bevy task to run physics

[Vrixyz]

It would be interesting to have physics running in a background task over a few frames, and using interpolation to smooth results.

We could have a physics world simulating at 30 or 20 fps ; while rendering would go at 60 fps.

TODO

• Study physx does its double buffering: when is the new buffer written exactly ?

Plan

flowchart TD
    A[Set to allow user to change physics data] --> COPY(Copy physics into background)
    COPY --> FORK(Start bevy task)
    FORK --> Loop{Elapsed time}
    FORK --> SIM["Rapier physics simulation (50ms ; 20fps)"]
    Loop -->|"-50ms"| BU["Bevy update (16.6ms ; 60FPS)"]
    BU --> Loop
    Loop -->|"+50ms"| J(join)
    SIM --> J(Join bevy task)
    J --> WRITE(Write physics from background task into bevy)
    WRITE --> A

Loading

Implementation

• Add a "background mode" to the plugin.
• step_simulation() is expected to provide the end result of our simulation, It makes sense for it to be the system reading the result of the background computation.
• Add a start_simulation system to dump physics state and start the simulation.
• re-order the apply_*_user_changes to be before that start_simulation system. Clarify in the docs that any value modified outside of expected set will be without effect.

Difficulties

Passing data out of ECS

• A naive approach is to copy the whole RapierContext ; but IslandSolver doesn´t implement Clone, and this goes quite deep ( to SolverConstraintsSet ) ; we should keep in mind simd and enhanced-determinism features compatibility.

  • we might not need SolverConstraintsSet (and physicsPipeline in general) so try to clone everything but that.

  • This should be possible but adding Clone to those traits should probably be behind a feature?

• Another approach is to unsafely copy 😱

• Another approach would be to mem::swap the RapierContext with an empty/incorrect one for the ECS, while the simulation is ongoing.

  • This could be made more sound by using an Option<RapierContext> + .take() 🤔

  Attempts

• background simulation: See bevy task to run physics #596

• interpolation experiments: Interpolation exploration Vrixyz/bevy_rapier#28