Use LinearRgba in bevy_gizmos internals (#12128)

# Objective

This PR arose as part of the migration process for `bevy_color`: see
#12056.

While examining how `bevy_gizmos` stores color types internally, I found
that rather than storing a `Color` internally, it actually stores a
`[f32;4]` for a linear RGB, type aliased to a `ColorItem`.

While we don't *have* to clean this up to complete the migration, now
that we have explicit strong typing for linear color types we should use
them rather than replicating this idea throughout the codebase.

## Solution

- Added `LinearRgba::NAN`, for when you want to do cursed rendering
things.
- Replaced the internal color representation in `bevy_gizmo`: this was
`ColorItem`, but is now `LinearRgba`.
- `LinearRgba` is now `Pod`, enabling us to use the same fast `bytemuck`
bit twiddling tricks that we were using before. This requires:

1. Forcing `LinearRgba` to be `repr(C)`
2. Implementing `Zeroable`, and unsafe trait which defines what the
struct looks like when all values are zero.
3. Implementing `Pod`, a marker trait with stringent safety requirements
that is required for "plain old data".

---------

Co-authored-by: Alice Cecile <alice.i.cecil@gmail.com>

crates/bevy_color/Cargo.toml

@@ -13,6 +13,7 @@ bevy_math = { path = "../bevy_math", version = "0.14.0-dev" }
 bevy_reflect = { path = "../bevy_reflect", version = "0.14.0-dev", features = [
   "bevy",
 ] }
+bytemuck = "1"
 serde = "1.0"
 thiserror = "1.0"
 wgpu = { version = "0.19.1", default-features = false }

crates/bevy_color/src/linear_rgba.rs

@@ -1,11 +1,13 @@
 use crate::{color_difference::EuclideanDistance, Alpha, Luminance, Mix, StandardColor};
 use bevy_math::Vec4;
 use bevy_reflect::{Reflect, ReflectDeserialize, ReflectSerialize};
+use bytemuck::{Pod, Zeroable};
 use serde::{Deserialize, Serialize};
 
 /// Linear RGB color with alpha.
 #[derive(Debug, Clone, Copy, PartialEq, Serialize, Deserialize, Reflect)]
 #[reflect(PartialEq, Serialize, Deserialize)]
+#[repr(C)]
 pub struct LinearRgba {
     /// The red channel. [0.0, 1.0]
     pub red: f32,
@@ -44,6 +46,18 @@ impl LinearRgba {
         alpha: 0.0,
     };
 
+    /// An invalid color.
+    ///
+    /// This type can be used to represent an invalid color value;
+    /// in some rendering applications the color will be ignored,
+    /// enabling performant hacks like hiding lines by setting their color to `INVALID`.
+    pub const NAN: Self = Self {
+        red: f32::NAN,
+        green: f32::NAN,
+        blue: f32::NAN,
+        alpha: f32::NAN,
+    };
+
     /// Construct a new [`LinearRgba`] color from components.
     pub const fn new(red: f32, green: f32, blue: f32, alpha: f32) -> Self {
         Self {
@@ -279,6 +293,33 @@ impl encase::private::CreateFrom for LinearRgba {
     }
 }
 
+/// A [`Zeroable`] type is one whose bytes can be filled with zeroes while remaining valid.
+///
+/// SAFETY: [`LinearRgba`] is inhabited
+/// SAFETY: [`LinearRgba`]'s all-zero bit pattern is a valid value
+unsafe impl Zeroable for LinearRgba {
+    fn zeroed() -> Self {
+        LinearRgba {
+            red: 0.0,
+            green: 0.0,
+            blue: 0.0,
+            alpha: 0.0,
+        }
+    }
+}
+
+/// The [`Pod`] trait is [`bytemuck`]'s marker for types that can be safely transmuted from a byte array.
+///
+/// It is intended to only be implemented for types which are "Plain Old Data".
+///
+/// SAFETY: [`LinearRgba`] is inhabited.
+/// SAFETY: [`LinearRgba`] permits any bit value.
+/// SAFETY: [`LinearRgba`] does not have padding bytes.
+/// SAFETY: all of the fields of [`LinearRgba`] are [`Pod`], as f32 is [`Pod`].
+/// SAFETY: [`LinearRgba`] is `repr(C)`
+/// SAFETY: [`LinearRgba`] does not permit interior mutability.
+unsafe impl Pod for LinearRgba {}
+
 impl encase::ShaderSize for LinearRgba {}
 
 #[cfg(test)]

crates/bevy_gizmos/Cargo.toml

@@ -17,6 +17,7 @@ webgpu = []
 bevy_pbr = { path = "../bevy_pbr", version = "0.14.0-dev", optional = true }
 bevy_sprite = { path = "../bevy_sprite", version = "0.14.0-dev", optional = true }
 bevy_app = { path = "../bevy_app", version = "0.14.0-dev" }
+bevy_color = { path = "../bevy_color", version = "0.14.0-dev" }
 bevy_ecs = { path = "../bevy_ecs", version = "0.14.0-dev" }
 bevy_math = { path = "../bevy_math", version = "0.14.0-dev" }
 bevy_asset = { path = "../bevy_asset", version = "0.14.0-dev" }

crates/bevy_gizmos/src/gizmos.rs

@@ -3,6 +3,7 @@
 use std::{iter, marker::PhantomData};
 
 use crate::circles::DEFAULT_CIRCLE_SEGMENTS;
+use bevy_color::LinearRgba;
 use bevy_ecs::{
     component::Tick,
     system::{Deferred, ReadOnlySystemParam, Res, Resource, SystemBuffer, SystemMeta, SystemParam},
@@ -19,14 +20,13 @@ use crate::{
 };
 
 type PositionItem = [f32; 3];
-type ColorItem = [f32; 4];
 
 #[derive(Resource, Default)]
 pub(crate) struct GizmoStorage<T: GizmoConfigGroup> {
-    pub list_positions: Vec<PositionItem>,
+    pub(crate) list_positions: Vec<PositionItem>,
-    pub list_colors: Vec<ColorItem>,
+    pub(crate) list_colors: Vec<LinearRgba>,
-    pub strip_positions: Vec<PositionItem>,
+    pub(crate) strip_positions: Vec<PositionItem>,
-    pub strip_colors: Vec<ColorItem>,
+    pub(crate) strip_colors: Vec<LinearRgba>,
     marker: PhantomData<T>,
 }
 
@@ -104,9 +104,9 @@ where
 #[derive(Default)]
 struct GizmoBuffer<T: GizmoConfigGroup> {
     list_positions: Vec<PositionItem>,
-    list_colors: Vec<ColorItem>,
+    list_colors: Vec<LinearRgba>,
     strip_positions: Vec<PositionItem>,
-    strip_colors: Vec<ColorItem>,
+    strip_colors: Vec<LinearRgba>,
     marker: PhantomData<T>,
 }
 
@@ -244,10 +244,8 @@ impl<'w, 's, T: GizmoConfigGroup> Gizmos<'w, 's, T> {
         }
         self.extend_strip_positions(positions);
         let len = self.buffer.strip_positions.len();
-        self.buffer
+        self.buffer.strip_colors.resize(len - 1, color.into());
-            .strip_colors
+        self.buffer.strip_colors.push(LinearRgba::NAN);
-            .resize(len - 1, color.as_linear_rgba_f32());
-        self.buffer.strip_colors.push([f32::NAN; 4]);
     }
 
     /// Draw a line in 3D made of straight segments between the points, with a color gradient.
@@ -287,11 +285,11 @@ impl<'w, 's, T: GizmoConfigGroup> Gizmos<'w, 's, T> {
 
         for (position, color) in points {
             strip_positions.push(position.to_array());
-            strip_colors.push(color.as_linear_rgba_f32());
+            strip_colors.push(color.into());
         }
 
         strip_positions.push([f32::NAN; 3]);
-        strip_colors.push([f32::NAN; 4]);
+        strip_colors.push(LinearRgba::NAN);
     }
 
     /// Draw a wireframe sphere in 3D made out of 3 circles around the axes.
@@ -583,14 +581,14 @@ impl<'w, 's, T: GizmoConfigGroup> Gizmos<'w, 's, T> {
     fn extend_list_colors(&mut self, colors: impl IntoIterator<Item = LegacyColor>) {
         self.buffer
             .list_colors
-            .extend(colors.into_iter().map(|color| color.as_linear_rgba_f32()));
+            .extend(colors.into_iter().map(LinearRgba::from));
     }
 
     #[inline]
     fn add_list_color(&mut self, color: LegacyColor, count: usize) {
         self.buffer
             .list_colors
-            .extend(iter::repeat(color.as_linear_rgba_f32()).take(count));
+            .extend(iter::repeat(LinearRgba::from(color)).take(count));
     }
 
     #[inline]

crates/bevy_gizmos/src/lib.rs

@@ -52,6 +52,7 @@ pub mod prelude {
 use aabb::AabbGizmoPlugin;
 use bevy_app::{App, Last, Plugin};
 use bevy_asset::{load_internal_asset, Asset, AssetApp, Assets, Handle};
+use bevy_color::LinearRgba;
 use bevy_core::cast_slice;
 use bevy_ecs::{
     component::Component,
@@ -309,7 +310,7 @@ struct LineGizmoUniform {
 #[derive(Asset, Debug, Default, Clone, TypePath)]
 struct LineGizmo {
     positions: Vec<[f32; 3]>,
-    colors: Vec<[f32; 4]>,
+    colors: Vec<LinearRgba>,
     /// Whether this gizmo's topology is a line-strip or line-list
     strip: bool,
 }