[css-color-4] clarify L=100 is a diffuse white, permit specular L values for HDR. Fixes w3c#289

Author: svgeesus

css-color-4/Overview.bs

@@ -115,6 +115,7 @@ Representing sRGB Colors: the <<color>> type</h2>
 	and can take any value between those two.
 
 	While all colors share an underlying storage format,
+	<!-- that is unlikely to be the case in future, with half floats etc being added to the OM -->
 	CSS contains several syntaxes for <em>specifying</em> <<color>> values.
 	Some directly specify the sRGB color,
 	such as the ''rgb()'' and ''rgba()'' functions
@@ -135,7 +136,7 @@ Representing sRGB Colors: the <<color>> type</h2>
 
 	Some operations work differently on <dfn export>achromatic</dfn> colors.
 	An <a>achromatic</a> color is a shade of gray:
-	in the RGB colorspace,
+	in the sRGB colorspace,
 	a color is <a>achromatic</a> if the red, green, and blue channels are all the same value;
 	in the HSL colorspace,
 	a color is <a>achromatic</a> if the saturation is ''0%'';
@@ -208,6 +209,7 @@ Resolving Color values</h2>
 
 		Issue: Gecko disagrees, and serializes any <<color>> with an alpha channel of 0 as transparent.
 		No other browser does that though.
+		<!-- needs WPT test -->
 
 		<li>
 		The computed value and used value of
@@ -1580,13 +1582,17 @@ Device-independent Colors: Lab and LCH</h2>
 
 	Lab is a rectangular coordinate system with a central Lightness axis.
 	L=0 is deep black (no light at all)
-	while L=100 is white
-	(D50 white, a standardized daylight spectrum with a color temperature of 5000K).
-	Usefully, L=50 is mid gray, by design:
+	while L=100 is a diffuse white
+	(the illuminant is D50 white, a standardized daylight spectrum with a color temperature of 5000K,
+	as reflected by a perfect diffuse reflector).
+	Values greater than 100 would correspond to specular highlights,
+	but their precise color is undefined in this specification.
+	Usefully, L=50 is mid gray, by design,
+	and equal increments in L are evenly spaced visually:
 	the Lab color space is intended to be <em>perceptually uniform</em>.
 	The a and b axes convey hue;
-	positive values along the a axis are red
-	while negative values are the complementary color, green.
+	positive values along the a axis are a purplish red
+	while negative values are the complementary color, a green.
 	Similarly, positive values along the b axis are yellow
 	and negative are the complementary blue/violet.
 	Desaturated colors have small values of a and b
@@ -1599,9 +1605,11 @@ Device-independent Colors: Lab and LCH</h2>
 	the values used in image editors which offer Lab editing,
 	and the value used by physical measurement devices such as spectrometers,
 	when they report measured colors in Lab.
-	Conversion from colors specified using other white points is called a chromatic adaptation transform,
+	Conversion from colors specified using other white points is called a <dfn export>chromatic adaptation transform</dfn>,
 	which models the changes in the human visual system as we adapt to a new lighting condition.
-	The Bradford algorithm is the industry standard chromatic adaptation transform,
+	The Bradford algorithm
+	<!-- add reference -->
+	is the industry standard chromatic adaptation transform,
 	and is easy to calculate as it is a simple matrix multiplication.
 
 
@@ -1649,6 +1657,9 @@ Specifying Lab and LCH: the lab() and lch() functional notations</h3>
 	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''.
+	Values less than ''0'' must be clipped to ''0''; values greater than
+	''100'' are permitted (for forwards compatibility with High Dynamic Range (HDR),
+	and must not be clipped.
 
 	The second and third arguments are the distances along the "a" and "b" axises
 	in the Lab colorspace,