bevy/crates/bevy_transform/src/lib.rs
James Liu eaeba0866d Parallelized transform propagation (#4775)
# Objective
Fixes #4697. Hierarchical propagation of properties, currently only Transform -> GlobalTransform, can be a very expensive operation. Transform propagation is a strict dependency for anything positioned in world-space. In large worlds, this can take quite a bit of time, so limiting it to a single thread can result in poor CPU utilization as it bottlenecks the rest of the frame's systems.

## Solution

 - Move transforms without a parent or a child (free-floating (Global)Transform) entities into a separate parallel system.
 - Chunk the hierarchy based on the root entities and process it in parallel with `Query::par_for_each_mut`. 
 - Utilize the hierarchy's specific properties introduced in #4717 to allow for safe use of `Query::get_unchecked` on multiple threads. Assuming each child is unique in the hierarchy, it is impossible to have an aliased `&mut GlobalTransform` so long as we verify that the parent for a child is the same one propagated from.

---

## Changelog
Removed: `transform_propagate_system` is no longer `pub`.
2022-11-21 18:18:38 +00:00

109 lines
4 KiB
Rust

#![warn(missing_docs)]
#![warn(clippy::undocumented_unsafe_blocks)]
#![doc = include_str!("../README.md")]
/// The basic components of the transform crate
pub mod components;
mod systems;
#[doc(hidden)]
pub mod prelude {
#[doc(hidden)]
pub use crate::{components::*, TransformBundle, TransformPlugin};
}
use bevy_app::prelude::*;
use bevy_ecs::prelude::*;
use bevy_hierarchy::ValidParentCheckPlugin;
use prelude::{GlobalTransform, Transform};
/// A [`Bundle`] of the [`Transform`] and [`GlobalTransform`]
/// [`Component`](bevy_ecs::component::Component)s, which describe the position of an entity.
///
/// * To place or move an entity, you should set its [`Transform`].
/// * To get the global transform of an entity, you should get its [`GlobalTransform`].
/// * For transform hierarchies to work correctly, you must have both a [`Transform`] and a [`GlobalTransform`].
/// * You may use the [`TransformBundle`] to guarantee this.
///
/// ## [`Transform`] and [`GlobalTransform`]
///
/// [`Transform`] is the position of an entity relative to its parent position, or the reference
/// frame if it doesn't have a parent.
///
/// [`GlobalTransform`] is the position of an entity relative to the reference frame.
///
/// [`GlobalTransform`] is updated from [`Transform`] in the systems labeled
/// [`TransformPropagate`](crate::TransformSystem::TransformPropagate).
///
/// This system runs in stage [`CoreStage::PostUpdate`](crate::CoreStage::PostUpdate). If you
/// update the [`Transform`] of an entity in this stage or after, you will notice a 1 frame lag
/// before the [`GlobalTransform`] is updated.
#[derive(Bundle, Clone, Copy, Debug, Default)]
pub struct TransformBundle {
/// The transform of the entity.
pub local: Transform,
/// The global transform of the entity.
pub global: GlobalTransform,
}
impl TransformBundle {
/// An identity [`TransformBundle`] with no translation, rotation, and a scale of 1 on all axes.
pub const IDENTITY: Self = TransformBundle {
local: Transform::IDENTITY,
global: GlobalTransform::IDENTITY,
};
/// Creates a new [`TransformBundle`] from a [`Transform`].
///
/// This initializes [`GlobalTransform`] as identity, to be updated later by the
/// [`CoreStage::PostUpdate`](crate::CoreStage::PostUpdate) stage.
#[inline]
pub const fn from_transform(transform: Transform) -> Self {
TransformBundle {
local: transform,
..Self::IDENTITY
}
}
}
impl From<Transform> for TransformBundle {
#[inline]
fn from(transform: Transform) -> Self {
Self::from_transform(transform)
}
}
/// Label enum for the systems relating to transform propagation
#[derive(Debug, Hash, PartialEq, Eq, Clone, SystemLabel)]
pub enum TransformSystem {
/// Propagates changes in transform to children's [`GlobalTransform`](crate::components::GlobalTransform)
TransformPropagate,
}
/// The base plugin for handling [`Transform`] components
#[derive(Default)]
pub struct TransformPlugin;
impl Plugin for TransformPlugin {
fn build(&self, app: &mut App) {
app.register_type::<Transform>()
.register_type::<GlobalTransform>()
.add_plugin(ValidParentCheckPlugin::<GlobalTransform>::default())
// add transform systems to startup so the first update is "correct"
.add_startup_system_to_stage(
StartupStage::PostStartup,
systems::sync_simple_transforms.label(TransformSystem::TransformPropagate),
)
.add_startup_system_to_stage(
StartupStage::PostStartup,
systems::propagate_transforms.label(TransformSystem::TransformPropagate),
)
.add_system_to_stage(
CoreStage::PostUpdate,
systems::sync_simple_transforms.label(TransformSystem::TransformPropagate),
)
.add_system_to_stage(
CoreStage::PostUpdate,
systems::propagate_transforms.label(TransformSystem::TransformPropagate),
);
}
}