Added a Grey trait, and implementations on baked-in colors. Fixes #13206 (#13237)

Added a Grey trait to allow colors to create a generic "grey" color.

This currently assumes the color spaces follow the same gradient, which
I'm pretty sure isn't true, but it should make a "grey-ish" color
relative to the provided intensity.

# Objective

- Implements #13206 

## Solution

- A small `Grey` trait was added and implemented for the common color
kinds.

## Testing

- Currently untested, unit tests exposed the non-linear relation between
colors. I am debating adding an example to show this, as I have no idea
what color space represents what relation of grey, and I figure others
may be similarly confused.

## Changelog

- The `Grey` trait was added, and the corresponding `grey` 

## BREAKING CHANGES

The const qualifier for LinearRGBA::gray was removed (the symbol still
exists via a trait, it's just not const anymore)

crates/bevy_color/src/color_ops.rs

@@ -42,6 +42,19 @@ pub trait Mix: Sized {
     }
 }
 
+/// Trait for returning a grayscale color of a provided lightness.
+pub trait Gray: Mix + Sized {
+    /// A pure black color.
+    const BLACK: Self;
+    /// A pure white color.
+    const WHITE: Self;
+
+    /// Returns a grey color with the provided lightness from (0.0 - 1.0). 0 is black, 1 is white.
+    fn gray(lightness: f32) -> Self {
+        Self::BLACK.mix(&Self::WHITE, lightness)
+    }
+}
+
 /// Methods for manipulating alpha values.
 pub trait Alpha: Sized {
     /// Return a new version of this color with the given alpha value.
@@ -112,6 +125,8 @@ pub(crate) fn lerp_hue(a: f32, b: f32, t: f32) -> f32 {
 
 #[cfg(test)]
 mod tests {
+    use std::fmt::Debug;
+
     use super::*;
     use crate::{testing::assert_approx_eq, Hsla};
 
@@ -145,4 +160,25 @@ mod tests {
         assert_approx_eq!(lerp_hue(350., 10., 0.5), 0., 0.001);
         assert_approx_eq!(lerp_hue(350., 10., 0.75), 5., 0.001);
     }
+
+    fn verify_gray<Col>()
+    where
+        Col: Gray + Debug + PartialEq,
+    {
+        assert_eq!(Col::gray(0.), Col::BLACK);
+        assert_eq!(Col::gray(1.), Col::WHITE);
+    }
+
+    #[test]
+    fn test_gray() {
+        verify_gray::<crate::Hsla>();
+        verify_gray::<crate::Hsva>();
+        verify_gray::<crate::Hwba>();
+        verify_gray::<crate::Laba>();
+        verify_gray::<crate::Lcha>();
+        verify_gray::<crate::LinearRgba>();
+        verify_gray::<crate::Oklaba>();
+        verify_gray::<crate::Oklcha>();
+        verify_gray::<crate::Xyza>();
+    }
 }

crates/bevy_color/src/hsla.rs

@@ -1,5 +1,5 @@
 use crate::{
-    Alpha, ColorToComponents, Hsva, Hue, Hwba, Lcha, LinearRgba, Luminance, Mix, Srgba,
+    Alpha, ColorToComponents, Gray, Hsva, Hue, Hwba, Lcha, LinearRgba, Luminance, Mix, Srgba,
     StandardColor, Xyza,
 };
 use bevy_math::{Vec3, Vec4};
@@ -119,6 +119,11 @@ impl Mix for Hsla {
     }
 }
 
+impl Gray for Hsla {
+    const BLACK: Self = Self::new(0., 0., 0., 1.);
+    const WHITE: Self = Self::new(0., 0., 1., 1.);
+}
+
 impl Alpha for Hsla {
     #[inline]
     fn with_alpha(&self, alpha: f32) -> Self {

crates/bevy_color/src/hsva.rs

@@ -1,5 +1,5 @@
 use crate::{
-    Alpha, ColorToComponents, Hue, Hwba, Lcha, LinearRgba, Mix, Srgba, StandardColor, Xyza,
+    Alpha, ColorToComponents, Gray, Hue, Hwba, Lcha, LinearRgba, Mix, Srgba, StandardColor, Xyza,
 };
 use bevy_math::{Vec3, Vec4};
 use bevy_reflect::prelude::*;
@@ -89,6 +89,11 @@ impl Mix for Hsva {
     }
 }
 
+impl Gray for Hsva {
+    const BLACK: Self = Self::new(0., 0., 0., 1.);
+    const WHITE: Self = Self::new(0., 0., 1., 1.);
+}
+
 impl Alpha for Hsva {
     #[inline]
     fn with_alpha(&self, alpha: f32) -> Self {

crates/bevy_color/src/hwba.rs

@@ -2,7 +2,9 @@
 //! in [_HWB - A More Intuitive Hue-Based Color Model_] by _Smith et al_.
 //!
 //! [_HWB - A More Intuitive Hue-Based Color Model_]: https://web.archive.org/web/20240226005220/http://alvyray.com/Papers/CG/HWB_JGTv208.pdf
-use crate::{Alpha, ColorToComponents, Hue, Lcha, LinearRgba, Mix, Srgba, StandardColor, Xyza};
+use crate::{
+    Alpha, ColorToComponents, Gray, Hue, Lcha, LinearRgba, Mix, Srgba, StandardColor, Xyza,
+};
 use bevy_math::{Vec3, Vec4};
 use bevy_reflect::prelude::*;
 
@@ -91,6 +93,11 @@ impl Mix for Hwba {
     }
 }
 
+impl Gray for Hwba {
+    const BLACK: Self = Self::new(0., 0., 1., 1.);
+    const WHITE: Self = Self::new(0., 1., 0., 1.);
+}
+
 impl Alpha for Hwba {
     #[inline]
     fn with_alpha(&self, alpha: f32) -> Self {

crates/bevy_color/src/laba.rs

@@ -1,5 +1,5 @@
 use crate::{
-    impl_componentwise_vector_space, Alpha, ColorToComponents, Hsla, Hsva, Hwba, LinearRgba,
+    impl_componentwise_vector_space, Alpha, ColorToComponents, Gray, Hsla, Hsva, Hwba, LinearRgba,
     Luminance, Mix, Oklaba, Srgba, StandardColor, Xyza,
 };
 use bevy_math::{Vec3, Vec4};
@@ -101,6 +101,11 @@ impl Mix for Laba {
     }
 }
 
+impl Gray for Laba {
+    const BLACK: Self = Self::new(0., 0., 0., 1.);
+    const WHITE: Self = Self::new(1., 0., 0., 1.);
+}
+
 impl Alpha for Laba {
     #[inline]
     fn with_alpha(&self, alpha: f32) -> Self {

crates/bevy_color/src/lcha.rs

@@ -1,5 +1,6 @@
 use crate::{
-    Alpha, ColorToComponents, Hue, Laba, LinearRgba, Luminance, Mix, Srgba, StandardColor, Xyza,
+    Alpha, ColorToComponents, Gray, Hue, Laba, LinearRgba, Luminance, Mix, Srgba, StandardColor,
+    Xyza,
 };
 use bevy_math::{Vec3, Vec4};
 use bevy_reflect::prelude::*;
@@ -122,6 +123,11 @@ impl Mix for Lcha {
     }
 }
 
+impl Gray for Lcha {
+    const BLACK: Self = Self::new(0.0, 0.0, 0.0000136603785, 1.0);
+    const WHITE: Self = Self::new(1.0, 0.0, 0.0000136603785, 1.0);
+}
+
 impl Alpha for Lcha {
     #[inline]
     fn with_alpha(&self, alpha: f32) -> Self {

crates/bevy_color/src/linear_rgba.rs

@@ -1,6 +1,6 @@
 use crate::{
     color_difference::EuclideanDistance, impl_componentwise_vector_space, Alpha, ColorToComponents,
-    Luminance, Mix, StandardColor,
+    Gray, Luminance, Mix, StandardColor,
 };
 use bevy_math::{Vec3, Vec4};
 use bevy_reflect::prelude::*;
@@ -121,18 +121,6 @@ impl LinearRgba {
         }
     }
 
-    /// Construct a new [`LinearRgba`] color with the same value for all channels and an alpha of 1.0.
-    ///
-    /// A value of 0.0 is black, and a value of 1.0 is white.
-    pub const fn gray(value: f32) -> Self {
-        Self {
-            red: value,
-            green: value,
-            blue: value,
-            alpha: 1.0,
-        }
-    }
-
     /// Return a copy of this color with the red channel set to the given value.
     pub const fn with_red(self, red: f32) -> Self {
         Self { red, ..self }
@@ -236,6 +224,11 @@ impl Mix for LinearRgba {
     }
 }
 
+impl Gray for LinearRgba {
+    const BLACK: Self = Self::BLACK;
+    const WHITE: Self = Self::WHITE;
+}
+
 impl Alpha for LinearRgba {
     #[inline]
     fn with_alpha(&self, alpha: f32) -> Self {

crates/bevy_color/src/oklaba.rs

@@ -1,6 +1,6 @@
 use crate::{
     color_difference::EuclideanDistance, impl_componentwise_vector_space, Alpha, ColorToComponents,
-    Hsla, Hsva, Hwba, Lcha, LinearRgba, Luminance, Mix, Srgba, StandardColor, Xyza,
+    Gray, Hsla, Hsva, Hwba, Lcha, LinearRgba, Luminance, Mix, Srgba, StandardColor, Xyza,
 };
 use bevy_math::{Vec3, Vec4};
 use bevy_reflect::prelude::*;
@@ -101,6 +101,11 @@ impl Mix for Oklaba {
     }
 }
 
+impl Gray for Oklaba {
+    const BLACK: Self = Self::new(0., 0., 0., 1.);
+    const WHITE: Self = Self::new(1.0, 0.0, 0.000000059604645, 1.0);
+}
+
 impl Alpha for Oklaba {
     #[inline]
     fn with_alpha(&self, alpha: f32) -> Self {

crates/bevy_color/src/oklcha.rs

@@ -1,6 +1,6 @@
 use crate::{
-    color_difference::EuclideanDistance, Alpha, ColorToComponents, Hsla, Hsva, Hue, Hwba, Laba,
+    color_difference::EuclideanDistance, Alpha, ColorToComponents, Gray, Hsla, Hsva, Hue, Hwba,
-    Lcha, LinearRgba, Luminance, Mix, Oklaba, Srgba, StandardColor, Xyza,
+    Laba, Lcha, LinearRgba, Luminance, Mix, Oklaba, Srgba, StandardColor, Xyza,
 };
 use bevy_math::{Vec3, Vec4};
 use bevy_reflect::prelude::*;
@@ -119,6 +119,11 @@ impl Mix for Oklcha {
     }
 }
 
+impl Gray for Oklcha {
+    const BLACK: Self = Self::new(0., 0., 0., 1.);
+    const WHITE: Self = Self::new(1.0, 0.000000059604645, 90.0, 1.0);
+}
+
 impl Alpha for Oklcha {
     #[inline]
     fn with_alpha(&self, alpha: f32) -> Self {

crates/bevy_color/src/srgba.rs

@@ -1,6 +1,6 @@
 use crate::color_difference::EuclideanDistance;
 use crate::{
-    impl_componentwise_vector_space, Alpha, ColorToComponents, LinearRgba, Luminance, Mix,
+    impl_componentwise_vector_space, Alpha, ColorToComponents, Gray, LinearRgba, Luminance, Mix,
     StandardColor, Xyza,
 };
 use bevy_math::{Vec3, Vec4};
@@ -314,6 +314,11 @@ impl EuclideanDistance for Srgba {
     }
 }
 
+impl Gray for Srgba {
+    const BLACK: Self = Self::BLACK;
+    const WHITE: Self = Self::WHITE;
+}
+
 impl ColorToComponents for Srgba {
     fn to_f32_array(self) -> [f32; 4] {
         [self.red, self.green, self.blue, self.alpha]

crates/bevy_color/src/xyza.rs

@@ -1,5 +1,5 @@
 use crate::{
-    impl_componentwise_vector_space, Alpha, ColorToComponents, LinearRgba, Luminance, Mix,
+    impl_componentwise_vector_space, Alpha, ColorToComponents, Gray, LinearRgba, Luminance, Mix,
     StandardColor,
 };
 use bevy_math::{Vec3, Vec4};
@@ -144,6 +144,11 @@ impl Mix for Xyza {
     }
 }
 
+impl Gray for Xyza {
+    const BLACK: Self = Self::new(0., 0., 0., 1.);
+    const WHITE: Self = Self::new(0.95047, 1.0, 1.08883, 1.0);
+}
+
 impl ColorToComponents for Xyza {
     fn to_f32_array(self) -> [f32; 4] {
         [self.x, self.y, self.z, self.alpha]

crates/bevy_core_pipeline/src/bloom/mod.rs

@@ -2,7 +2,7 @@ mod downsampling_pipeline;
 mod settings;
 mod upsampling_pipeline;
 
-use bevy_color::LinearRgba;
+use bevy_color::{Gray, LinearRgba};
 pub use settings::{BloomCompositeMode, BloomPrefilterSettings, BloomSettings};
 
 use crate::{