initial implementation

Cargo.toml

@@ -4,6 +4,6 @@ version = "0.1.0"
 authors = ["Robin Appelman <robin@icewind.nl>"]
 edition = "2018"
 
-# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
-
 [dependencies]
+rgb = "0.8"
+num-traits = "0.2"

src/lib.rs

@@ -1,7 +1,69 @@
+use num_traits::{cast, Bounded, Float, NumCast};
+use rgb::RGB;
+use std::ops::Div;
+
+/// Convert sRGB color to perceptual luminance
+///
+/// See [Wikipedia: Converting color to Grayscale](https://en.wikipedia.org/wiki/Grayscale#Converting_color_to_grayscale)
+pub fn luminance<T: Bounded + NumCast, F: Float + NumCast + Div>(color: RGB<T>) -> F {
+    let channels: [T; 3] = color.into();
+    let [r, g, b] = channels;
+    scale_channel::<T, F>(r) * cast(0.2126).unwrap()
+        + scale_channel::<T, F>(g) * cast(0.7152).unwrap()
+        + scale_channel::<T, F>(b) * cast(0.0722).unwrap()
+}
+
+fn scale_channel<T: Bounded + NumCast, F: Float + NumCast + Div>(channel: T) -> F {
+    let float: F = cast(channel).expect("Failed to convert rgb channel into float");
+    let relative =
+        float / cast(T::max_value()).expect("Max value to rgb channel doesn't fit into float");
+    if relative < cast(0.03928).unwrap() {
+        relative / cast(12.92).unwrap()
+    } else {
+        F::powf(
+            (relative + cast(0.055).unwrap()) / cast(1.055).unwrap(),
+            cast(2.4).unwrap(),
+        )
+    }
+}
+
+/// Calculate contrast between two colors as specified by [WCAG 2](http://www.w3.org/TR/WCAG20#contrast-ratiodef)
+pub fn contrast<T: Bounded + NumCast, F: Float + NumCast + Div>(a: RGB<T>, b: RGB<T>) -> F {
+    let luminance_a: F = luminance::<T, F>(a) + cast::<_, F>(0.05).unwrap();
+    let luminance_b: F = luminance::<T, F>(b) + cast::<_, F>(0.05).unwrap();
+
+    if luminance_a > luminance_b {
+        luminance_a / luminance_b
+    } else {
+        luminance_b / luminance_a
+    }
+}
+
 #[cfg(test)]
 mod tests {
+    use super::*;
+    use rgb::RGB8;
+
     #[test]
-    fn it_works() {
-        assert_eq!(2 + 2, 4);
+    fn test_contrast() {
+        let white = RGB8::from([255, 255, 255]);
+        let black = RGB8::from([0, 0, 0]);
+        let red = RGB8::from([255, 0, 0]);
+        let green = RGB8::from([0, 255, 0]);
+        let blue = RGB8::from([0, 0, 255]);
+        let yellow = RGB8::from([255, 255, 0]);
+
+        assert_eq!(luminance::<_, f32>(white), 1.0);
+        assert_eq!(luminance::<_, f32>(black), 0.0);
+
+        assert_eq!(contrast::<_, f32>(white, white), 1.0);
+        assert_eq!(contrast::<_, f32>(white, black), 20.999998);
+        assert_eq!(contrast::<_, f32>(black, white), 20.999998);
+
+        assert_eq!(contrast::<_, f32>(white, red), 3.9984765);
+        assert_eq!(contrast::<_, f32>(white, green), 1.3721902);
+        assert_eq!(contrast::<_, f32>(white, blue), 8.592471);
+        assert_eq!(contrast::<_, f32>(white, yellow), 1.0738392);
+        assert_eq!(contrast::<_, f32>(black, yellow), 19.556);
     }
 }