[css-color-hdr] Defined "media white" more clearly, #11346

Author: svgeesus

css-color-hdr-1/Overview.bs

@@ -151,20 +151,29 @@ One stop is a doubling of luminance.
 	  } 
 </xmp>
 
+<dfn export>Media white</dfn>
+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.
+
 In <dfn export>Standard Dynamic Range</dfn> (SDR),
-the media white (the color of a normal white background, 
-or of white text on a dark background)
-is the lightest possible color.
+media white is also
+lightest possible color,
+produced by full-intensity red, green, and blue.
 
 <div class="example" id="ex-SDR-dynamic-range">
 	For example, in [=sRGB=], 
+	under standard viewing conditions,
 	white is defined to have a luminance of 80 cd/m²
 	while black is defined to have a luminance of 0.2 cd/m².
 	Thus, the dynamic range is 8.6 stops.
 </div>
 
-For SDR, [=luminance=] is treated as <em>relative</em> to media white.
-Making the screen brighter does not change the dynamic range,
+It is common to adjust the overall brighness of the screen
+to acommodate user preference or viewing conditions 
+which are different to the standard ones.
+
+For SDR, making the screen brighter does not change the dynamic range,
 because the darkest color also gets brighter.
 
 <div class="example" id="ex-SDR-brighter-dynamic-range">
@@ -175,9 +184,9 @@ because the darkest color also gets brighter.
 </div>
 
 In <dfn export>High Dynamic Range</dfn> (HDR),
-brighter colors than media white can be displayed.
-For example, if media white on an HDR display is set to around 200 cd/m²
-it might be possible to display highlights at 1000 cd/m².
+brighter colors than [=media white=] can be displayed.
+For example, if media white on an HDR display is set to 203 cd/m²
+it might be possible to display small highlights at 1000 cd/m².
 Typically, the brightest colors can only be displayed
 on a small part of the display,
 and for a limited time.
@@ -190,25 +199,35 @@ This is because of energy usage and heating considerations.
 	Thus, the dynamic range is 14.3 stops.
 </div>
 
-For HDR, [=luminance=] is treated as <em>absolute</em>,
-rather than being relative to media white.
-Using a brighter screen increases the dynamic range,
+For HDR, using a brighter screen increases the dynamic range,
 while the luminance of media white remains constant.
 
 <div class="example" id="ex-HDR-PQ-dynamic-range">
 	For example, using the ''rec2100-pq'' colorspace 
 	defined in [[Rec_BT.2100]], 
-	the brightest peak white is defined to have a luminance of 10000 cd/m²
+	the brightest peak white is defined to have a luminance of 10,000 cd/m²
 	while the deepest black has a luminance of 0.001 cd/m².
 	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>
+so that that the media white level 
+of the inputs to compositing
+will end up at the media white level 
+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.
+This may result in a media white being displayed
+at a value lower or higher than 203 cd/m².
+
 Introducing Headroom {#introducing-headroom}
 --------------------------------------------
 
-	<em>This section is non-normative</em>
-
-	The luminance level 
+	The peak [=luminance=] level 
 	which an HDR display can produce 
 	varies greatly.
 
@@ -225,12 +244,12 @@ Introducing Headroom {#introducing-headroom}
 
 <div class="example" id="ex-low-headroom">
 	For example, a low-end HDR display
-	(which just meets the requirements of
+	(which <em>just</em> meets the requirements of
 	<a href="https://displayhdr.org/">DisplayHDR 400</a>, 
 	the lowest tier of Vesa Certified DisplayHDR conformance [[DisplayHDR]])
 	might display media white at 
 	up to 200 cd/m<sup>2</sup>
-	while their peak white is only
+	while the peak white is only
 	400 cd/m<sup>2</sup>
 	(at the brightest media white, only one stop of HDR headroom).
 </div>