[css-color-4] add diagram

Author: svgeesus

css-color-4/Overview.bs

@@ -2762,6 +2762,7 @@ Device-dependent CMYK Colors: the ''device-cmyk()'' function</h2>
 	This is not trivial, like the conversion from HSL or HWB to RGB;
 	the precise conversion depends on the precise characteristics of the output device.
 
+	<!-- hopelessly handwavy and untestable -->
 	If the user agent has information about the output device such that it believes
 	it can accurately convert the CMYK color to a correct RGB color,
 	the computed value of the ''device-cmyk()'' function must be that RGBA color.
@@ -2784,6 +2785,23 @@ Device-dependent CMYK Colors: the ''device-cmyk()'' function</h2>
 		to avoid any color-matching difficulties.
 	</div>
 
+	The naive conversion is necessarily approximate,
+	since it has no knowledge of the colorimetry of the inks,
+	the dot gain,
+	the colorimetry of the RGB space, and so on.
+
+	<figure>
+		<img src="images/macbeth-naive.svg" style="width: 60vmin">
+		<figcaption>
+			A color checker, used for ensuring color fidelity
+			in the print and photographic inductries.
+			Averaged measured Lab values are available for each patch.
+			The rectangles show the Lab values, converted to sRGB.
+			The circles show the Lab values, passed through an ICC profile to convert them to CMYK.
+			The CMYK value are then naively converted to sRGB.
+		</figcaption>
+	</figure>
+
 <h3 id='cmyk-rgb'>
 Converting Between Uncalibrated CMYK and RGB-Based Colors</h3>