Add slower lerp when tracking system swaps.

Author: Sebane1

src/IVRDriver.hpp

@@ -97,8 +97,10 @@ class IVRDriver : protected vr::IServerTrackedDeviceProvider {
   virtual std::optional<vr::DriverPose_t>
   GetExternalPoseForHand(bool left_hand) = 0;
 
-  /** Pose lerp speed (0–1) for smoothing VD/SlimeVR transitions. From driver config file. */
+  /** Pose lerp speed (0–1) for smoothing. From driver config file. */
   virtual float GetPoseLerpSpeed() = 0;
+  /** Slower lerp speed used when swapping VD ↔ SlimeVR to smooth the transition. */
+  virtual float GetPoseLerpSpeedOnSwap() = 0;
 
   virtual inline const char *const *GetInterfaceVersions() override {
     return vr::k_InterfaceVersions;

src/TrackerDevice.cpp

@@ -143,16 +143,22 @@ void SlimeVRDriver::TrackerDevice::Update() {
 
   // Target pose: controllers use external (VD/Steam Link) when available else SlimeVR; trackers use last SlimeVR pose.
   vr::DriverPose_t target = last_pose_atomic_.load();
+  bool have_external = false;
   if (is_controller_) {
     auto external = GetDriver()->GetExternalPoseForHand(is_left_hand_);
-    if (external.has_value())
+    have_external = external.has_value();
+    if (have_external)
       target = *external;
   }
-  // Lerp from current toward target to reduce jitter and smooth transitions.
+  // Use slower lerp when swapping VD ↔ SlimeVR to smooth the transition.
+  float lerp_t = GetDriver()->GetPoseLerpSpeed();
+  if (is_controller_ && (have_external != last_frame_had_external_))
+    lerp_t = GetDriver()->GetPoseLerpSpeedOnSwap();
+  last_frame_had_external_ = have_external;
   if (!smoothed_pose_.has_value())
     smoothed_pose_ = target;
   else
-    smoothed_pose_ = LerpPose(*smoothed_pose_, target, GetDriver()->GetPoseLerpSpeed());
+    smoothed_pose_ = LerpPose(*smoothed_pose_, target, lerp_t);
   GetDriver()->GetDriverHost()->TrackedDevicePoseUpdated(
       device_index_, *smoothed_pose_, sizeof(vr::DriverPose_t));
 }

src/TrackerDevice.hpp

@@ -70,6 +70,7 @@ class TrackerDevice : public IVRDevice {
   std::atomic<vr::DriverPose_t> last_pose_atomic_ =
       IVRDevice::MakeDefaultPose();
   std::optional<vr::DriverPose_t> smoothed_pose_;
+  bool last_frame_had_external_ = false;
   static vr::DriverPose_t LerpPose(const vr::DriverPose_t &from,
                                    const vr::DriverPose_t &to, float t);
 

src/VRDriver.cpp

@@ -495,6 +495,7 @@ void SlimeVRDriver::VRDriver::LoadDriverConfig() {
     if (auto v = obj["external_hand_max_radius_m"]; !v.error()) config_external_hand_max_radius_m_ = static_cast<float>(v.get_double());
     if (auto v = obj["stale_external_pose_frames"]; !v.error()) config_stale_external_pose_frames_ = static_cast<int>(v.get_int64());
     if (auto v = obj["pose_lerp_speed"]; !v.error()) config_pose_lerp_speed_ = static_cast<float>(v.get_double());
+    if (auto v = obj["pose_lerp_speed_on_swap"]; !v.error()) config_pose_lerp_speed_on_swap_ = static_cast<float>(v.get_double());
     if (auto v = obj["frozen_pose_position_epsilon_m"]; !v.error()) config_frozen_pose_position_epsilon_m_ = static_cast<float>(v.get_double());
     logger_->Log("Loaded driver config from {}", path);
   } catch (const simdjson::simdjson_error &) {
@@ -506,6 +507,10 @@ float SlimeVRDriver::VRDriver::GetPoseLerpSpeed() {
   return config_pose_lerp_speed_;
 }
 
+float SlimeVRDriver::VRDriver::GetPoseLerpSpeedOnSwap() {
+  return config_pose_lerp_speed_on_swap_;
+}
+
 SlimeVRDriver::SettingsValue
 SlimeVRDriver::VRDriver::GetSettingsValue(std::string key) {
   vr::EVRSettingsError err = vr::EVRSettingsError::VRSettingsError_None;

src/VRDriver.hpp

@@ -91,6 +91,7 @@ namespace SlimeVRDriver {
   float config_external_hand_max_radius_m_ = 1.7f;
   int config_stale_external_pose_frames_ = 1;
   float config_pose_lerp_speed_ = 0.8f;
+  float config_pose_lerp_speed_on_swap_ = 0.25f;
   float config_frozen_pose_position_epsilon_m_ = 0.005f;
   void LoadDriverConfig();
   void UpdateExternalControllerPoses();
@@ -101,5 +102,6 @@ namespace SlimeVRDriver {
   bool ExternalHandInFrontAndInRadius(const double hand_pos[3],
                                      const vr::TrackedDevicePose_t &hmd_pose) const;
   virtual float GetPoseLerpSpeed() override;
+  virtual float GetPoseLerpSpeedOnSwap() override;
 };
 }; // namespace SlimeVRDriver