Add luminance pixel format

src/image.rs

@@ -1,6 +1,8 @@
 pub mod linear_rgb;
+pub mod luminance;
 pub mod srgb;
 pub use linear_rgb::*;
+pub use luminance::*;
 pub use srgb::*;
 
 pub mod farbfeld;

src/image/luminance.rs

@@ -0,0 +1,79 @@
+use super::*;
+
+use ccvl_proc_macros::Pixel;
+
+#[derive(Clone, Copy, Debug, PartialEq, Pixel)]
+pub struct Luminance {
+	y: f32
+}
+
+const R_COEFF: f32 = 0.2126;
+const G_COEFF: f32 = 0.7152;
+const B_COEFF: f32 = 0.0722;
+
+impl Luminance {
+	pub fn from<T: Into<LinearRGB>>(pixel: T) -> Self {
+		Into::<Luminance>::into(Into::<LinearRGB>::into(pixel))
+	}
+
+	pub fn into<T: From<LinearRGB>>(self) -> T {
+		T::from(Into::<LinearRGB>::into(self))
+	}
+}
+
+impl From<LinearRGB> for Luminance {
+	fn from(rgb: LinearRGB) -> Self {
+		Self {
+			y: rgb.r * R_COEFF + rgb.g * G_COEFF + rgb.b * B_COEFF
+		}
+	}
+}
+
+impl Into<LinearRGB> for Luminance {
+	fn into(self) -> LinearRGB {
+		LinearRGB::new(self.y, self.y, self.y)
+	}
+}
+
+impl std::ops::Deref for Luminance {
+	type Target = f32;
+
+	fn deref(&self) -> &f32 {
+		&self.y
+	}
+}
+
+impl std::ops::DerefMut for Luminance {
+	fn deref_mut(&mut self) -> &mut f32 {
+		&mut self.y
+	}
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use proptest::prelude::*;
+
+    macro_rules! prop_assert_approx {
+        ( $act:expr, $exp:expr ) => {
+            prop_assert!(($act - $exp).abs() < 1e-6)
+        };
+    }
+
+	proptest! {
+		#[test]
+		fn to_rgb_copies_y_to_each_channel(y in 0.0f32..1.0) {
+			let res: LinearRGB = Luminance { y }.into();
+			prop_assert_eq!(res.r, y);
+			prop_assert_eq!(res.g, y);
+			prop_assert_eq!(res.b, y);
+		}
+
+		#[test]
+		fn approx_unchanged_from_rgb_round_trip(y in 0.0f32..1.0) {
+			let rgb: LinearRGB = Luminance { y }.into();
+			let res: Luminance = rgb.into();
+			prop_assert_approx!(res.y, y);
+		}
+	}
+}