[css-color-hdr] Added sample code for Jzazbz #9934

Author: svgeesus

css-color-hdr-1/ICtCp.js

@@ -1,6 +1,6 @@
 
 function XYZ_to_ICtCp (XYZ) {
-    // convert an array of absolute, D65 XYZ to the ICtCp form of LMS
+    // convert an array of D65 XYZ to ICtCp
 
     // The matrix below includes the 4% crosstalk components
     // and is from the procedure in the Dolby "What is ICtCp" paper"
@@ -10,7 +10,7 @@ function XYZ_to_ICtCp (XYZ) {
         [  0.0070797844607479,  0.0748396662186362,  0.8433265453898765 ],
     ];
 
-    let LMS = multiplyMatrices(M, XYZ);
+    let LMS = multiplyMatrices(M, XYZ.map(v => v * Yw));
     return LMStoICtCp(LMS);
 }
 
@@ -33,6 +33,7 @@ function LMStoICtCp (LMS) {
     ];
 
     // apply the PQ EOTF
+    // values scaled so [0, 10,000] maps to [0, 1]
     // we can't ever be dividing by zero because of the "1 +" in the denominator
     let PQLMS = LMS.map (function (val) {
         let num = c1 + (c2 * ((val / 10000) ** m1));

css-color-hdr-1/Overview.bs

@@ -655,7 +655,7 @@ Predefined color spaces for HDR: {#predefined-HDR}
 	the black level should be adjusted 
 	using the PLUGE test signal and procedure 
 	specified in [[Rec_BT.814]] Annex 4. 
-	
+
 	For HLG values in CSS, which uses wide-range, 
 	the black corresponds to code point 0.
 
@@ -1106,6 +1106,17 @@ and both place media white at a component value of 1.0.
 	highlight: js
 </pre>
 
+<h3 id="jzazbz_code">
+	Sample code for ''jzazbz''
+</h3>
+
+This uses the same signed power (spow) function defined above.
+
+<pre class="include-code lang-javascript">
+	path: jzazbz.js
+	highlight: js
+</pre>
+
 <h3 id="ictcp_code">
 	Sample code for ''ICtCp''
 </h3>
@@ -1116,7 +1127,7 @@ which is how the conversion is defined in [[!Rec_BT.2100]],
 this sample code proceeds directly from absolute CIE XYZ
 for compatibility with the other color conversion code.
 
-The 4% crosstalk matrix, and the rotation ,
+The 4% crosstalk matrix, and the hue rotation,
 are also built into the XYZ to LMS step,
 rather than being applied as three separate steps.
 

css-color-hdr-1/jzazbz.js

@@ -0,0 +1,61 @@
+function XYZ_to_Jzazbz (XYZ) {
+    // convert an array of D65 XYZ toJzAzBz
+    // such that [0,0,0] is black and
+    // media white is [0.2220, -0.00016, -0.0001]
+
+    const Yw = 203; // absolute luminance of media white
+    const M = [
+        [  0.41478972, 0.579999,  0.0146480 ],
+        [ -0.2015100,  1.120649,  0.0531008 ],
+        [ -0.0166008,  0.264800,  0.6684799 ],
+    ];
+    const b = 1.15;
+    const g = 0.66;
+
+    // First make XYZ absolute, not relative to media white
+    // Maximum luminance in PQ is 10,000 cd/m²
+    // BT.2048 says media white Y=203  cd/m²
+    let [Xa, Ya, Za] = XYZ.map(v => v * Yw);
+
+    // then modify X and Y, to minimize blue curvature
+    let Xm = b * Xa - (b - 1) * Za;
+    let Ym = g * Ya - (g - 1) * Xa;
+
+    // now move to LMS cone domain
+    let LMS = multiplyMatrices(M, [Xm, Ym, Za]);
+    return LMStoJzazbz(LMS);
+}
+
+function LMStoJzazbz(LMS) {
+
+    const M = [
+        [  0.5,       0.5,       0        ],
+        [  3.524000, -4.066708,  0.542708 ],
+        [  0.199076,  1.096799, -1.295875 ],
+    ];
+    const c1 = 3424 / 2 ** 12;
+    const c2 = 2413 / 2 ** 7;
+    const c3 = 2392 / 2 ** 7;
+    const n = 2610 / 2 ** 14;
+    const p = (1.7 * 2523) / 2 ** 5;    // compared to usual PQ, 1.7 scale
+    const d = -0.56;
+    const d0 = 1.6295499532821566e-11;  // tiny shift to move black back to 0
+
+
+    // PQ-encode LMS
+    let PQLMS = (
+        LMS.map(function (val) {
+            let num = c1 + c2 * spow((val / 10000), n);
+            let denom = 1 + c3 * spow((val / 10000), n);
+
+            return spow((num / denom), p);
+        })
+    );
+
+    // calculate Iz az bz
+    let [Iz, az, bz] = multiplyMatrices(M, PQLMS);
+    // now Jz from Iz
+    let Jz = ((1 + d) * Iz) / (1 + d * Iz) - d0;
+    return [Jz, az, bz];
+}
+