[Feature] Add root motion extraction and application

Sources/UntoldEngine/Animation/CompiledAnimationClip.swift

@@ -58,6 +58,13 @@ final class CompiledAnimationClip {
     let restRotations: [simd_quatf]
     let restScales: [simd_float3]
 
+    /// Root motion metadata: the skeleton's first parentless joint, and the
+    /// root channel's net displacement/yaw over one loop — used to correct
+    /// frame deltas across the loop wrap.
+    let rootJointIndex: Int?
+    let rootTranslationPerLoop: simd_float3
+    let rootYawPerLoop: Float
+
     init(clip: AnimationClip, skeleton: Skeleton) {
         name = clip.name
         jointCount = skeleton.jointPaths.count
@@ -96,5 +103,24 @@ final class CompiledAnimationClip {
         self.restTranslations = restTranslations
         self.restRotations = restRotations
         self.restScales = restScales
+
+        let rootIndex = skeleton.parentIndices.firstIndex(where: { $0 == nil })
+        rootJointIndex = rootIndex
+        if let rootIndex, channels[rootIndex].animated {
+            let rootChannel = channels[rootIndex]
+            if let first = rootChannel.translationValues.first, let last = rootChannel.translationValues.last {
+                rootTranslationPerLoop = last - first
+            } else {
+                rootTranslationPerLoop = .zero
+            }
+            if let first = rootChannel.rotationValues.first, let last = rootChannel.rotationValues.last {
+                rootYawPerLoop = wrapAngle(yawTwist(last).yaw - yawTwist(first).yaw)
+            } else {
+                rootYawPerLoop = 0
+            }
+        } else {
+            rootTranslationPerLoop = .zero
+            rootYawPerLoop = 0
+        }
     }
 }

Sources/UntoldEngine/Animation/RootMotion.swift

@@ -0,0 +1,204 @@
+//
+//  RootMotion.swift
+//  UntoldEngine
+//
+// Copyright (C) Untold Engine Studios
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at https://mozilla.org/MPL/2.0/.
+
+import Foundation
+import simd
+
+// Root motion: a locomotion clip authored with its root joint traveling
+// (a walk that actually moves forward) normally drags the mesh away from
+// the entity transform and snaps back when the clip loops. With root motion
+// enabled, the root's horizontal translation and yaw deltas are extracted
+// each frame and applied to the entity transform instead, and the pose is
+// grounded — the character moves through the world because its animation
+// says so. Vertical root motion, pitch, and roll stay in the pose (a
+// stumbling zombie still leans).
+//
+// Loop wrap is handled with the clip's precomputed per-loop root
+// displacement and yaw: when the sampled channel time wraps, the delta is
+// corrected by one full loop instead of snapping backward.
+// See docs/Architecture/animationPoseLayer.md.
+
+/// Per-entity root motion state.
+struct RootMotionState {
+    var isEnabled = false
+
+    /// Entity whose transform receives the extracted deltas — the entity
+    /// the public API was called on (the gameplay handle). Hierarchical
+    /// assets keep their AnimationComponent on a skinned descendant, but
+    /// games move the asset root.
+    var anchorEntity: EntityID = .invalid
+
+    /// Optional joint-path override; by default the skeleton's first
+    /// parentless joint drives root motion.
+    var rootJointPath: String?
+    var resolvedRootIndex: Int?
+
+    /// Last frame's raw (pre-strip) root sample, for delta extraction.
+    var hasPreviousSample = false
+    var previousTranslation = simd_float3.zero
+    var previousYaw: Float = 0
+    var previousTranslationTime: Float = 0
+    var previousRotationTime: Float = 0
+
+    /// Forget the sample history (clip switches, enable toggles); the next
+    /// frame re-baselines with a zero delta.
+    mutating func resetHistory() {
+        hasPreviousSample = false
+    }
+}
+
+// MARK: - Angle helpers
+
+/// Wraps an angle to (-π, π] so frame-to-frame yaw deltas never take the
+/// long way around.
+@inline(__always)
+func wrapAngle(_ angle: Float) -> Float {
+    var wrapped = fmod(angle + .pi, 2 * .pi)
+    if wrapped < 0 {
+        wrapped += 2 * .pi
+    }
+    return wrapped - .pi
+}
+
+/// Swing–twist decomposition about the +Y axis: returns the yaw angle and
+/// the twist quaternion, with the remainder (`q * twist⁻¹`) carrying pitch
+/// and roll. The twist is normalized to the shortest arc so yaw is always
+/// in (-π, π].
+@inline(__always)
+func yawTwist(_ q: simd_quatf) -> (yaw: Float, twist: simd_quatf) {
+    let projected = simd_float4(0, q.imag.y, 0, q.real)
+    let length = simd_length(projected)
+    guard length > 1e-8 else {
+        // A 180° rotation about a horizontal axis has no well-defined yaw.
+        return (0, simd_quatf(ix: 0, iy: 0, iz: 0, r: 1))
+    }
+    var twist = simd_quatf(vector: projected / length)
+    if twist.real < 0 {
+        twist = simd_quatf(vector: -twist.vector)
+    }
+    return (2 * atan2(twist.imag.y, twist.real), twist)
+}
+
+/// Grounds the root joint of a local pose: horizontal translation is zeroed
+/// and yaw removed (the entity transform owns both once root motion is on);
+/// vertical translation, pitch, and roll remain.
+@inline(__always)
+func stripRootMotion(from pose: inout PoseBuffer, rootIndex: Int) {
+    guard rootIndex >= 0, rootIndex < pose.jointCount else { return }
+    pose.translations[rootIndex].x = 0
+    pose.translations[rootIndex].z = 0
+    let (_, twist) = yawTwist(pose.rotations[rootIndex])
+    pose.rotations[rootIndex] = simd_normalize(pose.rotations[rootIndex] * twist.inverse)
+}
+
+// MARK: - Per-frame extraction
+
+/// Resolves which joint drives root motion, caching the answer until the
+/// state is reconfigured.
+func resolveRootMotionJointIndex(
+    state: inout RootMotionState,
+    skeleton: Skeleton,
+    compiledClip: CompiledAnimationClip
+) -> Int? {
+    if let cached = state.resolvedRootIndex {
+        return cached
+    }
+    let index: Int? = if let path = state.rootJointPath {
+        skeleton.jointPaths.firstIndex(of: path)
+    } else {
+        compiledClip.rootJointIndex
+    }
+    state.resolvedRootIndex = index
+    return index
+}
+
+/// Extracts this frame's root translation/yaw deltas from the raw sampled
+/// pose, applies them to the entity transform (character space rotated into
+/// the entity's current orientation), and grounds the pose's root joint.
+///
+/// Must run on the raw sampled pose, before transition offsets are applied,
+/// so the deltas come from the clip and transitions blend grounded poses.
+func applyRootMotion(
+    entityId: EntityID,
+    animationComponent: AnimationComponent,
+    skeleton: Skeleton,
+    compiledClip: CompiledAnimationClip,
+    clipDuration: Float,
+    clipSpeed: Float
+) {
+    guard animationComponent.rootMotion.isEnabled else { return }
+    guard let rootIndex = resolveRootMotionJointIndex(
+        state: &animationComponent.rootMotion,
+        skeleton: skeleton,
+        compiledClip: compiledClip
+    ), rootIndex < animationComponent.localPose.jointCount else { return }
+
+    let channel = compiledClip.channels[rootIndex]
+    guard channel.animated else { return }
+
+    let translation = animationComponent.localPose.translations[rootIndex]
+    let (yaw, _) = yawTwist(animationComponent.localPose.rotations[rootIndex])
+
+    // Channel-wrapped sample times, replicating the sampler's per-channel
+    // wrap, to detect when the clip looped between frames.
+    let channelTime = fmod(animationComponent.currentTime, clipDuration) * clipSpeed
+    let translationTime = wrappedChannelTime(channelTime, lastKeyTime: channel.translationTimes.last)
+    let rotationTime = wrappedChannelTime(channelTime, lastKeyTime: channel.rotationTimes.last)
+
+    let motionEntity = animationComponent.rootMotion.anchorEntity == .invalid
+        ? entityId
+        : animationComponent.rootMotion.anchorEntity
+
+    if animationComponent.rootMotion.hasPreviousSample {
+        var delta = translation - animationComponent.rootMotion.previousTranslation
+        if translationTime < animationComponent.rootMotion.previousTranslationTime {
+            delta += compiledClip.rootTranslationPerLoop
+        }
+
+        var yawDelta = yaw - animationComponent.rootMotion.previousYaw
+        if rotationTime < animationComponent.rootMotion.previousRotationTime {
+            yawDelta += compiledClip.rootYawPerLoop
+        }
+        yawDelta = wrapAngle(yawDelta)
+
+        let horizontal = simd_float3(delta.x, 0, delta.z)
+        if scene.get(component: LocalTransformComponent.self, for: motionEntity) != nil {
+            // LocalTransformComponent's default rotation is the zero
+            // quaternion (simd_quatf()), which rotates every vector to zero
+            // — treat it as identity so deltas survive on never-rotated
+            // entities.
+            var entityRotation = getRotationQuaternion(entityId: motionEntity)
+            if simd_length_squared(entityRotation.vector) < 1e-8 {
+                entityRotation = simd_quatf(ix: 0, iy: 0, iz: 0, r: 1)
+            }
+            if simd_length_squared(horizontal) > 0 {
+                translateBy(entityId: motionEntity, position: entityRotation.act(horizontal))
+            }
+            if yawDelta != 0 {
+                let yawRotation = simd_quatf(angle: yawDelta, axis: simd_float3(0, 1, 0))
+                rotateTo(entityId: motionEntity, rotation: simd_normalize(entityRotation * yawRotation))
+            }
+        }
+    }
+
+    animationComponent.rootMotion.previousTranslation = translation
+    animationComponent.rootMotion.previousYaw = yaw
+    animationComponent.rootMotion.previousTranslationTime = translationTime
+    animationComponent.rootMotion.previousRotationTime = rotationTime
+    animationComponent.rootMotion.hasPreviousSample = true
+
+    stripRootMotion(from: &animationComponent.localPose, rootIndex: rootIndex)
+}
+
+@inline(__always)
+private func wrappedChannelTime(_ time: Float, lastKeyTime: Float?) -> Float {
+    guard let lastKeyTime, lastKeyTime > 0 else { return 0 }
+    return fmod(time, lastKeyTime)
+}

Sources/UntoldEngine/ECS/Components.swift

@@ -203,6 +203,7 @@ public class AnimationComponent: Component {
     var hasPreviousPose = false
     var lastSampleDeltaTime: Float = 0
     var transition = PoseTransition()
+    var rootMotion = RootMotionState()
 
     public required init() {}
 
@@ -218,6 +219,7 @@ public class AnimationComponent: Component {
         hasPreviousPose = false
         lastSampleDeltaTime = 0
         transition = PoseTransition()
+        rootMotion = RootMotionState()
     }
 
     func getAllAnimationClips() -> [String] {

Sources/UntoldEngine/Systems/AnimationSystem.swift

@@ -189,6 +189,18 @@ private func updateAnimationSystem(deltaTime: Float) {
             into: &animationComponent.localPose
         )
 
+        // Root motion runs on the raw sampled pose, before transition
+        // offsets: deltas come from the clip, transitions blend grounded
+        // poses.
+        applyRootMotion(
+            entityId: entity,
+            animationComponent: animationComponent,
+            skeleton: skeletonComponent.skeleton,
+            compiledClip: compiledClip,
+            clipDuration: animationClip.duration,
+            clipSpeed: animationClip.speed
+        )
+
         // Transitions decay in real time, independent of playback speed.
         animationComponent.transition.apply(
             to: &animationComponent.localPose,
@@ -326,9 +338,44 @@ public func changeAnimation(entityId: EntityID, name: String, transitionHalflife
         animationComponent.currentAnimation = animationClip
         animationComponent.currentTime = 0
         animationComponent.pause = withPause
+        // Re-baseline root motion on the new clip; the first frame after a
+        // switch contributes no delta.
+        animationComponent.rootMotion.resetHistory()
     }
 }
 
+/// Enables or disables root motion for the entity (or its descendants that
+/// carry an `AnimationComponent`). While enabled, the root joint's
+/// horizontal translation and yaw drive the entity transform instead of the
+/// pose; vertical motion, pitch, and roll stay in the pose. By default the
+/// skeleton's first parentless joint is the root; pass `rootJointPath` to
+/// designate a different joint.
+public func setRootMotionEnabled(entityId: EntityID, enabled: Bool, rootJointPath: String? = nil) {
+    let animationComponents = animationComponentsForEntityOrDescendants(entityId: entityId)
+    guard animationComponents.isEmpty == false else {
+        handleError(.noAnimationComponent, entityId)
+        return
+    }
+
+    for (_, animationComponent) in animationComponents {
+        animationComponent.rootMotion.isEnabled = enabled
+        animationComponent.rootMotion.rootJointPath = rootJointPath
+        animationComponent.rootMotion.anchorEntity = entityId
+        animationComponent.rootMotion.resolvedRootIndex = nil
+        animationComponent.rootMotion.resetHistory()
+    }
+}
+
+public func isRootMotionEnabled(entityId: EntityID) -> Bool {
+    let targetEntityId = resolveEntityWithAnimationComponent(entityId: entityId) ?? entityId
+    guard let animationComponent = scene.get(component: AnimationComponent.self, for: targetEntityId) else {
+        handleError(.noAnimationComponent, entityId)
+        return false
+    }
+
+    return animationComponent.rootMotion.isEnabled
+}
+
 /// Captures inertialization offsets for a clip switch. Falls back to a hard
 /// cut (no transition) when there is nothing to blend from: no clip playing,
 /// no pose displayed yet, no skeleton, or a zero halflife.
@@ -372,6 +419,20 @@ private func beginAnimationTransition(
         into: &animationComponent.transition.scratchTargetNext
     )
 
+    // With root motion enabled the displayed pose is grounded, so the
+    // incoming clip's samples must be grounded too — otherwise the captured
+    // offset would reintroduce the horizontal root displacement.
+    if animationComponent.rootMotion.isEnabled,
+       let rootIndex = resolveRootMotionJointIndex(
+           state: &animationComponent.rootMotion,
+           skeleton: skeleton,
+           compiledClip: compiledClip
+       )
+    {
+        stripRootMotion(from: &animationComponent.transition.scratchTarget, rootIndex: rootIndex)
+        stripRootMotion(from: &animationComponent.transition.scratchTargetNext, rootIndex: rootIndex)
+    }
+
     // Copy the scratch poses out (COW, no allocation) so the mutating
     // begin() call does not overlap a read of the same property.
     let targetPose = animationComponent.transition.scratchTarget