some more work on conversions

svgeesus · svgeesus · commit 0ac2c5ca94fc · 2019-02-21T16:14:51.000+02:00
diff --git a/css-color-4/LCH-examples.html b/css-color-4/LCH-examples.html
@@ -0,0 +1,55 @@
+<html>
+    <meta charset="utf-8"></meta>
+	<script src="math.js"></script>
+	<script src="conversions.js"></script>
+	<script>
+	// convert a sampling of
+    // several RGB colorspaces
+    // to Lab and LCH
+
+    console.log("sRGB");
+
+	for (var r =0; r <=1; r+=0.5) {
+        for (var g =0; g <=1; g+=0.5) {
+            for (var b =0; b <=1; b+=0.5) {
+                console.log("r=", r, " g=", g, "b=", b);
+                var lab = D65_to_D50(XYZ_to_Lab(lin_sRGB_to_XYZ(lin_sRGB([r, g, b]))));
+		        console.log("Lab=",lab);
+                console.log("LCH=", Lab_to_LCH(lab));
+            }
+        }
+    };
+
+    console.log("image-p3");
+
+    // I am a bad person
+
+    for (var r =0; r <=1; r+=0.5) {
+        for (var g =0; g <=1; g+=0.5) {
+            for (var b =0; b <=1; b+=0.5) {
+                console.log("r=", r, " g=", g, "b=", b);
+                var lab = D65_to_D50(XYZ_to_Lab(lin_P3_to_XYZ(lin_P3([r, g, b]))));
+		        console.log("Lab=",lab);
+                console.log("LCH=", Lab_to_LCH(lab));
+            }
+        }
+    };
+
+    console.log("rec2020");
+
+    // write-once paste-often
+
+    for (var r =0; r <=1; r+=0.5) {
+        for (var g =0; g <=1; g+=0.5) {
+            for (var b =0; b <=1; b+=0.5) {
+                console.log("r=", r, " g=", g, "b=", b);
+                var lab = D65_to_D50(XYZ_to_Lab(lin_2020_to_XYZ(lin_2020([r, g, b]))));
+		        console.log("Lab=",lab);
+                console.log("LCH=", Lab_to_LCH(lab));
+            }
+        }
+    };
+
+
+	</script>
+</html>
diff --git a/css-color-4/conversions.js b/css-color-4/conversions.js
@@ -199,9 +199,9 @@ function XYZ_to_Lab(XYZ) {
 	var f = xyz.map(value => value > ε ? Math.cbrt(value) : (κ * value + 16)/116);
 
 	return [
-		(116 * f[1]) - 16, // L
+		(116 * f[1]) - 16, 	 // L
 		500 * (f[0] - f[1]), // a
-		200 * (f[1] - f[2]) // b
+		200 * (f[1] - f[2])  // b
 	];
 }
 
@@ -231,10 +231,11 @@ function Lab_to_XYZ(Lab) {
 
 function Lab_to_LCH(Lab) {
 	// Convert to polar form
+	var hue = Math.atan2(Lab[2], Lab[1]) * 180 / Math.PI;
 	return [
 		Lab[0], // L is still L
 		Math.sqrt(Math.pow(Lab[1], 2) + Math.pow(Lab[2], 2)), // Chroma
-		Math.atan2(Lab[2], Lab[1]) * 180 / Math.PI // Hue, in degrees
+		hue >= 0 ? hue : hue + 360 // Hue, in degrees [0 to 360)
 	];
 }
 
