more P3 and 2020 stuff

Author: svgeesus

css-color/Overview.bs

@@ -1735,6 +1735,42 @@ The ITU Rec.2020[[2020]] colorspace has the following characteristics:
 	<tr><th>Transfer function</th><td>see below</td></tr>
 </table>
 
+Rec.2020 defines two transfer functions, one for 10 bits per component 
+and one for 12 bits per component. Both use the same equation as sRGB, but with 
+the constants at higher precision to cope with 10 or 12-bit components:
+
+<pre> E < β ? 4.5 * E : α * Math.pow(E, 0.45) - (α - 1);
+</pre>	
+
+For 10-bit, α=1.099 and β=0.018. For 12-bit, α=1.0993 and β=0.0181. 
+<!-- or give the full values, since this will be done in double precision float by JS anyway -->
+
+For both predefined color spaces, conversion to Lab requires several steps, 
+although in practice all but the first step are linear calculations and can be combined.</p>
+
+<!-- make these steps links to the appropriate section in a calculations appendix -->
+<ol><li>Convert from gamma-corrected RGB to linear-light RGB (undo gamma correction)
+	<li>Convert from linear RGB to CIE XYZ
+	<li>Convert from a D65 whitepoint (used by both P3 and Rec.2020) to the D50 whitepoint used 
+		in Lab, with the Bradford transform
+	<li>Convert D50-adapted XYZ to Lab
+</ol>
+
+Conversion from Lab to P3 or Rec.2020 also requires multiple steps, and again in practice all 
+but the last step are linear calculations and can be combined.
+
+<ol>
+	<li>Convert Lab to (D50-adapted) XYZ
+	<li>Convert from a D50 whitepoint (used by Lab) to the D65 whitepoint used 
+		in sRGB, with the Bradford transform
+	<li>Convert from (D65-adapted) CIE XYZ to linear RGB
+	<li>Convert from linear-light RGB to RGB (do gamma correction)
+</ol>
+
+Implementations may choose to implement these steps in some other way (for example, 
+using an ICC profile with relative colorimetric rendering intent) provided the results 
+are the same for colors inside the source and destination gamuts.
+
 <h3 id="at-profile">Specifying a color profile: the ''profile'' at-rule</h3>
 <!-- we agreed to call it profile rather than color-profile or icc-profile, for brevity -->