[css-color-4] Use 64 bit accurate matrices for ProPhoto, w3c#7675

Author: svgeesus

css-color-4/conversions.js

@@ -152,24 +152,24 @@ function gam_ProPhoto(RGB) {
 }
 
 function lin_ProPhoto_to_XYZ(rgb) {
-	// convert an array of linear-light prophoto-rgb values to CIE XYZ
-	// using  D50 (so no chromatic adaptation needed afterwards)
-	// http://www.brucelindbloom.com/index.html?Eqn_RGB_XYZ_Matrix.html
+	// convert an array of linear-light prophoto-rgb values to CIE D50 XYZ
+	// matrix cannot be expressed in rational form, but is calculated to 64 bit accuracy
+	// see https://github.com/w3c/csswg-drafts/issues/7675
 	var M = [
-		[ 0.7977604896723027,  0.13518583717574031,  0.0313493495815248     ],
-		[ 0.2880711282292934,  0.7118432178101014,   0.00008565396060525902 ],
-		[ 0.0,                 0.0,                  0.8251046025104601     ]
+		[ 0.79776664490064230,  0.13518129740053308,  0.03134773412839220 ],
+		[ 0.28807482881940130,  0.71183523424187300,  0.00008993693872564 ],
+		[ 0.00000000000000000,  0.00000000000000000,  0.82510460251046020 ]
 	];
 
 	return multiplyMatrices(M, rgb);
 }
 
 function XYZ_to_lin_ProPhoto(XYZ) {
-	// convert XYZ to linear-light prophoto-rgb
+	// convert D50 XYZ to linear-light prophoto-rgb
 	var M = [
-	  	[  1.3457989731028281,  -0.25558010007997534,  -0.05110628506753401 ],
-	  	[ -0.5446224939028347,   1.5082327413132781,    0.02053603239147973 ],
-	  	[  0.0,                  0.0,                   1.2119675456389454  ]
+		[  1.34578688164715830, -0.25557208737979464, -0.05110186497554526 ],
+        [ -0.54463070512490190,  1.50824774284514680,  0.02052744743642139 ],
+        [  0.00000000000000000,  0.00000000000000000,  1.21196754563894520 ]
 	];
 
 	return multiplyMatrices(M, XYZ);