bevy_color: Added Color Conversion Mermaid Diagram (#12147)

Shows relationships between color spaces and explains what files should
contain which conversions.

# Objective

- Provide documentation for maintainers and users on how color space
conversion is implemented.

## Solution

- Created a mermaid diagram documenting the relationships between
various color spaces. This diagram also includes links to defining
articles, and edges include links to conversion formulae.
- Added a `conversion.md` document which is included in the
documentation of each of the color spaces. This ensures it is readily
visible in all relevant contexts.


## Notes

The diagram is in the Mermaid (`.mmd`) format, and must be converted
into an SVG file (or other image format) prior to use in Rust
documentation. I've included a link to
[mermaid.live](https://mermaid.live) as an option for doing such
conversion in an appropriate README.

Below is a screenshot of the documentation added.


![Capture](https://github.com/bevyengine/bevy/assets/2217286/370a65f2-6dd4-4af7-a99b-3763832d1b8a)

crates/bevy_color/docs/conversion.md

@@ -0,0 +1,16 @@
+# Conversion
+
+Conversion between the various color spaces is achieved using Rust's native [From] trait. Because certain color spaces are defined by their transformation to and from another space, these [From] implementations reflect that set of definitions.
+
+```rust
+# use bevy_color::{Srgba, LinearRgba};
+let color = Srgba::rgb(0.5, 0.5, 0.5);
+
+// Using From explicitly
+let linear_color = LinearRgba::from(color);
+
+// Using Into
+let linear_color: LinearRgba = color.into();
+```
+
+For example, the [sRGB](crate::Srgba) space is defined by its relationship with [Linear RGB](crate::LinearRgba), and [HWB](crate::Hwba) by its with [sRGB](crate::Srgba). As such, it is the responsibility of [sRGB](crate::Srgba) to provide [From] implementations for [Linear RGB](crate::LinearRgba), and [HWB](crate::Hwba) for [sRGB](crate::Srgba). To then provide conversion between [Linear RGB](crate::LinearRgba) and [HWB](crate::Hwba) directly, [HWB](crate::Hwba) is responsible for implementing these conversions, delegating to [sRGB](crate::Srgba) as an intermediatory. This ensures that all conversions take the shortest path between any two spaces, and limit the proliferation of domain specific knowledge for each color space to their respective definitions.

crates/bevy_color/docs/diagrams/README.md

@@ -0,0 +1,3 @@
+# Creating Mermaid Diagrams
+
+Mermaid diagrams (`.mmd` files) can be converted to SVG using various tools. The simplest to work with is [mermaid.live](https://mermaid.live/), which is a HTML web app. When editing `.mmd` files, make sure to regenerate the associated SVGs.

crates/bevy_color/docs/diagrams/model_graph.mmd

@@ -0,0 +1,22 @@
+%%{ init: { 'theme': 'dark', 'flowchart': { 'curve': 'stepAfter', 'padding': 30 }, 'themeCSS': '.label foreignObject { overflow: visible; }' } }%%
+flowchart LR
+    lRGB(<a href='https://en.wikipedia.org/wiki/Rgb'>Linear<br/>sRGB</a>)
+    Oklab(<a href='https://oklch.com/'>Oklab</a>)
+    Oklch(<a href='https://oklch.com/'>Oklch</a>)
+    XYZ(<a href='https://en.wikipedia.org/wiki/XYZ_color'>XYZ</a>)
+    Lab(<a href='https://en.wikipedia.org/wiki/Lab_color'>Lab</a>)
+    Lch(<a href='https://en.wikipedia.org/wiki/CIELAB_color_space#Cylindrical_model'>Lch</a>)
+    sRGB(<a href='https://en.wikipedia.org/wiki/Srgb'>sRGB</a>)
+    HWB(<a href='https://en.wikipedia.org/wiki/HWB_color_model'>HWB</a>)
+    HSV(<a href='https://en.wikipedia.org/wiki/HSL_and_HSV'>HSV</a>)
+    HSL(<a href='https://en.wikipedia.org/wiki/HSL_and_HSV'>HSL</a>)
+    GPU <--> lRGB
+    lRGB <--<a href='https://bottosson.github.io/posts/oklab/#converting-from-linear-srgb-to-oklab'>Conversion</a>--> Oklab
+    Oklab <--<a href='https://bottosson.github.io/posts/oklab/#the-oklab-color-space'>Conversion</a>--> Oklch
+    lRGB <--<a href='http://www.brucelindbloom.com/index.html?Eqn_RGB_XYZ_Matrix.html'>Conversion</a>--> XYZ
+    XYZ <--<a href='http://www.brucelindbloom.com/index.html?Eqn_XYZ_to_Lab.html'>Conversion</a>--> Lab
+    Lab <--<a href='http://www.brucelindbloom.com/index.html?Eqn_Lab_to_LCH.html'>Conversion</a>--> Lch
+    lRGB <--<a href='https://en.wikipedia.org/wiki/SRGB#From_sRGB_to_CIE_XYZ'>Conversion</a>--> sRGB
+    sRGB <--<a href='http://alvyray.com/Papers/CG/HWB_JGTv208.pdf'>Conversion</a>--> HWB
+    HWB <--<a href='http://alvyray.com/Papers/CG/HWB_JGTv208.pdf'>Conversion</a>--> HSV
+    HSV <--<a href='https://en.wikipedia.org/wiki/HSL_and_HSV#Interconversion'>Conversion</a>--> HSL

crates/bevy_color/src/color.rs

@@ -6,6 +6,10 @@ use serde::{Deserialize, Serialize};
 ///
 /// This is useful when you need to store a color in a data structure that can't be generic over
 /// the color type.
+#[doc = include_str!("../docs/conversion.md")]
+/// <div>
+#[doc = include_str!("../docs/diagrams/model_graph.svg")]
+/// </div>
 #[derive(Debug, Clone, Copy, PartialEq, Serialize, Deserialize, Reflect)]
 #[reflect(PartialEq, Serialize, Deserialize)]
 pub enum Color {

crates/bevy_color/src/hsla.rs

@@ -4,6 +4,10 @@ use serde::{Deserialize, Serialize};
 
 /// Color in Hue-Saturation-Lightness (HSL) color space with alpha.
 /// Further information on this color model can be found on [Wikipedia](https://en.wikipedia.org/wiki/HSL_and_HSV).
+#[doc = include_str!("../docs/conversion.md")]
+/// <div>
+#[doc = include_str!("../docs/diagrams/model_graph.svg")]
+/// </div>
 #[derive(Debug, Clone, Copy, PartialEq, Serialize, Deserialize, Reflect)]
 #[reflect(PartialEq, Serialize, Deserialize)]
 pub struct Hsla {

crates/bevy_color/src/hsva.rs

@@ -4,6 +4,10 @@ use serde::{Deserialize, Serialize};
 
 /// Color in Hue-Saturation-Value (HSV) color space with alpha.
 /// Further information on this color model can be found on [Wikipedia](https://en.wikipedia.org/wiki/HSL_and_HSV).
+#[doc = include_str!("../docs/conversion.md")]
+/// <div>
+#[doc = include_str!("../docs/diagrams/model_graph.svg")]
+/// </div>
 #[derive(Debug, Clone, Copy, PartialEq, Serialize, Deserialize, Reflect)]
 #[reflect(PartialEq, Serialize, Deserialize)]
 pub struct Hsva {

crates/bevy_color/src/hwba.rs

@@ -8,6 +8,10 @@ use serde::{Deserialize, Serialize};
 
 /// Color in Hue-Whiteness-Blackness (HWB) color space with alpha.
 /// Further information on this color model can be found on [Wikipedia](https://en.wikipedia.org/wiki/HWB_color_model).
+#[doc = include_str!("../docs/conversion.md")]
+/// <div>
+#[doc = include_str!("../docs/diagrams/model_graph.svg")]
+/// </div>
 #[derive(Debug, Clone, Copy, PartialEq, Serialize, Deserialize, Reflect)]
 #[reflect(PartialEq, Serialize, Deserialize)]
 pub struct Hwba {

crates/bevy_color/src/laba.rs

@@ -5,6 +5,10 @@ use bevy_reflect::{Reflect, ReflectDeserialize, ReflectSerialize};
 use serde::{Deserialize, Serialize};
 
 /// Color in LAB color space, with alpha
+#[doc = include_str!("../docs/conversion.md")]
+/// <div>
+#[doc = include_str!("../docs/diagrams/model_graph.svg")]
+/// </div>
 #[derive(Debug, Clone, Copy, PartialEq, Serialize, Deserialize, Reflect)]
 #[reflect(PartialEq, Serialize, Deserialize)]
 pub struct Laba {

crates/bevy_color/src/lcha.rs

@@ -3,6 +3,10 @@ use bevy_reflect::{Reflect, ReflectDeserialize, ReflectSerialize};
 use serde::{Deserialize, Serialize};
 
 /// Color in LCH color space, with alpha
+#[doc = include_str!("../docs/conversion.md")]
+/// <div>
+#[doc = include_str!("../docs/diagrams/model_graph.svg")]
+/// </div>
 #[derive(Debug, Clone, Copy, PartialEq, Serialize, Deserialize, Reflect)]
 #[reflect(PartialEq, Serialize, Deserialize)]
 pub struct Lcha {

crates/bevy_color/src/lib.rs

@@ -50,11 +50,10 @@
 //!
 //! See also the [Wikipedia article on color spaces](https://en.wikipedia.org/wiki/Color_space).
 //!
-//! # Conversions
-//!
-//! Each color space can be converted to and from the others using the [`From`] trait. Not all
-//! possible combinations of conversions are provided, but every color space has a conversion to
-//! and from [`Srgba`] and [`LinearRgba`].
+#![doc = include_str!("../docs/conversion.md")]
+//! <div>
+#![doc = include_str!("../docs/diagrams/model_graph.svg")]
+//! </div>
 //!
 //! # Other Utilities
 //!

crates/bevy_color/src/linear_rgba.rs

@@ -5,6 +5,10 @@ use bytemuck::{Pod, Zeroable};
 use serde::{Deserialize, Serialize};
 
 /// Linear RGB color with alpha.
+#[doc = include_str!("../docs/conversion.md")]
+/// <div>
+#[doc = include_str!("../docs/diagrams/model_graph.svg")]
+/// </div>
 #[derive(Debug, Clone, Copy, PartialEq, Serialize, Deserialize, Reflect)]
 #[reflect(PartialEq, Serialize, Deserialize)]
 #[repr(C)]

crates/bevy_color/src/oklaba.rs

@@ -6,6 +6,10 @@ use bevy_reflect::{Reflect, ReflectDeserialize, ReflectSerialize};
 use serde::{Deserialize, Serialize};
 
 /// Color in Oklaba color space, with alpha
+#[doc = include_str!("../docs/conversion.md")]
+/// <div>
+#[doc = include_str!("../docs/diagrams/model_graph.svg")]
+/// </div>
 #[derive(Debug, Clone, Copy, PartialEq, Serialize, Deserialize, Reflect)]
 #[reflect(PartialEq, Serialize, Deserialize)]
 pub struct Oklaba {

crates/bevy_color/src/srgba.rs

@@ -6,6 +6,10 @@ use serde::{Deserialize, Serialize};
 use thiserror::Error;
 
 /// Non-linear standard RGB with alpha.
+#[doc = include_str!("../docs/conversion.md")]
+/// <div>
+#[doc = include_str!("../docs/diagrams/model_graph.svg")]
+/// </div>
 #[derive(Debug, Clone, Copy, PartialEq, Serialize, Deserialize, Reflect)]
 #[reflect(PartialEq, Serialize, Deserialize)]
 pub struct Srgba {

crates/bevy_color/src/xyza.rs

@@ -3,6 +3,10 @@ use bevy_reflect::{Reflect, ReflectDeserialize, ReflectSerialize};
 use serde::{Deserialize, Serialize};
 
 /// [CIE 1931](https://en.wikipedia.org/wiki/CIE_1931_color_space) color space, also known as XYZ, with an alpha channel.
+#[doc = include_str!("../docs/conversion.md")]
+/// <div>
+#[doc = include_str!("../docs/diagrams/model_graph.svg")]
+/// </div>
 #[derive(Debug, Clone, Copy, PartialEq, Serialize, Deserialize, Reflect)]
 #[reflect(PartialEq, Serialize, Deserialize)]
 pub struct Xyza {