Ensure Okhsl and Okhsv return undefined hues for achromatic colors (#517

)

* Ensure Okhsl and Okhsv return undefined hues for achromatic colors

Fixes #516

* Provide variables for epsilon values

* Update to use null and rework Okhsl saturation handling

src/spaces/okhsl.js

@@ -376,8 +376,8 @@ function okhslToOklab (hsl, lmsToRgb,  okCoeff) {
 
 	let [h, s, l] = hsl;
 	let L = toeInv(l);
-	let a = 0.0;
-	let b = 0.0;
+	let a = null;
+	let b = null;
 	h = constrain(h) / 360.0;
 
 	if (L !== 0.0 && L !== 1.0 && s !== 0) {
@@ -423,6 +423,10 @@ function okhslToOklab (hsl, lmsToRgb,  okCoeff) {
 function oklabToOkhsl (lab, lmsToRgb, okCoeff) {
 	// Oklab to Okhsl.
 
+	// Epsilon for lightness should approach close to 32 bit lightness
+	// Epsilon for saturation just needs to be sufficiently close when denoting achromatic
+	let εL = 1e-7;
+	let εS = 1e-4;
 	let L = lab[0];
 	let s = 0.0;
 	let l = toe(L);
@@ -458,7 +462,22 @@ function oklabToOkhsl (lab, lmsToRgb, okCoeff) {
 		}
 	}
 
-	return [constrain(h * 360), s, l];
+	const achromatic = Math.abs(s) < εS;
+	if (achromatic || l === 0.0 || Math.abs(1 - l) < εL) {
+		h = null;
+		// Due to floating point imprecision near lightness of 1, we can end up
+		// with really high around white, this is to provide consistency as
+		// saturation can be really high for white due this imprecision.
+		if (!achromatic) {
+			s = 0.0;
+		}
+	}
+
+	else {
+		h = constrain(h * 360);
+	}
+
+	return [h, s, l];
 }
 
 

src/spaces/okhsv.js

@@ -47,8 +47,8 @@ function okhsvToOklab (hsv, lmsToRgb, okCoeff) {
 	h = constrain(h) / 360.0;
 
 	let l = toeInv(v);
-	let a = 0.0;
-	let b = 0.0;
+	let a = null;
+	let b = null;
 
 	// Avoid processing gray or colors with undefined hues
 	if (l !== 0.0 && s !== 0.0) {
@@ -95,6 +95,8 @@ function okhsvToOklab (hsv, lmsToRgb, okCoeff) {
 function oklabToOkhsv (lab, lmsToRgb, okCoeff) {
 	// Oklab to Okhsv.
 
+	// Epsilon for saturation just needs to be sufficiently close when denoting achromatic
+	let ε = 1e-4;
 	let l = lab[0];
 	let s = 0.0;
 	let v = toe(l);
@@ -134,7 +136,15 @@ function oklabToOkhsv (lab, lmsToRgb, okCoeff) {
 		s = (s0 + tMax) * cv / ((tMax * s0) + tMax * k * cv);
 	}
 
-	return [constrain(h * 360), s, v];
+	if (Math.abs(s) < ε || v === 0.0) {
+		h = null;
+	}
+
+	else {
+		h = constrain(h * 360);
+	}
+
+	return [h, s, v];
 }
 
 

test/conversions.js

@@ -965,7 +965,7 @@ const tests = {
 				{
 					name: "sRGB white to Okhsl",
 					args: "white",
-					expect: [180, 0.6519721306444567, 1.0000000000000002],
+					expect: [ null, 0.0, 1.0000000000000002 ],
 				},
 				{
 					name: "sRGB red to Okhsl",
@@ -1000,7 +1000,7 @@ const tests = {
 				{
 					name: "sRGB black to Okhsl",
 					args: "black",
-					expect: [0.0, 0.0, 0.0],
+					expect: [ null, 0.0, 0.0 ],
 				},
 			],
 		},
@@ -1013,7 +1013,7 @@ const tests = {
 				{
 					name: "sRGB white to Okhsv",
 					args: "white",
-					expect: [ 180, 1.3189507366749435e-15, 1.0000000000000007 ],
+					expect: [ null, 1.3189507366749435e-15, 1.0000000000000007 ],
 				},
 				{
 					name: "sRGB red to Okhsv",
@@ -1048,7 +1048,7 @@ const tests = {
 				{
 					name: "sRGB black to Okhsv",
 					args: "black",
-					expect: [0.0, 0.0, 0.0],
+					expect: [ null, 0.0, 0.0],
 				},
 			],
 		},