[css-color-hdr] Added non-normative introduction to HDR headroom

Author: svgeesus

css-color-hdr/Overview.bs

@@ -88,6 +88,89 @@ Value Definitions {#values}
 	also accept the <a>CSS-wide keywords</a> as their property value.
 	For readability they have not been repeated explicitly.
 
+Controlling Dynamic Range {#controlling-dynamic-range}
+==========================
+
+Introducing Headroom {#introducing-headroom}
+--------------------------------------------
+
+	<em>This section is non-normative</em>
+
+	The luminance level 
+	which an HDR display can produce 
+	varies greatly.
+	The highest levels (peak luminance)
+	are also typically constrained 
+	to small areas of the screen
+	and short durations of time.
+
+	The amount by which peak white is greater than media white
+	is termed the <dfn export>HDR headroom</dfn>,
+	and depends on the level of media white,
+	user preference,
+	and viewing conditions.
+
+	It is typically expressed in photographic stops,
+	where one stop means a doubling of luminance.
+
+<div class="example" id="ex-low-headroom">
+	For example, some lower-end HDR displays 
+	might display media white at 
+	up to 200 cd/m<sup>2</sup>
+	while their peak white is only
+	400 cd/m<sup>2</sup>
+	(at the brightest media white, only one stop of HDR headroom).
+</div>
+
+<div class="example" id="ex-high-headroom">
+	For example, a high-end HDR display 
+	might display media white at 
+	up to 400 cd/m<sup>2</sup>
+	while it's peak white is
+	1600 cd/m<sup>2</sup>.
+
+	In dim viewing conditions,
+	with media white set to 100 cd/m<sup>2</sup>,
+	this gives four stops (16x) of HDR headroom.
+</div>
+
+<p id="headroom-privacy">
+	The Web platform does not currently
+	directly expose the level of headroom of a display,
+	because it depends on viewing conditions
+	and real-time, unquantized headroom information
+	would be a tracking vector
+	(for example, detecting that a user moved outside
+	and that it is a sunny day).
+</p>
+
+<p id="constrained-headroom">
+	It is not always desirable 
+	to use the maximum available level
+	of HDR headroom.
+	CSS provides a way to control,
+	in broad terms,
+	how much headroom is desired;
+	and this level can vary by element
+	and over time.
+</p>
+
+<div class="example" id="ex-photo-grid">
+	For example, consider a Web page displaying a grid of photographs.
+	Some of these are HDR, including some very high dynamic range images
+	such as a shot straight into the sun.
+	It would be tiring to view the set of photos together
+	at full dynamic range.
+	At the same time, 
+	squashing everything to the SDR range 
+	would not adequately represent
+	the character of individual photos.
+	The design goal is to show "enough" HDR
+	to allow meaningful evaluation of the group of photos.
+</div>
+
+
+
 Predefined color spaces for HDR: {#predefined-HDR}
 =====================