[css-color-4] correct prophoto transfer function, fix #5229

Author: svgeesus

css-color-4/Overview.bs

@@ -2448,11 +2448,20 @@ Predefined colorspaces: ''srgb'', ''display-p3'', ''a98-rgb'', ''prophoto-rgb'',
 					<tr><th>Green chromaticity</th><td>0.159597</td><td>0.840403</td></tr>
 					<tr><th>Blue chromaticity</th><td>0.036598</td><td>0.000105</td></tr>
 					<tr><th>White chromaticity</th><td>0.345704</td><td>0.358540</td></tr>
-					<tr><th>Transfer function</th><td colspan="2">1/1.800 </td></tr>
+					<tr><th>Transfer function</th><td colspan="2">see below </td></tr>
 					<tr><th>White luminance</th><td colspan="2">160.0 to 640.0 cd/m<sup>2</sup></td></tr>
 					<tr><th>Black luminance</th><td colspan="2">See text</td></tr>
 				</table>
 
+				<pre class="lang-javascript">
+					var Cl;
+					if (C <= (16/512))
+						Cl = C / 16;
+					else
+						Cl = Math.pow(C, 1.8);
+					</pre>
+					C is the red, green or blue component.
+
 		<dt><dfn>rec2020</dfn>
 		<dd>
 			The ''rec2020'' [[!Rec.2020]] colorspace accepts three numeric parameters,

css-color-4/conversions.js

@@ -107,8 +107,9 @@ function lin_ProPhoto(RGB) {
 	// to linear light (un-companded) form.
 	// Transfer curve is gamma 1.8 with a small linear portion
 	// TODO for negative values, extend linear portion on reflection of axis, then add pow below that
+	const Et2 = 16/512;
 	return RGB.map(function (val) {
-		if (val < 0.031248) {
+		if (val < Et2) {
 			return val / 16;
 		}
 
@@ -121,8 +122,9 @@ function gam_ProPhoto(RGB) {
 	// to gamma corrected form
 	// Transfer curve is gamma 1.8 with a small linear portion
 	// TODO for negative values, extend linear portion on reflection of axis, then add pow below that
+	const Et = 1/512;
 	return RGB.map(function (val) {
-		if (val > 0.001953) {
+		if (val > Et) {
 			return Math.pow(val, 1/1.8);
 		}