[css-color-4] update prophoto-rgb description

Author: svgeesus

css-color-4/Overview.bs

@@ -2599,12 +2599,16 @@ Predefined colorspaces: ''srgb'', ''display-p3'', ''a98-rgb'', ''prophoto-rgb'',
 				The ''prophoto-rgb'' colorspace accepts three numeric parameters,
 				representing the red, green, and blue channels of the color,
 				with each having a valid range of [0, 1]. The transfer curve is
-				a gamma function with a value of 1/1.8.
+				a gamma function with a value of 1/1.8,
+				and a small linear portion near black.
 				The white point is D50, the same as is used by CIE Lab. Thus,
 				conversion to Lab does not require the chromatic adaptation step.
 
-				The ProPhoto RGB space uses primaries chosen to allow
-				a wide color gamut and to minimise hue shifts under tonal manipulation.
+				The ProPhoto RGB space uses hyper-saturated,
+				non physically realizable primaries.
+				These were chosen to allow
+				a wide color gamut and in particular,
+				to minimise hue shifts under tonal manipulation.
 				It is often used in digital photography as a wide gamut
 				colorspace for the master version of
 				photographic images. The ''prophoto-rgb'' colorspace allows CSS to
@@ -2637,6 +2641,20 @@ Predefined colorspaces: ''srgb'', ''display-p3'', ''a98-rgb'', ''prophoto-rgb'',
 					</pre>
 					C is the red, green or blue component.
 
+				<figure>
+					<img src="images/prophoto-prim-sec.svg" style="width: 80.68vmin"
+					alt="diagram of rec2020 primaries and secondaries in LCH">
+					<figcaption>Visualization of the protphoto-rgb 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.
+					prophoto-rgb primaries and secondaries have much higher Chroma,
+					but much of this ultrawide gamut
+					does not correspond to physically realizable colors.
+					</figcaption>
+				</figure>
+
+
 		<dt><dfn>rec2020</dfn>
 		<dd>
 			The ''rec2020'' [[!Rec.2020]] colorspace accepts three numeric parameters,