[css-color-hdr] Remove absolute and relative HDR concepts, stress scene-referred and display-referred. Anchor media white, and allow color re-rendering #10460

Author: svgeesus

css-color-hdr-1/Overview.bs

@@ -151,7 +151,8 @@ One stop is a doubling of luminance.
 	  } 
 </xmp>
 
-<dfn export>Media white</dfn>
+<dfn export>Media white</dfn>,
+also called HDR Reference White,
 is the color of a normal white background, 
 or of white text on a dark background.
 It can be comfortably viewed, over the whole screen.
@@ -210,8 +211,12 @@ while the luminance of media white remains constant.
 	Thus, the encoded dynamic range is 23.3 stops.
 </div>
 
-When [[#Compositing-SDR-HDR]], the media white for SDR content
-should be anchored at <strong>203 cd/m²</strong>
+Issue: add a diagram showing SDR and HDR dynamic ranges on a log scale
+
+When [[#Compositing-SDR-HDR]], 
+and for color space conversion,
+the media white for SDR content
+should be anchored at <strong>203 cd/m²</strong> [[!Rpt_BT.2408]]
 so that that the media white level 
 of the inputs to compositing
 will end up at the media white level 
@@ -220,7 +225,7 @@ of the combined signal.
 When displaying HDR (or mixed SDR and HDR) content,
 for displays which are less capable than the reference mastering display,
 and for viewing conditions different to the standard ones,
-a <dfn export>color re-rendering</dfn> step will be performed.
+a <dfn export>color re-rendering</dfn> step ((OOTF) will be performed.
 This may result in a media white being displayed
 at a value lower or higher than 203 cd/m².
 
@@ -477,11 +482,6 @@ Predefined color spaces for HDR: {#predefined-HDR}
 
 	ITU Reference 2100 is used for HDR 4k and 8k television.
 
-	In contrast to SDR color spaces and some HDR color spaces,
-	the PQ values are <em>absolute</em> rather than relative.
-
-	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]]):
 
@@ -547,16 +547,18 @@ Predefined color spaces 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. Media white is 203 cd/m² [[!Rpt_BT.2408]].
+	Linear-light RGB signals are converted to PQ encoded as follows.
+	The maximum encodable value (peak, small-area white) in PQ is 10,000 cd/m². 
+	Media white is 203 cd/m² [[!Rpt_BT.2408]].
 
 	<!-- 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 = 203;	// the absolute luminance of diffuse white, cd/m²
-		var x = Er * Yw / 10000; 	// absolute luminance of peak white is 10,000 cd/m².
+		var Yw = 203;	// the luminance of diffuse white, cd/m²
+		var x = Er * Yw / 10000; 	// luminance of peak white is 10,000 cd/m².
 		const n = 2610 / (2 ** 14);
 		const m = 2523 / (2 ** 5);
 		const c1 = 3424 / (2 ** 12);
@@ -579,8 +581,8 @@ Predefined color spaces for HDR: {#predefined-HDR}
 		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 = 203;			// the absolute luminance of diffuse white, cd/m²
-		var Ea = x * 10000; 	// absolute luminance, [0, 10,000].
+		var Yw = 203;			// the luminance of diffuse white, cd/m²
+		var Ea = x * 10000; 	// luminance, [0, 10,000].
 		var Er = x * 10000 / Yw; 	// luminance relative to diffuse white, [0, 70 or so].
 	</pre>
 
@@ -705,7 +707,7 @@ Predefined color spaces for HDR: {#predefined-HDR}
 	regardless of the actual bit depth (10 or 12 bits per component).
 
 	The color with red, green and blue all at 1.0 
-	represents HDR Reference White with an absolute luminance of 
+	represents [=media white=] with a luminance of 
 	203 cd/m².
 
 	<pre class="lang-css">color(rec2100-linear 1 1 1)</pre>
@@ -716,7 +718,7 @@ Predefined color spaces for HDR: {#predefined-HDR}
 		<pre class="lang-css">color(rec2100-linear 9.852 9.852 9.852)</pre>
 
 		represents a white highlight 
-		with an absolute luminance
+		with a luminance
 		of 9.852 × 203 = 1,999 cd/m².
 	</div>
 
@@ -952,19 +954,20 @@ are correctly handled
 and not clipped or gamut mapped until
 the final transfer to the device color space.
 
-Relative HDR, using the HLG transfer function,
+HDR using the HLG transfer function,
 must map SDR media white
 to the same luminance as is used
 to display the 75% HLG value. [[!SMPTE-ST-2084]]
 
 For further details, see also tables 3 and 4 in ITU Rpt_BT.2408-0 [[!Rpt_BT.2408]]
 
-Absolute HDR, using the PQ transfer function,
+HDR using the PQ transfer function,
 should map SDR media white
 to 203 cd/m², the same luminance as is used
-to display the 58% PQ value. [[!SMPTE-ST-2084]]
+to display the 58% PQ value. [[!SMPTE-ST-2084]].
+
 However, implementations may chose to incorporate
-a color re-rendering step (OOTF)
+a color re-rendering step
 to account for non-reference viewing conditions.
 
 <!--