Avian Interpolation

A general-purpose Transform interpolation plugin for fixed timesteps in Avian Physics for the Bevy engine.

examples/split_screen_comparison.rs:

comparison.mp4

Note: The interpolation on the left is smoother in reality, blame the video recording software ;)

Why do I need interpolation?

Ever had your character jitter around when making the camera follow them? This plugin may be for you!

For a full explanation, see Bevy's physics_in_fixed_timestep example. The short version is that on fast enough machines, your game will update its rendered frame more often than it will update its physics simulation. This means that sometimes e.g. your camera will be moved around without any physics objects being updated this frame. This will lead to the physics object's movement looking chppy and jittery, like on the right window in the video above.

There are a number of ways in which we can deal with this, and the easiest is interpolation. By letting the visuals of the physics objects intentionally lag a tiiiiiny bit behind, we can smoothly interpolate between the last two values, leading to smooth and correct visuals, at the cost of the rendered objects being behind the underlying physics objects by a bit. How much? Well, long story short, expect the physics to be ahead of the graphics by a single digit millisecond value. For most games, that is not noticeable at all and will just "magically" make the game more smooth, like on the left window in the video above :)

Usage

Add the plugin to your project. Replace 3d with 2d if you are using Avian 2D.

cargo add avian_interpolation3d --git https://github.com/janhohenheim/avian_interpolation

it's not on crates.io yet because I'm waiting for a new Avian release, as this was made targeting the main branch. This means you also need to use the main branch of Avian. Again, replace 3d with 2d if you are using Avian 2D.

cargo add avian3d --git https://github.com/Jondolf/avian

Now, add AvianInterpolationPlugin to your app after PhysicsPlugins and everything Just Works™:

App::new()
    .add_plugins((
        DefaultPlugins,
        // Disabling SyncPlugin is optional, but will get you a performance boost.
        PhysicsPlugins::default().build().disable::<SyncPlugin>(),
        AvianInterpolationPlugin::default(),
    ))
    .run();

And that's it! The Transform component of all moving objects will now be interpolated after the physics simulation. This means that the new Transform will be available in Update for rendering, spatial sound, moving your camera, etc. The interpolation source will be their Position and Rotation.

Limitations

• Disables transform syncing, i.e. Avian's feature of translating Transform to its internal representation and vice versa.
  • In practice, this means that you can not directly modify the Transform component of any rigid body anymore. Use Position and Rotation instead. Transform is a purely aesthetic component and should not be modified for physics. Depending on your point of view, this is actually a feature ;)
  • You can still read the Transform of anything just as you would always do, if you want.
  • If you still want to have your Transform changed as if you had transform syncing enabled, set InterpolateTransformFields::translation or InterpolateTransformFields::rotation to InterpolationMode::Last for that entity. This will use the last available physics transform as the interpolation source instead.
• Assumes PhysicsSchedule is left at its default value of FixedPostUpdate.
• Assumes that all entities with Position will also have Rotation and vice versa.
• Assumes RigidBodys will not form hierarchies with other RigidBodys.
• Assumes Rigidbody::Static objects will not move.
• Will not interpolate scales for you

Differences to bevy_transform_interpolation

• bevy_transform_interpolation works with Transform in general, while this plugin works only for Avian.
• This plugin makes the results of the interpolation available for systems in Update, which is nicer to work with than bevy_transform_interpolation's PostUpdate.
• By the above features and limitations, this plugin is less memory-intensive and does fewer checks per entity. I didn't do any benchmarks, but it should be faster. Blazingly fast, some may say.
• For most use-cases, this should work as a drop-in replacement for bevy_transform_interpolation as long as you don't mutate rigid bodies' Transforms by hand.

Version Compatibility

avian_interpolation	avian	bevy
main	main	0.14