docs/hil_quickstart.md

Android / DJI HIL quickstart

What runs where

DJI aircraft-side Simulator + physical remote controller
    -> OpenFly Go on Android -> pose/session messages -> UE/AirSim
    <- Android virtual camera <- rendered JPEG frames <- Gaussian scene

DJI Simulator is the motion authority. UE follows its state and renders the virtual camera; it does not run a second flight dynamics model for the HIL vehicle. Android handles control and safety actions. run_expo_east.sh opens HIL mode by default. Its optional --preview mode uses AirSim SimpleFlight and is not a hardware-in-the-loop session.

1. Prepare the devices

• Linux PC with the extracted OpenFlySplat UE runtime and an active scene. The Expo East package already contains one; the scene-free package requires ./convert_nanogs_ply.sh /path/to/model.ply --crop-mode none first.
• Android phone running the matching OpenFly Go client: MSDK V4 for the Mini 2 setup, or MSDK V5 for the Mini 4 Pro setup. The Android app is a separate companion, not included in the UE executable. Source builds require your own DJI application key.
• Compatible DJI aircraft and its paired remote controller. Connect the phone to the remote controller by USB and confirm that the app sees the aircraft.

Use a stationary bench setup with the propellers removed. Confirm that the app reports DJI Simulator active before issuing any simulated takeoff/control command. A healthy network connection alone does not mean simulation is active.

2. Start the UE host

If run_expo_east.sh is already open, continue to phone connection below; the HIL host is already running. Otherwise, close any software-only preview and run from Linux/OpenFlySplatUE:

./run_openfly_hil.sh --settings NanoGSConverter/Config/OpenFlyHil.json

The scene opens with the HIL monitor. Before connecting the phone, a waiting state is expected. To show both HIL and renderer controls, add --ui All.

3. Connect the phone

Open the app's UE HIL panel and choose either:

Mode	Steps
Android hotspot / Android 热点直连	Enable the phone hotspot, connect the PC to it, then start HIL in the app. Discovery uses UDP; do not hard-code a hotspot gateway address.
Existing LAN / 已有局域网	Join the same LAN on phone and PC, enter the PC's LAN IP as the UE address, then start HIL.

Keep the default UDP port 30020 and image TCP port 30022 unless both ends are configured differently. Starting HIL attempts to start/adopt the DJI Simulator session. If this does not complete, use the app's DJI Simulator start/status control and inspect its message. Never force a simulator restart while its virtual aircraft is airborne.

Select UE in the HIL camera-source selector to display the rendered image instead of the physical DJI camera. Check all three indicators before moving:

1. UE peer/heartbeat connected.
2. DJI Simulator active with fresh state and nonzero POSE updates.
3. TCP image connected with an advancing frame counter.

The hardware controller can then operate the simulated aircraft. Finish by landing the virtual aircraft, stopping the simulator/HIL session in the phone, and closing the UE application.

4. Network directions

Receiver	Port	Traffic
UE PC	UDP 30020	Android HELLO, POSE, heartbeat and PING
Android phone	UDP 30021	UE heartbeat, PONG and safety EVENT notifications
Android phone	TCP 30022	UE initiates the connection and sends framed JPEG images

Allow these directions on the trusted local network. AirSim RPC port 41451 is separate and is not needed for phone discovery. No ADB connection is required for the runtime data path. This HIL transport has no authentication/encryption; do not expose it to the public Internet.

5. Troubleshooting

Symptom	Check
Waiting for phone / 等待手机连接	Same network, correct UE LAN IP, UDP 30020/30021, and hotspot/client isolation.
Image online, waiting for DJI Simulator	Network works. Check aircraft/remote USB connection and simulator state in the app.
Pose online, no image	Phone TCP 30022 listener, firewall and UE camera-source selection. UE connects to the phone, not the reverse.
Two UE windows or port-in-use error	Close the software preview or another HIL process before launching again.
Sky only / missing GS scene	Confirm that the package has Content/NanoGSData/RuntimeActive/active_scene.runtime.json and its referenced tree directory. Restart UE after conversion.
Spawn is inside geometry	Close UE, adjust the scene's ground/start settings and reconvert; do not resolve a bad spawn by starting a hardware control session.

After a network timeout, the UE monitor enters a waiting state. Restore the network and let the phone reconnect; check fresh simulator state and image frames again before resuming.

Scene origin and rendering

The scene converter generates a PlayerStart and a hidden floor. The runtime uses that PlayerStart as the common AirSim/HIL origin. The floor is not a building collision mesh: visible GS surfaces do not automatically provide physical collision geometry. Start height is measured above estimated local ground; do not treat the PLY origin as ground level.

The profile requests 1440x1080 JPEG, quality 92, up to 30 FPS. V4 and V5 expose different DJI simulator state APIs; a phone POSE transmit setting is not proof of an equally fast fresh hardware state stream. For the recorded configuration and measured rates, see HIL_PERFORMANCE_CN.md in the source documentation.