[css-color-hdr] Defined the term "Absolute D65 CIE XYZ"

Author: svgeesus

css-color-hdr-1/Overview.bs

@@ -603,7 +603,24 @@ Mixing Dynamic Range Limits: the ''dynamic-range-limit-mix()'' function {#dynami
 	on a display with HDR headroom 1:
 
 	X = 243.664, Y = 275.713, Z = 244.000
+	</div>
+
+	Colors are interpolated in <dfn export>Absolute D65 CIE XYZ</dfn>.
+
+	This is derived from ''xyz-d65''
+	by multiplying each color component by 203 [[Rpt_BT.2408]]
+	to convert from relative units (Y of white is 1, regardless of actual brightness)
+	to absolute units of cd/m², also clled "nits".
+	In the <a href="color-conversion-code">sample code</a>,
+	this constant is denoted <var ignore=''>Yw</var>,
+	the luminance of HDR Reference White.
+
+	<div class="example" id="ex-rel-to-abs">
+		For example, the css color ''white'' is 
+		<pre class="lang-css">color(xyz-d65 0.9505 1 1.089)</pre>
 
+		In absolute D65 XYX this is
+		[192.9426, 203, 221.0787]
 	</div>
 
 <h3 id="headroom-interpolation">
@@ -615,8 +632,8 @@ Mixing Dynamic Range Limits: the ''dynamic-range-limit-mix()'' function {#dynami
 	giving result color <var>cxyz</var>,
 	when the target headroom is <var>H</var>:
 
-	1. Let <var>c1xyz</var> be <var>c1</var> converted to Absolute D65 CIE XYZ
-	2. Let <var>c2xyz</var> be <var>c2</var> converted to Absolute D65 CIE XYZ
+	1. Let <var>c1xyz</var> be <var>c1</var> converted to [=Absolute D65 CIE XYZ=]
+	2. Let <var>c2xyz</var> be <var>c2</var> converted to [=Absolute D65 CIE XYZ=]
 	3. Let <var> w1</var> = clamp((<var>H</var> - <var>H2</var>) / (<var>H1</var> - <var>H2</var>), 0, 1)
 	4. Let <var> w2</var> = clamp((<var>H</var> - <var>H1</var>) / (<var>H2</var> - <var>H1</var>), 0, 1)
 		Note that <var> w2</var> = 1 - <var> w1</var>