Correct setup for replicating with Avian

[cBournhonesque]

Avian uses 2 extra components Position and Rotation, which it syncs to/from Transform in its PhysicsSet::SyncPlugin.

There are multiple ways of settings replication with avian, and its tricky to gets things right.

1. Recommended

• Replicate Position/Rotation

  app.register_component::<Position>(ChannelDirection::ServerToClient)
      .add_prediction(ComponentSyncMode::Full)
      .add_interpolation_fn(position::lerp)
      .add_correction_fn(position::lerp);

  app.register_component::<Rotation>(ChannelDirection::ServerToClient)
      .add_prediction(ComponentSyncMode::Full)
      .add_interpolation_fn(rotation::lerp)
      .add_correction_fn(rotation::lerp);

• Run VisualInterpolation on Transform

app.add_interpolation::<Transform>(ComponentSyncMode::None);
app.add_interpolation_fn::<Transform>(TransformLinearInterpolation::lerp);

I like this setup because it makes sense that VisualInterpolation is applied directly to Transform, which is the component which affects the visual position of entities.
Also you don't have to modify the SyncPlugin of avian because by default it runs in FixedPostUpdate

The system order is:

• [FixedPostUpdate] Physics -> UpdateHistory -> Correction -> Sync (Position->Transform) -> UpdateVisualInterpolation (Transform)
• [PostUpdate] VisuallyInterpolate

Note that you cannot replicate Transform because the server will send the PostUpdate value of Transform which is computed after the Position->Transform Sync, and we update the history using the values before Sync, so you will get constant rollbacks.

Lightyear already provides the correct ordering constraints for this setup.

2. Only use Position/Rotation

• Replicate Position/Rotation
• Run VisualInterpolation on Position/Rotation

Since you run VisualInterpolation on Position/Rotation, you will have to run avian's SyncPlugin in PostUpdate

The system order is:

• [FixedPostUpdate] UpdateHistory -> Correction -> UpdateVisualInterpolation (Position)
• [PostUpdate] VisuallyInterpolatePosition -> SyncPlugin -> TransformPropagate
  (Note that lightyear already provides ordering constraints for this setup:

  lightyear/lightyear/src/utils/avian3d.rs

  Line 50 in 0f51455

  InterpolationSet::VisualInterpolation,

  )

3. Only use Transform

I think this could also work well, it would require to use the Transform->Position SyncPlugin.
I think I had some issues because Transform is updated multiple times (in VisualInterpolation, in TransformPropagate)
so you might have to run an extra SyncPlugin step in PreUpdate after RestoreVisualInterpolation but before CheckRollback.

It seemed tricky to get right so I haven't investigated further.

[blueforesticarus]

Is there a way to sync Transform if Position does not exist? ie. for non-rigid bodies?

[cBournhonesque]

In that case I would probably add Transform to the protocol, but you would have to be careful to use https://docs.rs/lightyear/latest/lightyear/shared/replication/components/struct.DisabledComponents.html#method.disable
to prevent Transform from being replicated on entities rigid_bodies...

I would need to investigate a bit more why simply replicating Transform didn't work well for me