[css-color] Better define lab()/lch() argument syntax and handling. Fix lch()'s hue to use <<hue>>. Add an explanatory note to gray(). Fixes w3c#272.

Author: tabatkins

css-color/Overview.bs

@@ -1530,7 +1530,8 @@ Device-independent Colors: Lab and LCH</h2>
 	LCH has the same L axis as Lab,
 	but uses polar coordinates C (chroma) and H (hue).
 	C is the geometric distance from the L axis
-	and H is the angle, in degrees, from the positive a axis.
+	and H is the angle from the positive a axis,
+	with positive angles being more clockwise.
 
 	Note: The Lightness axis in Lab should not be confused with the L axis in HSL.
 	For example, in HSL, the sRGB colors blue (#00F) and yellow (#FF0)
@@ -1552,30 +1553,53 @@ Specifying Lab and LCH: the lab() and lch() functional notations</h3>
 	<dfn>lab()</dfn> = lab( <<number>> <<number>> <<number>> [ / <<alpha-value>> ]? )
 	</pre>
 
-	The first argument specifies the CIE Lightness,
-	the second argument is a and the third is b.
-	L is constrained to the range [0, 100]
-	while a and b are signed values and theoretically unbounded
+	The first argument specifies the CIE Lightness.
+	This is typically a number between ''0''
+	(representing black)
+	and ''100''
+	(representing white),
+	similar to the lightness argument of ''hsl()''.
+	However, CIE Lightness <em>can</em> exceed this range on some systems,
+	with extra-bright whites using a lightness up to ''400''.
+
+	The second and third arguments are the distances along the "a" and "b" axises
+	in the Lab colorspace,
+	as described in the previous section.
+	These values are signed
+	(allow both positive and negative values)
+	and theoretically unbounded
 	(but in practice do not exceed ±160).
+
 	There is an optional fourth alpha value,
-	separated by a slash.
+	separated by a slash,
+	and interpreted identically to the <<alpha-value>> in ''rgb()''.
 
 	<pre class='prod'>
-	<dfn>lch()</dfn> = lch( <<number>> <<number>> <<number>> [ / <<alpha-value>> ]? )
+	<dfn>lch()</dfn> = lch( <<number>> <<number>> <<hue>> [ / <<alpha-value>> ]? )
 	</pre>
 
 	The first argument specifies the CIE Lightness,
-	the second argument is C and the third is H.
-	L is constrained to the range [0, 100].
-	C is an unsigned number, theoretically unbounded
-	(but in practice does not exceed 230).
-	H is constrained to the range [0, 360).
-	There is an optional fourth alpha value, separated by a slash.
-
-	Issue: Need to define handling for out of range numbers.
-	Clip L, wrap H?
-	Have also seen L up to 400,
-	specifically for high dynamic range situations.
+	interpreted identically to the Lightness argument of ''lab()''.
+
+	The second argument is the chroma
+	(roughly representing the "amount of color").
+	Its minimum useful value is ''0'',
+	while its maximum is theoretically unbounded
+	(but in practice does not exceed ''230'').
+	If the provided value is negative,
+	it is clamped to ''0''.
+
+	The third argument is the hue.
+	It's interpreted identically to the <<hue>> argument of ''hsl()'',
+	but doesn't map hues to angles in the same way.
+	Instead, ''0deg'' represents red (pointing along the positive "a" axis),
+	''90deg'' represents yellow (the positive "b" axis),
+	''180deg'' represents green (the negative "a" axis),
+	and ''270deg'' represents blue (the negative "b" axis).
+
+	There is an optional fourth alpha value,
+	separated by a slash,
+	and interpreted identically to the <<alpha-value>> in ''rgb()''.
 
 	Issue: Need to decide what, if anything, to do for high dynamic range on luminance.
 
@@ -1651,6 +1675,9 @@ Specifying Grays: the ''gray()'' functional notation</h2>
 	equal to the CIE Lightness,
 	while the second optional argument specifies the alpha channel of the gray.
 
+	Note: In other words,
+	''gray(a / b)'' is equal to ''lab(a 0 0 / b)''.
+
 <h3 id='gray-to-rgb'>
 Converting gray colors to sRGB  colors</h3>