Interpolate camera and skater presentation between fixed ticks

crates/skate-game/src/animation.rs

@@ -105,7 +105,8 @@ fn bind(
     }
 }
 fn present(
-    skater: Res<SkaterRuntime>,
+    history: Res<crate::presentation::Presentation>,
+    time: Res<Time<Fixed>>,
     animation: Res<AnimationStatus>,
     mut nodes: Query<&mut Transform>,
 ) {
@@ -116,15 +117,19 @@ fn present(
     // rotated -90 degrees about X relative to the native frames. Both files'
     // world positions are Y-up. This is a skin basis change, not a physics turn.
     let basis = render_basis();
+    let Some((previous, current)) = history.pair() else { return; };
+    let alpha = time.overstep_fraction().clamp(0.0, 1.0);
     for binding in &animation.bindings {
-        let global = native_matrix(skater.render_pose[binding.bone]) * basis;
-        let local = if let Some(parent) = binding.parent_bone {
-            (native_matrix(skater.render_pose[parent]) * basis).inverse() * global
-        } else {
-            global
+        // Blend bone-local rotations, not matrix entries or independent world
+        // positions: joints retain their hierarchy while limbs turn.
+        let local = |snapshot: &crate::presentation::Snapshot| {
+            let global = snapshot.bones[binding.bone] * basis;
+            Transform::from_matrix(if let Some(parent) = binding.parent_bone {
+                (snapshot.bones[parent] * basis).inverse() * global
+            } else { global })
         };
         if let Ok(mut transform) = nodes.get_mut(binding.entity) {
-            *transform = Transform::from_matrix(local);
+            *transform = crate::presentation::blend(local(previous), local(current), alpha);
         }
     }
 }

crates/skate-game/src/app.rs

@@ -90,6 +90,7 @@ pub(crate) fn build(
     .add_plugins((
         input::InputPlugin,
         PhysicsPlugin,
+        crate::presentation::PresentationPlugin,
         assets::GameAssetsPlugin,
         animation::AnimationPlugin,
         world::WorldPlugin,

crates/skate-game/src/camera.rs

@@ -48,19 +48,17 @@ fn spawn(mut commands: Commands, config: Res<Config>) {
 }
 
 fn present(mut runtime: ResMut<CameraRuntime>, windows: Query<&Window>,
+    history: Res<crate::presentation::Presentation>, time: Res<Time<Fixed>>,
     mut cameras: Query<(&mut Camera, &mut Transform, &mut Projection), With<GameplayCamera>>) {
     if let Ok(window) = windows.single() {
         runtime.set_aspect_ratio(window.width() / window.height());
     }
-    let Some(frame) = runtime.frame else { return; };
+    let Some((previous, current)) = history.pair() else { return; };
+    let alpha = time.overstep_fraction().clamp(0.0, 1.0);
     for (mut camera, mut transform, mut projection) in &mut cameras {
-        let [right, up, at] = frame.basis.columns.map(Vec3::from_array);
-        // World rendering retains nativeXYZ. Native camera At points forward;
-        // Bevy camera looks along local-Z, hence the two-axis basis conversion.
-        transform.rotation = Quat::from_mat3(&Mat3::from_cols(-right, up, -at));
-        transform.translation = Vec3::new(frame.position[0], frame.position[1], frame.position[2]);
+        *transform = crate::presentation::blend(previous.camera, current.camera, alpha);
         if let Projection::Perspective(p) = &mut *projection {
-            p.fov = frame.field_of_view_degrees.to_radians();
+            p.fov = previous.fov + (current.fov - previous.fov) * alpha;
         }
         camera.is_active = true;
     }

crates/skate-game/src/main.rs

@@ -14,6 +14,7 @@ mod verification;
 mod performance;
 mod graphics_menu;
 mod render_capacity;
+mod presentation;
 mod world;
 mod skate_world;
 

crates/skate-game/src/physics.rs

@@ -336,15 +336,16 @@ mod difficulty_tests;
 
 fn present(
     physics: Res<GamePhysics>,
-    skater: Res<SkaterRuntime>,
+    history: Res<crate::presentation::Presentation>,
+    time: Res<Time<Fixed>>,
     mut roots: Query<&mut Transform, With<PlayerRoot>>,
 ) {
     if physics.failed {
         return;
     }
+    let Some((previous, current)) = history.pair() else { return; };
     for mut root in &mut roots {
-        *root = Transform::from_matrix(crate::animation::native_matrix(
-            skater.animated_skeleton.roots.animation_to_world,
-        ));
+        *root = crate::presentation::blend(previous.root, current.root,
+            time.overstep_fraction().clamp(0.0, 1.0));
     }
 }

crates/skate-game/src/presentation.rs

@@ -0,0 +1,82 @@
+//! Render-only snapshots of completed fixed ticks. Camera, root and skin use
+//! the same interval; interpolated values never feed back into simulation.
+use bevy::prelude::*;
+use crate::{app::SimulationSet, camera::CameraRuntime, physics::SkaterRuntime};
+use skate_core::camera::CameraFrame;
+
+pub(crate) struct PresentationPlugin;
+impl Plugin for PresentationPlugin {
+    fn build(&self, app: &mut App) {
+        app.init_resource::<Presentation>()
+            .add_systems(FixedUpdate, capture.after(SimulationSet::Physics));
+    }
+}
+
+#[derive(Clone)]
+pub(crate) struct Snapshot {
+    pub root: Transform,
+    pub bones: Vec<Mat4>,
+    pub camera: Transform,
+    pub fov: f32,
+}
+
+#[derive(Resource, Default)]
+pub(crate) struct Presentation {
+    pair: Option<(Snapshot, Snapshot)>,
+    generation: u64,
+    period: std::time::Duration,
+}
+impl Presentation {
+    pub fn pair(&self) -> Option<(&Snapshot, &Snapshot)> {
+        self.pair.as_ref().map(|(a, b)| (a, b))
+    }
+}
+
+fn capture(skater: Res<SkaterRuntime>, camera: Res<CameraRuntime>,
+    time: Res<Time<Fixed>>, mut history: ResMut<Presentation>) {
+    if skater.pose_generation == history.generation { return; }
+    let Some(frame) = camera.frame else { return; };
+    let next = Snapshot {
+        root: Transform::from_matrix(crate::animation::native_matrix(
+            skater.animated_skeleton.roots.animation_to_world)),
+        bones: skater.render_pose.iter().copied()
+            .map(crate::animation::native_matrix).collect(),
+        camera: camera_transform(frame),
+        fov: frame.field_of_view_degrees.to_radians(),
+    };
+    // Snap teleports/cuts and cadence changes instead of sweeping through the
+    // world or blending snapshots across different simulation-clock periods.
+    let reset = frame.discontinuity
+        || skater.player_input.processed.flags_2472 & (1 << 10) != 0
+        || history.period != time.timestep();
+    if let Some((previous, current)) = history.pair.as_mut() {
+        if reset || current.bones.len() != next.bones.len() {
+            *previous = next.clone();
+        } else {
+            std::mem::swap(previous, current);
+        }
+        *current = next;
+    } else {
+        history.pair = Some((next.clone(), next));
+    }
+    history.generation = skater.pose_generation;
+    history.period = time.timestep();
+}
+
+fn camera_transform(frame: CameraFrame) -> Transform {
+    let [right, up, at] = frame.basis.columns.map(Vec3::from_array);
+    Transform {
+        translation: Vec3::new(frame.position[0], frame.position[1], frame.position[2]),
+        // Native At is forward; Bevy cameras look along local -Z.
+        rotation: Quat::from_mat3(&Mat3::from_cols(-right, up, -at)).normalize(),
+        ..default()
+    }
+}
+
+pub(crate) fn blend(a: Transform, b: Transform, alpha: f32) -> Transform {
+    Transform {
+        translation: a.translation.lerp(b.translation, alpha),
+        rotation: a.rotation.normalize().slerp(b.rotation.normalize(), alpha),
+        scale: a.scale.lerp(b.scale, alpha),
+    }
+}

docs/RENDER_INTERPOLATION.md

@@ -0,0 +1,24 @@
+# Render interpolation
+
+The Bevy presentation layer retains two completed simulation snapshots and
+interpolates them with `Time<Fixed>::overstep_fraction()` in `Update`. Capture
+runs after the physics system on every fixed tick, including catch-up ticks.
+Frames without a new simulation tick still advance the interpolation fraction.
+
+Snapshots contain the animation root, final physical skin hierarchy (including
+the board/trucks/wheels), and camera transform/FOV. Bone transforms are converted
+to skin-local transforms at each endpoint before translation/scale lerp and
+quaternion slerp. The root and camera use that same fraction. These rendered
+transforms are not read back by the native simulation or camera controllers.
+
+Initialization, camera discontinuities, the existing camera-reset flag and
+simulation-period changes collapse the history to the newest snapshot. There
+is no boxcar camera filter, additional smoothing window, or extrapolation.
+Interpolation introduces one fixed interval of presentation delay (normally
+about 16.7 ms). This is a renderer feature, not a claim about retail behavior.
+
+Reference: Bevy's official fixed-timestep presentation example:
+https://bevy.org/examples-webgpu/movement/physics-in-fixed-timestep/
+
+Validation: `Build.ps1` succeeded; no automated or gameplay tests were run,
+as requested. Gameplay smoothness remains for the user to assess.