use lab colorspace

Author: Tony Crisci

README.md

@@ -10,7 +10,7 @@ Naming things is one of the hard things in computer science.
 
 Colornamer-rs is a library written in Rust that helps you come up with a good name for a color.
 
-It does this by converting the color to RGB colorspace and then finding the nearest neighbor in a list of colors.
+It does this by converting the color to [Lab color space](https://en.wikipedia.org/wiki/Lab_color_space) and using the Delta E formula to compare the color difference in a list of colors with known names and finding the nearest neighbor.
 
 ## Example
 
@@ -38,14 +38,16 @@ The color names are derived from several lists:
 * Pantone
 * ntc - an [astounding collection](http://chir.ag/projects/ntc/) of over 1500 named colors.
 
-## TODO
-
-* Use [Lab color space](https://en.wikipedia.org/wiki/Lab_color_space) instead of RGB.
-
 ## Acknowledgements
 
 This library is based on the wonderful [color-namer](https://github.com/colorjs/color-namer) library for nodejs.
 
+Color difference formulas were found at [easyrgb.com](http://www.easyrgb.com/en/math.php).
+
+Further reading:
+
+* [Wikipedia article for color distance](https://en.wikipedia.org/wiki/Color_difference).
+
 ## License
 
 MIT (see LICENSE)

src/color.rs

@@ -81,11 +81,52 @@ pub fn color_from_triplet(name: &'static str, t: (u8, u8, u8)) -> Color {
 }
 
 impl Color {
+	/// http://www.easyrgb.com/en/math.php
+	fn to_lab(&self) -> (f32, f32, f32) {
+		let xyz_normalize = |c: f32| {
+			let c_normal = c / 255.0;
+			if c_normal > 0.04045 {
+				((c_normal + 0.055) / 1.055).powf(2.4)
+			} else {
+				c_normal / 12.92
+			}
+		};
+
+		let r = xyz_normalize(self.r);
+		let g = xyz_normalize(self.g);
+		let b = xyz_normalize(self.b);
+
+		let x = r * 0.4124 + g * 0.3576 + b * 0.1805;
+		let y = r * 0.2126 + g * 0.7152 + b * 0.0722;
+		let z = r * 0.0193 + g * 0.1192 + b * 0.9505;
+
+		let lab_normalize = |c: f32| {
+			if c > 0.008856 {
+				c.powf(1.0 / 3.0)
+			} else {
+				7.787 * c + 16.0 / 116.0
+			}
+		};
+
+		let lx = lab_normalize(x);
+		let ly = lab_normalize(y);
+		let lz = lab_normalize(z);
+
+		let l = 116.0 * ly - 16.0;
+		let a = 500.0 * (lx - ly);
+		let b = 200.0 * (ly - lz);
+
+		(l, a, b)
+	}
+
 	pub fn distance(&self, other: &Self) -> f32 {
-		let dr = self.r - other.r;
-		let dg = self.g - other.g;
-		let db = self.b - other.b;
+		let (sl, sa, sb) = self.to_lab();
+		let (ol, oa, ob) = other.to_lab();
+
+		let dl = sl - ol;
+		let da = sa - oa;
+		let db = sb - ob;
 
-		dr*dr + dg*dg + db*db
+		(dl*dl + da*da + db*db).sqrt()
 	}
 }