lightness

src/Palette/Palette.gd

@@ -288,12 +288,29 @@ func sort(option: Palettes.SortOptions) -> void:
 		Palettes.SortOptions.VALUE:
 			sort_method = func(a: PaletteColor, b: PaletteColor): return a.color.v < b.color.v
 		Palettes.SortOptions.LIGHTNESS:
+			# Code inspired from:
+			# gdlint: ignore=max-line-length
+			# https://github.com/bottosson/bottosson.github.io/blob/master/misc/colorpicker/colorconversion.js#L519
 			sort_method = func(a: PaletteColor, b: PaletteColor):
-				var rgb_a: Array[float] = [a.color.r, a.color.g, a.color.b]
-				var rgb_b: Array[float] = [b.color.r, b.color.g, b.color.b]
-				var luminance_a = (rgb_a.max() + rgb_a.min()) / 2.0
-				var luminance_b = (rgb_b.max() + rgb_b.min()) / 2.0
-				return luminance_a < luminance_b
+				# function that returns OKHSL luminance
+				var lum : Callable = func(c: Color):
+					var l = 0.4122214708 * (c.r) + 0.5363325363 * (c.g) + 0.0514459929 * (c.b)
+					var m = 0.2119034982 * (c.r) + 0.6806995451 * (c.g) + 0.1073969566 * (c.b)
+					var s = 0.0883024619 * (c.r) + 0.2817188376 * (c.g) + 0.6299787005 * (c.b);
+					var l_ = pow(l, 1/3.0);
+					var m_ = pow(m, 1/3.0);
+					var s_ = pow(s, 1/3.0);
+					var oklab_l = 0.2104542553 * l_ + 0.7936177850 * m_ - 0.0040720468 * s_
+					# calculating toe
+					var k_1 = 0.206
+					var k_2 = 0.03
+					var k_3 = (1+k_1)/(1+k_2)
+					return 0.5 * (
+						k_3 * oklab_l - k_1
+						+ sqrt(
+							(k_3 * oklab_l - k_1) * (k_3 * oklab_l - k_1) + 4 * k_2 * k_3 * oklab_l)
+						)
+				return lum.call(a.color.srgb_to_linear()) < lum.call(b.color.srgb_to_linear())
 		Palettes.SortOptions.RED:
 			sort_method = func(a: PaletteColor, b: PaletteColor): return a.color.r < b.color.r
 		Palettes.SortOptions.GREEN: