Velocity MVP. Protobuf Bridge and Messages Update for Velocity Data Exchange.

server/core/src/main/java/dev/slimevr/VRServer.kt

@@ -182,6 +182,21 @@ class VRServer @JvmOverloads constructor(
 		instance = this
 	}
 
+	/**
+	 * TODO: The design of this method is chosen for future expandability in case we want to have more complex velocity policies that depend on tracker properties or other config values.
+	 * Initiates the velocity policy application process for the specified [Tracker] (or all trackers if null).
+	 *
+	 * This method serves as the initiator.
+	 * The actual policy logic is not handled here,
+	 * but is delegated to [dev.slimevr.config.VRConfig.applyVelocityPolicy].
+	 */
+	private fun applyVelocityPolicyTo(tracker: Tracker?) {
+		val targets = tracker?.let { listOf(it) } ?: trackers
+		for (t in targets) {
+			configManager.vrConfig.applyVelocityPolicy(t)
+		}
+	}
+
 	fun hasBridge(bridgeClass: Class<out Bridge?>): Boolean {
 		for (bridge in bridges) {
 			if (bridgeClass.isAssignableFrom(bridge.javaClass)) {
@@ -226,6 +241,9 @@ class VRServer @JvmOverloads constructor(
 			refreshTrackersDriftCompensationEnabled()
 			configManager.vrConfig.writeTrackerConfig(tracker)
 			configManager.saveConfig()
+
+			// Requires a fresh TrackerConfig on Update, so executed after we save a new state.
+			applyVelocityPolicyTo(tracker)
 		}
 	}
 

server/core/src/main/java/dev/slimevr/config/VRConfig.kt

@@ -58,6 +58,8 @@ class VRConfig {
 
 	val trackingChecklist: TrackingChecklistConfig = TrackingChecklistConfig()
 
+	var velocityConfig: VelocityConfig = VelocityConfig()
+
 	val vrcConfig: VRCConfig = VRCConfig()
 
 	init {
@@ -104,6 +106,17 @@ class VRConfig {
 		return config
 	}
 
+	/**
+	 * Applies the velocity policy to the given [Tracker].
+	 *
+	 * This method determines whether the tracker—be it a physical sensor or a computed virtual device—is
+	 * permitted to calculate and broadcast velocity data.
+	 * It resolves the active [VelocityConfig] value, updating [Tracker.allowVelocity] accordingly.
+	 */
+	fun applyVelocityPolicy(tracker: Tracker) {
+		tracker.allowVelocity = velocityConfig.sendDerivedVelocity
+	}
+
 	fun readTrackerConfig(tracker: Tracker) {
 		if (tracker.userEditable) {
 			val config = getTracker(tracker)
@@ -119,6 +132,7 @@ class VRConfig {
 					.readFilteringConfig(filters, tracker.getRotation())
 			}
 		}
+		applyVelocityPolicy(tracker)
 	}
 
 	fun writeTrackerConfig(tracker: Tracker?) {

server/core/src/main/java/dev/slimevr/config/VelocityConfig.kt

@@ -0,0 +1,10 @@
+package dev.slimevr.config
+
+/**
+ * Allows to enable/disable sending of optional derived velocity data via Protobuf.
+ * Enables Natural Locomotion Support
+ * May create overprediction in certain titles causing excessive jitter when moving upper body.
+ */
+class VelocityConfig {
+	var sendDerivedVelocity: Boolean = false // Disables derived velocity for all trackers. Driver zeroes out velocity if nothing is returned in protobuf message.
+}

server/core/src/main/java/dev/slimevr/tracking/processor/skeleton/HumanSkeleton.kt

@@ -1171,6 +1171,7 @@ class HumanSkeleton(
 			it.position = trackerBone.getTailPosition()
 			it.setRotation(trackerBone.getGlobalRotation() * trackerBone.rotationOffset.inv())
 			it.dataTick()
+			it.updateDerivedVelocity(System.nanoTime())
 		}
 	}
 

server/core/src/main/java/dev/slimevr/tracking/trackers/Tracker.kt

@@ -75,6 +75,11 @@ class Tracker @JvmOverloads constructor(
 	 */
 	val allowMounting: Boolean = false,
 
+	/**
+	 * If true, the tracker will send Derived Velocity.
+	 */
+	var allowVelocity: Boolean = false,
+
 	val isHmd: Boolean = false,
 
 	/**
@@ -105,7 +110,19 @@ class Tracker @JvmOverloads constructor(
 	// IMU: +z forward, +x left, +y up
 	// SlimeVR: +z backward, +x right, +y up
 	private var _acceleration = Vector3.NULL
+	private var _velocity = Vector3.NULL
 	private var _magVector = Vector3.NULL
+
+	/**
+	 * Velocity state server-side differentiation based on sent poses
+	 */
+	private data class VelocityState(
+		var prevTimeNs: Long = 0L,
+		var prevPos: Vector3 = Vector3(0f, 0f, 0f),
+	)
+
+	private var velocityState: VelocityState? = null
+
 	var position = Vector3.NULL
 	val resetsHandler: TrackerResetsHandler = TrackerResetsHandler(this)
 	val filteringHandler: TrackerFilteringHandler = TrackerFilteringHandler()
@@ -329,6 +346,42 @@ class Tracker @JvmOverloads constructor(
 		}
 	}
 
+	/**
+	 * Updates the derived velocity of the tracker by differentiating position over time.
+	 *
+	 * This method enforces the [allowVelocity] policy and checks for valid position data before
+	 * proceeding. If conditions are met, it calculates velocity based on the displacement since the
+	 * last update, applying a sanity check on the time delta to filter out noise and ensure data stability.
+	 *
+	 */
+	fun updateDerivedVelocity(nowNs: Long) {
+		if (!allowVelocity || !hasPosition) {
+			velocityState = null
+			_velocity = Vector3.NULL
+			return
+		}
+
+		val pos = position
+		val state = velocityState ?: VelocityState().also {
+			velocityState = it
+		}
+
+		if (state.prevTimeNs != 0L) {
+			val dt = (nowNs - state.prevTimeNs) * 1e-9
+			if (dt in 1e-4..0.25) {
+				_velocity = Vector3(
+					((pos.x - state.prevPos.x) / dt).toFloat(),
+					((pos.y - state.prevPos.y) / dt).toFloat(),
+					((pos.z - state.prevPos.z) / dt).toFloat(),
+				)
+			} else {
+				_velocity = Vector3.NULL
+			}
+		}
+		state.prevTimeNs = nowNs
+		state.prevPos = pos
+	}
+
 	/**
 	 * Gets the identity-adjusted tracker rotation after the resetsHandler's corrections
 	 * (identity reset, drift and identity mounting).
@@ -405,6 +458,24 @@ class Tracker @JvmOverloads constructor(
 		this._acceleration = vec
 	}
 
+	/**
+	 * Sets the derived velocity of the tracker.
+	 */
+	fun setVelocity(vec: Vector3) {
+		this._velocity = if (allowVelocity) vec else Vector3.NULL
+	}
+
+	/**
+	 * Gets the derived velocity of the tracker.
+	 */
+	fun getVelocity(): Vector3 = _velocity
+
+	/**
+	 * True if the tracker has valid velocity data.
+	 */
+	val hasVelocity: Boolean
+		get() = allowVelocity && _velocity != Vector3.NULL
+
 	/**
 	 * True if the raw rotation is coming directly from an IMU (no cameras or lighthouses)
 	 * For example, flex sensor trackers are not considered as IMU trackers (see TrackerDataType)

server/desktop/src/main/java/dev/slimevr/desktop/platform/ProtobufBridge.kt

@@ -82,6 +82,7 @@ abstract class ProtobufBridge(@JvmField protected val bridgeName: String) : ISte
 		target.position = source.position
 		target.setRotation(source.getRotation())
 		target.status = source.status
+		target.setVelocity(source.getVelocity())
 		target.batteryLevel = source.batteryLevel
 		target.batteryVoltage = source.batteryVoltage
 		target.dataTick()
@@ -103,23 +104,38 @@ abstract class ProtobufBridge(@JvmField protected val bridgeName: String) : ISte
 
 	@VRServerThread
 	protected fun writeTrackerUpdate(localTracker: Tracker?) {
-		val builder = ProtobufMessages.Position.newBuilder().setTrackerId(
-			localTracker!!.id,
-		)
-		if (localTracker.hasPosition) {
-			val pos = localTracker.position
+		val tracker = localTracker ?: return
+
+		val builder = ProtobufMessages.Position.newBuilder()
+			.setTrackerId(tracker.id)
+
+		if (tracker.hasPosition) {
+			val pos = tracker.position
 			builder.setX(pos.x)
 			builder.setY(pos.y)
 			builder.setZ(pos.z)
 		}
-		if (localTracker.hasRotation) {
-			val rot = localTracker.getRotation()
+
+		if (tracker.hasRotation) {
+			val rot = tracker.getRotation()
 			builder.setQx(rot.x)
 			builder.setQy(rot.y)
 			builder.setQz(rot.z)
 			builder.setQw(rot.w)
 		}
-		sendMessage(ProtobufMessage.newBuilder().setPosition(builder).build())
+
+		if (tracker.allowVelocity) {
+			val vel = tracker.getVelocity()
+			builder.setVx(vel.x)
+			builder.setVy(vel.y)
+			builder.setVz(vel.z)
+		}
+
+		sendMessage(
+			ProtobufMessage.newBuilder()
+				.setPosition(builder)
+				.build(),
+		)
 	}
 
 	@VRServerThread
@@ -169,6 +185,16 @@ abstract class ProtobufBridge(@JvmField protected val bridgeName: String) : ISte
 					),
 
 				)
+			if (positionMessage.hasVx()) {
+				tracker
+					.setVelocity(
+						Vector3(
+							positionMessage.vx,
+							positionMessage.vy,
+							positionMessage.vz,
+						),
+					)
+			}
 			tracker.dataTick()
 		}
 	}