[css-color-4] More even-handed description of clipping, showing some cases which give acceptable results w3c#10579

Author: svgeesus

css-color-4/Overview.bs

@@ -5400,6 +5400,11 @@ Clipping</h4>
 	The simplest and least acceptable method
 	is simply to clip the component values
 	to the displayable range.
+
+	Since the motivation for this method is speed,
+	clipping is commonly done on gamma-encoded values
+	rather than converting them to linear-light.
+
 	This changes the proportions of
 	the three primary colors (for an RGB display),
 	resulting in a hue shift.
@@ -5423,6 +5428,64 @@ Clipping</h4>
 		a substantial change of 69°.
 	</div>
 
+	When colors are not too far out of gamut,
+	clipping can give acceptable results.
+	This is particularly true for darker (or negative) 
+	component values.
+
+	<div class="example" id="ex-gamut-clip-good">
+		For example,
+		consider the color <code>color(rec2020 0.54 0.9 0)</code>
+		which is <code>oklch(80.72% 0.3296 141.6)</code>.
+
+		Converting to p3 colorspace, the negative blue value shows
+		the color is out of gamut:
+		<code>color(display-p3 0.3265 0.9165 -0.1262)</code>
+		which in linear-light is 
+		<code>color(display-p3-linear 0.0871 0.8205 -0.0146)</code>.
+
+		Clipping the gamma-encoded p3 color to the p3 gamut gives
+		<code>color(display-p3 0.3265 0.9165 0)</code>
+		which in linear-light is 
+		<code>color(display-p3-linear 0.0871 0.8205 0)</code>
+		which, for comparison, is 
+		<code>oklch(80.79% 0.3221 142.3)</code>.
+
+		This is a good result; 
+		the hue angle and lightness have barely changed
+		but the chroma is somewhat reduced, as expected.
+     
+		In terms of percentages of linear-light red green and blue,
+		the red and green are identical 
+		while the blue is -1.46% higher.
+	</div>
+
+	<div class="example" id="ex-gamut-clip-acceptable">
+		For example,
+		consider the color <code>color(prophoto-rgb 0.2 1.0 0.1)</code>
+		which is <code>oklch(85.07% 0.4873 151.4)</code>
+
+		Converting to p3 colorspace, the color is significantly
+		out of gamut:
+		<code>color(display-p3 -0.5782 1.067 -0.2363)</code>
+		which in linear-light is 
+		<code>color(display-p3-linear -0.2937 1.158 -0.0456)</code>.
+
+		Clipping the gamma-encoded p3 color to the p3 gamut gives
+		<code>color(display-p3 0 1 0)</code>
+		which in linear-light is, again, 
+		<code>color(display-p3-linear 0 1 0)</code>
+    	(component values of exactly 0 or 1 are unaffected by gamma encoding)
+		which, for comparison, is 
+		<code>oklch(84.88% 0.3685 145.6)</code>.
+
+		A less good but still visually acceptable result. 
+		Here the hue is more affected, a 5.8° change.
+
+		In terms of percentages of linear-light red green and blue,
+		red is 57% higher, green is 6.7% lower and blue is 23% higher.
+	</div>
+
 <h4 id="GM-closest">
 Closest Color (MINDE)</h4>