[css-color-5] second premul example

Author: svgeesus

css-color-5/Overview.bs

@@ -806,13 +806,34 @@ is divided by the interpolated alpha value.
 	and
 	[37.2% 15.6% 38.4%]
 	before interpolation.
+
 	The midpoint of linearly interpolating these colors
 	would be [23.4% 10.2% 38.8%]
 	which, with an alpha value of 0.5,
 	is <span class="swatch" style="--color: rgb(46.8%, 21.4%, 77.6%, 0.5)"></span> rgb(46.8% 20.4% 77.6% / 0.5)
 	when premultiplication is undone.
 </div>
 
+<div class="example" id="ex-premultiplied-lab">
+	To interpolate two colors in lab
+	<span class="swatch" style="--color: rgb(76%, 62%, 03%, 0.4)"></span> rgb(76% 62% 03% / 0.4)
+	and
+	<span class="swatch" style="--color: rgb(91.56%, 3.87%, 74.09%, 0.6)"></span> color(display-p3 0.84 0.19 0.72 / 0.6)
+	they are first converted to lab
+	<span class="swatch" style="--color: rgb(76%, 62%, 03%, 0.4)"></span> lab(66.927% 4.873 68.622 / 0.4)
+	<span class="swatch" style="--color: rgb(91.56%, 3.87%, 74.09%, 0.6)"></span> lab(53.503% 82.672 -33.901 / 0.6)
+	then the L, a and b coordinates are premultiplied before interpolation
+	[26.771% 1.949 27.449]
+	and
+	[32.102% 49.603 -20.341].
+
+	The midpoint of linearly interpolating these would be
+	[29.4365% 25.776 3.554]
+	which, with an alpha value of 0.5,
+	is <span class="swatch" style="--color: rgb(87.604%, 38.956%, 51.753%, 0.5)"></span> lab(58.873% 51.552 7.108) / 0.5)
+	when premultiplication is undone.
+</div>
+
 Issue(445):
 
 Hue interpolation {#hue-interpolation}