[css-color] update gray() section per SF meeting

Author: svgeesus

css-color/Overview.bs

@@ -1464,55 +1464,7 @@ Examples of HWB Colors</h3>
 		</table>
 	</div>
 
-
-<h2 id='grays'>
-Specifying Grays: the ''gray()'' functional notation</h2>
-
-	Grays are a very special set of colors.
-	They're fully desaturated
-	(lacking any actual color at all),
-	which means that specifying a gray with any of the other notations requires specifying some redundant information:
-	if specifying the color with ''rgb()'', all three channels are identical;
-	if specifying the color with ''hsl()'', the hue is irrelevant and the saturation is locked to ''0%''.
-
-	The ''gray()'' functional notation simplifies specifying this common set of colors,
-	so that only the necessary information is required.
-
-	<pre class='prod'>
-	<dfn>gray()</dfn> = gray( [<<number>> | <<percentage>>] [, <<alpha-value>>]? )
-	</pre>
-
-	The first argument specifies the shade of gray,
-	while the second optional argument specifies the alpha channel of the color.
-
-	''gray(A)'' represents the same color as ''rgb(A,A,A)''.
-	''gray(A,B)'' represents the same color as ''rgb(A,A,A,B)''.
-
-	<div class='issue lang-javascript'>
-		<a>gray()</a> should have a keyword arg that opts it into smarter modes.
-		"luminance", for example, instead of being a simple expansion to ''rgb()'',
-		could actually compute the gray with the specified luminance.
-		The sRGB power curve means that grays are much darker than they "should" be;
-		50% gray has a luminance of .21, for example, rather than .5.
-		<!-- that seems to be confusing gamma correction, linear light intensity (luminance), 
-		and perceptual lightness. -->
-
-		Reversing luminance to color is easy for grays:
-		if L &lt; .0774, <code>x * 12.92</code>;
-		otherwise, <code>(x ^ (5/12)) * 1.055 - .055</code>.
-		<!-- check, where does .0774 come from, should be .031308 -->
-
-		Alternately, base it on relative contrast ratios somehow?
-		#757575 is the gray that lives exactly between white and black, in contrast-ratio space.
-
-		Or use the L portion of Lab, where L=50 is a mid gray.
-
-		<!-- citation required -->
-		Taking 0% to be black, 100% to be white, and 50% to be #757575,
-		you convert between contrast-ratio space and luminance space
-		with <code>L = 21^p</code>,
-		then find the gray with that luminance.
-	</div>
+	
 
 <h2 id='lab-colors'>
 Device-independent Colors: Lab and LCH</h2>
@@ -1652,6 +1604,39 @@ Converting LCH colors to Lab  colors</h3>
 	<li>L is the same
 </ol>
 
+<h2 id='grays'>
+Specifying Grays: the ''gray()'' functional notation</h2>
+
+<p class="issue">As suggested at San Francisco, keep the syntax here but make
+	gray(number, alpha?) an alias to Lab with a=b=0. However, this should be 
+	</p>
+	 
+	Grays are fully desaturated colors. The ''gray()'' functional notation simplifies 
+	specifying this common set of colors, so that only a single numerical parameter 
+	is required, and so that gray(50%) is a visual mid-gray (perceptually equidistant 
+	between black and white).
+	
+
+	<pre class='prod'>
+	<dfn>gray()</dfn> = gray( <<number>>  [, <<alpha-value>>]? )
+	</pre>
+
+	The first argument specifies the shade of gray,
+	while the second optional argument specifies the alpha channel of the gray.
+
+<h3 id='gray-to-rgb'>
+Converting gray colors to sRGB  colors</h3>
+
+<p>Conversion from gray to sRGB  requires multiple steps; in practice all 
+but the last step are linear calculations and can be combined.</p>
+
+<ol>
+	<li>Convert to D65-adapted Lab by setting L to the gray value, a and b to 0
+	<li>Convert Lab to (D65-adapted) XYZ (<em>No</em> Bradford transform)
+	<li>Convert from (D65-adapted) CIE XYZ to linear sRGB
+	<li>Convert from linear-light sRGB to sRGB (do gamma correction)
+</ol>
+
 <h2 id='icc-colors'>
 Profiled, Device-dependent Colors</h2>