[css-color-4] Draft text for carried forward missing values, w3c#7536

Author: svgeesus

css-color-4/Overview.bs

@@ -4257,7 +4257,7 @@ Color Interpolation</h2>
 	Interpolation between <<color>> values
 	occurs by first
 	converting them to a given color space
-	which will be referred to as the 'interpolation space' below,
+	which will be referred to as the <dfn export>interpolation color space</dfn> below,
 	and then
 	linearly interpolating each component of the computed value of the color
 	separately.
@@ -4311,7 +4311,7 @@ Color Space for Interpolation</h3>
 	</div>
 
 	Mixing or otherwise combining colors
-	has different results depending on the color space used.
+	has different results depending on the [=interpolation color space=] used.
 	Thus, different color spaces may be more appropriate for each interpolation use case.
 
 	* In some cases, the result of physically mixing two colored lights is desired.
@@ -4328,7 +4328,7 @@ Color Space for Interpolation</h3>
 
 	These features are collectively termed the <dfn export>host syntax</dfn>.
 	The host syntax should define
-	what the default interpolation space should be for each case,
+	what the default [=interpolation color space=]  should be for each case,
 	and optionally provide syntax for authors to override this default.
 	If such syntax is part of a property value, it should use a <<color-interpolation-method>> token,
 	defined below for easy reference from other specifications.
@@ -4375,7 +4375,7 @@ Color Space for Interpolation</h3>
 	for at least one of their colors.
 
 	If the colors to be interpolated are outside the gamut
-	of the interpolation space,
+	of the [=interpolation color space=] ,
 	then once converted to that space,
 	they will contain out of range values.
 
@@ -4384,15 +4384,102 @@ Color Space for Interpolation</h3>
 <h3 id="interpolation-missing">
 Interpolating with Missing Components</h3>
 
-	If a color with a [=missing component=]
+	In the course of converting the two colors 
+	to the [=interpolation color space=],
+	any [=missing components=]
+	will be replaced with the value 0.
+
+	Thus, the first stage in interpolating two colors
+	is to classify any [=missing components=]
+	in the input colors,
+	and compare them to the components of the 
+	[=interpolation color space=].
+	If any analogous [=missing components=] are found,
+	they will be <strong>carried forward</strong>
+	and re-inserted in the converted color
+	before linear interpolation takes place.
+
+	The analogous components are as follows:
+
+	<table>
+		<tr>
+			<th>Category</th>
+			<th>Components</th>
+		</tr>
+		<tr>
+			<td>Reds</td>
+			<td>r,x</td>
+		</tr>
+		<tr>
+			<td>Greens</td>
+			<td>g,y</td>
+		</tr>
+		<tr>
+			<td>Blues</td>
+			<td>b,z</td>
+		</tr>
+		<tr>
+			<td>Lightness</td>
+			<td>L</td>
+		</tr>
+		<tr>
+			<td>Colorfulness</td>
+			<td>C, S</td>
+		</tr>
+		<tr>
+			<td>Hue</td>
+			<td>H</td>
+		</tr>
+	</table>
+
+	Note: for the purposes of this classification,
+	the XYZ spaces are considered super-saturated RGB spaces.
+	Also, despite Saturation being Lightness-dependent,
+	it falls in the same category as Chroma here.
+	The Whiteness and Blackness components of HWB
+	have no analogs in other color spaces.
+
+	<div class="example" id="ex-analogous-hue">
+		For example, if these two colors
+		are to be interpolated in OKLCH,
+		the missing hue in the CIE LCH color
+		is analogous to the hue component of OKLCH
+		and will be carried forward
+		while the missing blue component
+		in the second color
+		is not analogous to any OKLCH component
+		and will not be carried forward:
+
+		<pre>
+			<span class="swatch" style="--color: rgb(46.647% 46.628% 46.633%)"></span> lch(50% 0.02 none)</span>
+			<span class="swatch" style="--color: rgb(71.559% 49.813% 0%)"></span> color(display-p3 0.7 0.5 none)</span>
+		</pre>
+
+		which convert to
+
+		<pre>
+			<span class="swatch" style="--color: rgb(46.647% 46.628% 46.633%)"></span> oklch(56.897% 0.0001 0)</span>
+			<span class="swatch" style="--color: rgb(71.559% 49.813% 0%)"></span> oklch(63.612% 0.1522 78.748)</span>
+		</pre>
+
+		and with carried forward [=missing component=] re-inserted,
+		the two colors to be interpolated are:
+
+		<pre>
+			<span class="swatch" style="--color: rgb(46.647% 46.628% 46.633%)"></span> oklch(56.897% 0.0001 none)</span>
+			<span class="swatch" style="--color: rgb(71.559% 49.813% 0%)"></span> oklch(63.612% 0.1522 78.748)</span>
+		</pre>
+	</div>
+
+	If a color with a carried forward [=missing component=]
 	is interpolated
 	with another color
 	which is not missing that component,
 	the [=missing component=]
 	is treated as having
-	the other color's component value.
+	the <em>other color's</em> component value.
 
-	<div class="example" id="ex-">
+	<div class="example" id="ex-oklch-missing-hue">
 		For example, if these two colors are interpolated,
 		the second of which has a missing hue:
 
@@ -4412,7 +4499,7 @@ Interpolating with Missing Components</h3>
 
 		<pre>
 			<span class="swatch" style="--color: rgb(86.6% 62.7% 86.7%)"></span> oklch(78.3% 0.108 326.5)
-			<span class="swatch" style="--color: rgb(48.4% 0% 22.8%)"></span>oklch(39.2% 0.4 0)
+			<span class="swatch" style="--color: rgb(48.4% 0% 22.8%)"></span> oklch(39.2% 0.4 0)
 		</pre>
 	</div>