70ish] for HDR
+		var Yw = 140;	// the absolute luminance of diffuse white, cd/m²
+		var x = Er * Yw / 10000; 	// absolute luminance of peak white is 10,000 cd/m².
+		const n = 2610 / (2 ** 14);
+		const m = 2523 / (2 ** 5);
+		const c1 = 3424 / (2 ** 12);
+		const c2 = 2413 / (2 ** 7);
+		const c3 = 2392 / (2 ** 7);
+		xPQ = (((c1 + (c2 * (x ** n))) / (1 + (c3 * (x ** n)))) ** m);
+
+	</pre>
+		xPQ is the "gamma-corrected" (OETF-adjusted) signal [0, 1].
+
+	PQ encoded values are converted to linear-light as follows:
+
+	<!-- from BT.2100 Table 4 p.4 Reference EOTF -->
+
+	<pre class="lang-javascript">
+		var xPQ;			// the red, green or blue PQ-encoded component, [0, 1]
+		const ninv = (2 ** 14) / 2610;
+		const minv = (2 ** 5) / 2523;
+		const c1 = 3424 / (2 ** 12);
+		const c2 = 2413 / (2 ** 7);
+		const c3 = 2392 / (2 ** 7);
+		var x = (((Math.max(((xPQ ** minv) - c1), 0) / (c2 - (c3 * (xPQ ** minv)))) ** ninv);
+		var Yw = 140;			// the absolute luminance of diffuse white, cd/m²
+		var Ea = x * 10000; 	// absolute luminance, [0, 10,000].
+		var Er = x * 10000 / Yw; 	// luminance relative to diffuse white, [0, 70 or so].
+	</pre>
+
 
 <dfn export>rec2100-hlg</dfn> {#valdef-color-rec2100-hlg}
 ---------------------------
@@ -195,7 +239,7 @@ Predefined colorspaces for HDR: {#predefined-HDR}
 		<tr><th>White chromaticity</th><td>0.3127</td><td>0.3290</td></tr>
 		<tr><th>Transfer function</th><td colspan="2">Hybrid log Gamma</td></tr>
 		<tr><th>Peak white luminance</th><td colspan="2">12 times reference white</td></tr>
-		<tr><th>Black luminance</th><td colspan="2">depends on reference white</td></tr>
+		<tr><th>Black luminance</th><td colspan="2">depends on reference white, see text</td></tr>
 	</table>
 
 	Issue: add other examples
@@ -221,6 +265,7 @@ Predefined colorspaces for HDR: {#predefined-HDR}
 	Linear-light RGB signals are converted to HLG encoded as follows [[Rec.2390]]:
 
 	<!-- From BT.2390 p.26
+		confirmed with Froehlich p.43
 	-->
 
 	<pre class="lang-javascript">