[css-color-4] ad 2020 diagram

Author: svgeesus

css-color-4/Overview.bs

@@ -1971,9 +1971,6 @@ Specifying Lab and LCH: the ''lab()'' and ''lch()'' functional notations</h3>
 	''180deg'' points along the negative "a" axis (toward greenish cyan),
 	and ''270deg'' points along the negative "b" axis (toward sky blue).
 
-
-	<!-- add diagram showing hue angles in LCH for RGB primaries and secondaries in sRGB, P3, and Rec.2020 -->
-
 	There is an optional fourth alpha value,
 	separated by a slash,
 	and interpreted identically to the <<alpha-value>> in ''rgb()''.
@@ -2222,7 +2219,11 @@ Predefined colorspaces: ''srgb'', ''display-p3'', ''a98-rgb'', ''prophoto-rgb''
 			representing the red, green, and blue channels of the color,
 			with each having a valid range of [0, 1].
 			It uses the same primary chromaticities as [[!DCI-P3]],
-			but with a D65 whitepoint and the same transfer curve as sRGB.
+			but with a D65 whitepoint, and the same transfer curve as sRGB.
+
+			Modern displays, TVs, laptop screens and phone screens
+			are able to display all, or nearly all,
+			of the display-p3 gamut.
 
 			It has the following characteristics:
 
@@ -2320,6 +2321,8 @@ Predefined colorspaces: ''srgb'', ''display-p3'', ''a98-rgb'', ''prophoto-rgb''
 			representing the red, green, and blue channels of the color,
 			with each having a valid range of [0, 1]. ITU Reference 2020 is used for High Definition, 4k and 8k television.
 
+			Current displays are unable to reproduce the full gamut of rec2020. Coverage is expected to increase over time as displays improve.
+
 			It has the following characteristics:
 
 			<table>
@@ -2336,6 +2339,17 @@ Predefined colorspaces: ''srgb'', ''display-p3'', ''a98-rgb'', ''prophoto-rgb''
 			"In typical production practice the encoding function of image sources is adjusted so that the final picture has the desired look, as viewed on a reference monitor having the reference decoding function of Recommendation ITU-R BT.1886, in the reference viewing environment defined in Recommendation ITU-R BT.2035."<br/>
 			The transfer function (1886) for reference Rec.2020 displays is gamma 2.4 [[!Rec.2020]]
 
+			<figure>
+				<img src="images/2020-prim-sec.svg" style="width: 60vmin"
+				alt="diagram of rec2020 primaries and secondaries in LCH">
+				<figcaption>Visualization of the rec2020 colorspace in LCH. The primaries and secondaries are shown
+				(but in sRGB, not in the correct colors).
+				For comparison, the sRGB primaries and secondaries
+				are also shown, as dashed circles.
+				rec2020 primaries have much higher Chroma.
+				</figcaption>
+			</figure>
+
 	</dl>
 
 <h4 id="predefined-to-lab">