Merge pull request w3c#557 from cabanier/master

svgeesus · web-flow · commit 216f0372f20c · 2016-11-12T22:05:32.000-05:00
[css-color] Changes to CSS color spec from Adobe (see discussion thread)
diff --git a/.gitignore b/.gitignore
@@ -13,6 +13,5 @@ cssom-view/Overview.src.html
 *.orig
 Thumbs.db
 
-*/Overview.html
 
 node_modules
diff --git a/css-color/Overview.bs b/css-color/Overview.bs
@@ -152,7 +152,10 @@ Notes On Using Colors</h3>
 
 	An <dfn export>untagged image</dfn> is an image that is not explicitly assigned a color profile, as defined by the image format.
 
-	Note that this rule does not apply to videos, since untagged video should be presumed to be in CCIR 601.
+	Note that this rule does not apply to videos, since untagged video should be presumed to be in ITU.
+	* At below 720p, it is Recommendation ITU-R BT.601 [[!ITU-R-BT.601]]
+	* At 720p, it is SMPTE ST 296 (same colorimetry as 709) [[!SMPTE-ST-296]]
+	* At 1080p, it is Recommendation ITU-R BT.709 [[!ITU-R-BT.709]]
 
 	Issue:
 		Really?  Isn't HD video in Rec.709? Shouldn't video be consistent with images?
@@ -1613,7 +1616,7 @@ Converting sRGB colors to Lab colors</h3>
 
 	<!-- make these steps links to the appropriate section in a calculations appendix -->
 	<ol>
-		<li>Convert from sRGB to linear-light sRGB (undo gamma correction)
+		<li>Convert from sRGB to linear-light sRGB (undo gamma encoding)
 		<li>Convert from linear sRGB to CIE XYZ
 		<li>Convert from a D65 whitepoint (used by sRGB) to the D50 whitepoint used
 			in Lab, with the Bradford transform
@@ -1633,7 +1636,7 @@ Converting Lab colors to sRGB  colors</h3>
 		<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 sRGB
-		<li>Convert from linear-light sRGB to sRGB (do gamma correction)
+		<li>Convert from linear-light sRGB to sRGB (do gamma encoding)
 	</ol>
 
 <h3 id='lab-to-lch'>
@@ -1691,7 +1694,7 @@ Converting gray colors to sRGB  colors</h3>
 		<li>Convert Lab to XYZ
 		<li>Adapt from D50 to D65 (Bradford transform)
 		<li>Convert from (D65-adapted) CIE XYZ to linear sRGB
-		<li>Convert from linear-light sRGB to sRGB (do gamma correction)
+		<li>Convert from linear-light sRGB to sRGB (do gamma encoding)
 	</ol>
 
 <h2 id='icc-colors'>
@@ -1714,7 +1717,7 @@ Profiled, Device-dependent Colors</h2>
 	or an RGB colorspace (such as ProPhoto <!-- ref -->, <!-- would be good to mention AdobeRGB 1998 here too, need to check trademark and copyright issues first --> widely used by photographers), or any other color or monochrome output device which has been characterized.
 	In addition, for convenience,
 	CSS provides two predefined RGB color spaces:
-	DCI P3 [[!DCI-P3]],
+	image-p3 [[!DCI-P3]],
 	which is a wide gamut space typical of current wide-gamut monitors,
 	and Rec. 2020 [[[!Rec.2020]],
 	which is a ultra-wide gamut space capable of representing almost all visible real-world colors.
@@ -1770,7 +1773,7 @@ Profiled, Device-dependent Colors</h2>
 
 	* An optional <<ident>> denoting the colorspace.
 		This can be one of the predefined colorspaces
-		(such as ''dci-p3''),
+		(such as ''image-p3''),
 		or one defined by a ''@color-profile'' rule.
 		If omitted, it defaults to the predefined ''srgb'' color profile.
 
@@ -1819,7 +1822,7 @@ Profiled, Device-dependent Colors</h2>
 	identical to ''rgb(0 0 0)''.
 
 <h3 id="predefined">
-Predefined colorspaces: ''srgb'', ''dci-p3'', and ''rec2020''.</h3>
+Predefined colorspaces: ''srgb'', ''image-p3'', and ''rec2020''.</h3>
 
 	The following colorspaces are predefined for use in the ''color()'' function.
 	They can be used without any ''@color-profile'' rule.
@@ -1830,35 +1833,50 @@ Predefined colorspaces: ''srgb'', ''dci-p3'', and ''rec2020''.</h3>
 	<dl dfn-type=value dfn-for="color()">
 		<dt><dfn>srgb</dfn>
 		<dd>
-			The ''srgb'' [[!DCI-P3]] colorspace accepts three numeric parameters,
+			The ''srgb'' [[!SRGB]] colorspace accepts three numeric parameters,
 			representing the red, green, and blue channels of the color,
 			with each having a valid range of [0, 1].
 
 			It is the default colorspace for CSS,
 			identical to specifying a color with the ''rgb()'' function.
 
-		<dt><dfn>dci-p3</dfn>
+			It has the following characteristics:
+
+			<table>
+				<thead><td></td><td>x</td><td>y</td></thead>
+				<tr><th>Red chromaticity</th><td>0.640</td><td>0.330</td></tr>
+				<tr><th>Green chromaticity</th><td>0.300</td><td>0.600</td></tr>
+				<tr><th>Blue chromaticity</th><td>0.150</td><td>0.060</td></tr>
+				<tr><th>White chromaticity</th><td>0.3127</td><td>3290</td></tr>
+				<tr><th>Transfer function</th><td>see below</td></tr>
+			</table>
+
+			<pre class="lang-javascript">
+			var Cl;
+			if (C <= 0.04045)
+				Cl = C / 12.92;
+			else
+				Cl = Math.pow((C + 0.055) / 1.055, 2.4);
+			</pre>
+			C is the red, green or blue component.
+
+		<dt><dfn>image-p3</dfn>
 		<dd>
-			The ''dci-p3'' [[!DCI-P3]] colorspace accepts three numeric parameters,
+			The ''image-p3'' [[!DCI-P3]] colorspace accepts three numeric parameters,
 			representing the red, green, and blue channels of the color,
 			with each having a valid range of [0, 1].
 
 			It has the following characteristics:
 
 			<table>
-				<tr><th>Red chromaticity</th><td>x=0.680 y=0.320</td></tr>
-				<tr><th>Green chromaticity</th><td>x=0.265 y=0.690</td></tr>
-				<tr><th>Blue chromaticity</th><td>x=0.150 y=0.060</td></tr>
-				<tr><th>White chromaticity</th><td>x=0.3127 y=0.3290 (D65)</td></tr>
-				<tr><th>Transfer function</th><td>1/2.6</td></tr>
+				<thead><td></td><td>x</td><td>y</td></thead>
+				<tr><th>Red chromaticity</th><td>0.680</td><td>0.320</td></tr>
+				<tr><th>Green chromaticity</th><td>0.265</td><td>0.690</td></tr>
+				<tr><th>Blue chromaticity</th><td>0.150</td><td>0.060</td></tr>
+				<tr><th>White chromaticity</th><td>0.3127</td><td>3290</td></tr>
+				<tr><th>Transfer function</th><td>same as srgb</td></tr>
 			</table>
 
-			Issue: The defining spec for dci-p3 is not freely available. It costs USD 240.
-				There is a Wikipedia article, which is non-authoritative and of unknown accuracy
-				(particularly for the transfer function and viewing conditions).
-
-			Issue: DCI transfer function for P3 is gamma 1/2.6 while Apple P3 displays use gamma 1/2.2.
-
 		<dt><dfn>rec2020</dfn>
 		<dd>
 			The ''rec2020'' [[!Rec.2020]] colorspace accepts three numeric parameters,
@@ -1872,18 +1890,14 @@ Predefined colorspaces: ''srgb'', ''dci-p3'', and ''rec2020''.</h3>
 				<tr><th>Green chromaticity</th><td>x=0.170 y=0.797</td></tr>
 				<tr><th>Blue chromaticity</th><td>x=0.131 y=0.046</td></tr>
 				<tr><th>White chromaticity</th><td>x=0.3127 y=0.3290 (D65)</td></tr>
-				<tr><th>Transfer function</th><td>see below</td></tr>
+				<tr><th>Transfer function</th><td>1/2.4 (see note)</td></tr>
 			</table>
 
-			''rec2020'' transfer function has the same form as the one for sRGB, but with
-			the constants at higher precision to cope with 10 or 12-bit components:
+			Note: Rec2020 references a different transfer curve for cameras. However
+			this curve is never used in production cameras or 2020 displays.<br/>
+			"In typical production practice the encoding function of image sources is adjusted so that the final picture has the desired look, as viewed on a reference monitor having the reference decoding function of Recommendation ITU-R BT.1886, in the reference viewing environment defined in Recommendation ITU-R BT.2035."<br/>
+			The transfer function (1886) for reference Rec.2020 displays is gamma 2.4 [[!Rec.2020]]
 
-			<pre class="lang-javascript">
-			E < β ? 4.5 * E : α * Math.pow(E, 0.45) - (α - 1);
-			</pre>
-
-			Where α=1.09929682680944  and β=0.018053968510807
-			<!-- full-precision constants here, not the rounded ones for 10 or 12bit components -->.
 	</dl>
 
 <h4 id="predefined-to-lab">
@@ -1895,9 +1909,9 @@ Converting predefined colorspaces to Lab</h4>
 
 	<!-- 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 gamma-corrected RGB to linear-light RGB (undo gamma encoding)
 		<li>Convert from linear RGB to CIE XYZ
-		<li>Convert from a D65 whitepoint (used by both dci-p3 and rec2020) to the D50 whitepoint used in Lab,
+		<li>Convert from a D65 whitepoint (used by both image-p3 and rec2020) to the D50 whitepoint used in Lab,
 			with the Bradford transform
 		<li>Convert D50-adapted XYZ to Lab
 	</ol>
@@ -1907,15 +1921,15 @@ Converting predefined colorspaces to Lab</h4>
 <h4 id="lab-to-predefined">
 Converting Lab to predefined colorspaces</h4>
 
-	Conversion from Lab to dci-p3 or rec2020 also requires multiple steps,
+	Conversion from Lab to image-p3 or rec2020 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)
+		<li>Convert from linear-light RGB to RGB (do gamma encoding)
 	</ol>
 
 	Implementations may choose to implement these steps in some other way
@@ -1936,7 +1950,7 @@ Specifying a color profile: the ''color-profile'' at-rule</h3>
 
 	The <<custom-ident>> gives the <a>color profile's</a> name.
 	All of the predefined colorspace keywords
-	(''srgb'', ''dci-p3'', ''rec2020'')
+	(''srgb'', ''image-p3'', ''rec2020'')
 	are excluded from this <<custom-ident>>,
 	as they're predefined by this specification and always available.
 
@@ -2653,9 +2667,6 @@ Issue: The code below is only for sRGB, and duplicates the
 			Otherwise, set the channel's value to
 			<code>((channel + 0.055) / 1.055) ^ 2.4</code>.
 
-			Note: This reverses the logarithmic power scaling of the sRGB gamut,
-			so the value of the channel is  linear with respect to the amount of light produced.
-
 		<li>
 			The luminance is:
 
@@ -2741,11 +2752,7 @@ Transparency: the 'opacity' property</h2>
 Simple alpha compositing</h3>
 
 	When drawing, implementations must handle alpha
-	according to the rules in <a href="https://www.w3.org/TR/2003/REC-SVG11-20030114/masking.html#SimpleAlphaBlending">Section 14.2 Simple alpha compositing</a> of [[!SVG11]].
-	(If the <a property spec=svg2>color-interpolation</a>
-	or 'color-rendering' properties mentioned in that section
-	are not implemented or do not apply, implementations must act as though they have their initial values.)
-
+	according to the rules in <a href="https://www.w3.org/TR/compositing-1/#simplealphacompositing">Section 5.1 Simple alpha compositing</a> of [[!Compositing]].
 
 <h2 id='color-adjust'>
 Preserving Colors in Different-Capability Devices: the 'color-adjust' property</h2>
@@ -2846,6 +2853,8 @@ Default Style Rules</h2>
 
 <h2 id="color-conversion-code">Sample code for color conversions</h2>
 
+<em>This section is not normative.</em>
+
 <pre class="lang-javascript">
 	<!-- imported 11 May 2016-->
 // sRGB-related functions
@@ -2900,15 +2909,19 @@ function XYZ_to_lin_sRGB(XYZ) {
 	return math.multiply(M, XYZ).valueOf();
 }
 
-// DCI P3-related functions
+// image-p3-related functions
 
 
 function lin_P3(RGB) {
-	// convert an array of DCI P3 RGB values in the range 0.0 - 1.0
+	// convert an array of image-p3 RGB values in the range 0.0 - 1.0
 	// to linear light (un-companded) form.
 
 	return RGB.map(function (val) {
-		return Math.pow(val, 2.6);
+		if (val < 0.04045) {
+			return val / 12.92;
+		}
+
+		return Math.pow((val + 0.055) / 1.055, 2.4);
 	});
 }
 
@@ -2917,7 +2930,11 @@ function gam_P3(RGB) {
 	// to gamma corrected form
 
 	return RGB.map(function (val) {
-			return Math.pow(val, 1/2.6);
+		if (val > 0.0031308) {
+			return 1.055 * Math.pow(val, 1/2.4) - 0.055;
+		}
+
+		return 12.92 * val;
 	});
 }
 
@@ -2951,31 +2968,17 @@ function XYZ_to_lin_P3(XYZ) {
 function lin_2020(RGB) {
 	// convert an array of Rec. 2020 RGB values in the range 0.0 - 1.0
 	// to linear light (un-companded) form.
-	const α = 1.09929682680944 ;
-	const β = 0.018053968510807;
-
 	return RGB.map(function (val) {
-		if (val < β * 4.5 ) {
-			return val / 4.5;
-		}
-
-		return Math.pow((val + α -1 ) / α, 2.4);
+		return Math.pow(val, 2.4);
 	});
 }
 //check with standard this really is 2.4 and 1/2.4, not 0.45 was wikipedia claims
 
 function gam_2020(RGB) {
 	// convert an array of linear-light Rec. 2020 RGB  in the range 0.0-1.0
 	// to gamma corrected form
-	const α = 1.09929682680944 ;
-	const β = 0.018053968510807;
-
 	return RGB.map(function (val) {
-		if (val > β ) {
-			return α * Math.pow(val, 1/2.4) - (α - 1);
-		}
-
-		return 4.5 * val;
+			return Math.pow(val, 1/2.4);
 	});
 }
 
diff --git a/css-color/biblio.json b/css-color/biblio.json
@@ -0,0 +1,14 @@
+{
+    "ITU-R-BT.601": {
+        "href": "https://www.itu.int/rec/R-REC-BT.601/en",
+        "title": "Recommendation ITU-R BT.601"
+    },
+    "ITU-R-BT.709": {
+       "title": "Recommendation ITU-R BT.709",
+       "href": "https://www.itu.int/rec/R-REC-BT.709/en"
+    },
+    "SMPTE-ST-296": {
+       "title": "SMPTE ST 296",
+       "href": "http://ieeexplore.ieee.org/document/7291722/"
+    }
+}
