[css-color-4] Expand analogous components to analogous sets to minimize none → 0 conversions #10210

Resolution from #10210 (comment)

Author: LeaVerou

css-color-4/Overview.bs

@@ -4989,6 +4989,15 @@ Interpolating with Missing Components</h3>
 	before premultiplication, and
 	before linear interpolation takes place.
 
+	Similarly, if every component of an [=analogous set=]
+	(defined below)
+	in the original color
+	is a [=missing component=],
+	they are all [=carried forward=]
+	and re-inserted
+	in the corresponding [=analogous set=]
+	of the [=interpolation color space=].
+
 	The <dfn export>analogous components</dfn>  are as follows:
 
 	<table>
@@ -5041,6 +5050,41 @@ Interpolating with Missing Components</h3>
 	The Whiteness and Blackness components of HWB
 	have no analogs in other color spaces.
 
+	Additionally, for any two color spaces,
+	the components that remain
+	after removing all [=analogous components=]
+	form an <dfn export>analogous set</dfn> of components.
+
+	Note: Because the full set of all color components
+	is the [=analogous set=] that remains
+	when there are no individual [=analogous components=],
+	a color with all color components [=missing=]
+	will have all color components [=missing=]
+	in the [=interpolation color space=] as well.
+
+	<div class="example" id="ex-analogous-set">
+		When converting
+		<code class="lang-css">lab(50% none none)</code> to LCH
+		for interpolation,
+		Lightness is individually [=analogous components|analogous=].
+		The remaining components
+		(''a'' and ''b'' in Lab; ''C'' and ''H'' in LCH)
+		form an [=analogous set=].
+		Since both ''a'' and ''b'' are [=missing=],
+		both ''C'' and ''H'' are carried forward as [=missing=],
+		giving <code class="lang-css">lch(50% none none)</code>
+		rather than <code class="lang-css">lch(50% 0 0)</code>.
+
+		Similarly,
+		<code class="lang-css">rgb(none none none / 50%)</code>
+		converted to OKLab for interpolation
+		yields <code class="lang-css">oklab(none none none / 50%)</code>,
+		because the three color components
+		form the [=analogous set=]
+		(there are no individual [=analogous components=]
+		between sRGB and OKLab).
+	</div>
+
 	<wpt pathprefix="css/css-images/">
 		gradient/gradient-none-interpolation.html
 	</wpt>
@@ -7905,6 +7949,9 @@ Changes</h2>
 <h3 id="changes-from-20240213">Changes since the <a href="https://www.w3.org/TR/2024/CRD-css-color-4-20240213/">Candidate Recommendation Draft of 13 Feb 2024</a></h3>
 
 <ul>
+	<li>Expanded the concept of analogous components to analogous sets of components, to minimize ''none'' → ''0'' conversions
+		(<a href="https://github.com/w3c/csswg-drafts/issues/10210">Issue 10210</a>)
+	</li>
 	<!-- to 21 Apr 2025 -->
 	<li>Clarified that inside the color property, it is the resolved inherited value (not the raw inherited value) that is used</li>
 	<li>Listed categories of colors, such as those that resolve to sRGB or support legacy color syntax</li>