[css-values-4] Per WG resolution, define <ratio> interpolation as over the log of the ratio. Closes w3c#4953

Author: tabatkins

css-values-4/Overview.bs

@@ -1313,22 +1313,49 @@ Ratios: the <<ratio>> type</h3>
 Combination of <<ratio>></h4>
 
 	The interpolation of a <<ratio>> is defined
-	by converting each <<ratio>> to an equivalent form
-	with a ''1'' as its second value
-	(so a ratio of ''3 / 2'' would become ''1.5 / 1''),
-	then interpolating the numbers directly.
+	by converting each <<ratio>> to a number
+	by dividing the first value by the second
+	(so a ratio of ''3 / 2'' would become ''1.5''),
+	taking the logarithm of that result
+	(so the ''1.5'' would become approximately ''0.176''),
+	then interpolating those values.
+	The result during the interpolation
+	is converted back to a <<ratio>>
+	by inverting the logarithm,
+	then interpreting the result as a <<ratio>>
+	with the result as the first value and ''1'' as the second value.
 
 	<div class='example'>
 		For example,
 		halfway through a linear interpolation from ''5 / 1'' to ''3 / 2'',
-		the result is the ratio ''3.25 / 1'',
-		because 3.25 is halfway between 5 and 1.5.
+		the result is approximately the ratio ''2.73 / 1''
+		(roughly ''11 / 4'', slightly taller than a ''3 / 1'' ratio):
 
-		Note that this means the results are <em>not</em> scale-dependent;
-		interpolating from ''5 / 1'' to ''300 / 200''
-		gives the same results as the preceding example.
+		<pre>
+		start  = log(5);   // ≈ 0.69897
+		end    = log(1.5); // ≈ 0.17609
+		interp = 0.69897*.5 + 0.17609*.5; // ≈ 0.43753
+		final  = 10^interp; // ≈ 2.73
+		</pre>
 	</div>
 
+	Note: Interpolating over the logarithm of the ratio
+	means the results are scale-independent
+	(''5 / 1'' to ''300 / 200'' would give the same results as above),
+	that they're symmetrical over "wide" and "tall" variants
+	(interpolating from ''1 / 5'' to ''2 / 3'' would give
+	a ratio approximately equal to ''1 / 2.73'' at the halfway point),
+	and that they're symmetrical over whether the width is fixed and the height is based on the ratio
+	or vice versa.
+	These properties are not shared by many other possible interpolation strategies.
+
+	Note: Due to the properties of logarithms,
+	any log can be used;
+	the example here uses base-10 log,
+	but if, say, the natural log and e was used,
+	the intermediate results would be different
+	but the final result would be the same.
+
 	Addition of <<ratio>>s is not possible.
 
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