HoloLens 2 Sensor Streaming

HoloLens 2 server software and Python client library for streaming sensor data via TCP. Created to stream HoloLens data in real-time to a Linux machine for research purposes but also works on Windows and OS X. The server is offered as a standalone application (appxbundle) or Unity plugin (dll).

Supported interfaces

• Research Mode Visible Light Cameras (640x480 @ 30 FPS, Grayscale, H264 or HEVC encoded)
  • Left Front
  • Left Left
  • Right Front
  • Right Right
• Research Mode Depth
  • AHAT (512x512 @ 45 FPS, 16-bit Depth + 16-bit AB, H264 or HEVC encoded or Lossless* Zdepth for Depth)
  • Long Throw (320x288 @ 5 FPS, 16-bit Depth + 16-bit AB, PNG encoded)
• Research Mode IMU
  • Accelerometer (m/s^2)
  • Gyroscope (deg/s)
  • Magnetometer
• Front Camera (1920x1080 @ 30 FPS, RGB, H264 or HEVC encoded)
• Microphone (2 channels @ 48000 Hz, 16-bit PCM, AAC encoded)
• Spatial Input (30 Hz)
  • Head Tracking
  • Eye Tracking
  • Hand Tracking
• Spatial Mapping
• Scene Understanding
• Voice Input
• Extended Eye Tracking (30, 60, or 90 FPS)
• Extended Audio (Microphone + Application audio, 2 channels @ 48000 Hz, 16-bit PCM, AAC encoded)

Additional features

• Download calibration data (e.g., camera intrinsics, extrinsics, undistort maps) for the Front Camera and Research Mode sensors (except RM IMU Magnetometer).
• Optional per-frame pose for the Front Camera and Research Mode sensors.
• Support for Mixed Reality Capture (Holograms in Front Camera video).
• Client can configure the bitrate and properties of the H264, HEVC, and AAC encoded streams.
• Client can configure the resolution and framerate of the Front Camera. See here for a list of supported configurations.
• Client can configure the focus, white balance, and exposure of the Front Camera. See here.
• Frame timestamps can be converted to Windows FILETIME (UTC) for external synchronization. See here.
• Client can send messages to a Unity application using the plugin.
• C++ client library.
• MATLAB client (MEX).

Preparation

Before using the server software configure your HoloLens as follows:

0. Update your HoloLens: Settings -> Update & Security -> Windows Update.
1. Enable developer mode: Settings -> Update & Security -> For developers -> Use developer features.
2. Enable device portal: Settings -> Update & Security -> For developers -> Device Portal.
3. Enable research mode: Refer to the Enabling Research Mode section in HoloLens Research Mode.

Please note that enabling Research Mode on the HoloLens increases battery usage.

Installation (sideloading)

The server application is distributed as a single appxbundle file and can be installed using one of the three following methods.

Method 1

1. On your HoloLens, open Microsoft Edge and navigate to this repository.
2. Download the latest appxbundle.
3. Open the appxbundle and tap Install.

Method 2

1. Download the latest appxbundle.
2. Go to the Device Portal (type the IP address of your HoloLens in the address bar of your preferred web browser) and upload the appxbundle to the HoloLens (System -> File explorer -> Downloads).
3. On your HoloLens, open the File Explorer and locate the appxbundle. Tap the appxbundle file to open the installer and tap Install.

Method 3

1. Download the latest appxbundle.
2. Go to the Device Portal and navigate to Views -> Apps. Under Deploy apps, select Local Storage, click Browse, and select the appxbundle.
3. Click Install, wait for the installation to complete, then click Done.

You can find the server application (hl2ss) in the All apps list.

Permissions

The first time the server runs it will ask for the necessary permissions to access sensor data. If there are any issues please verify that the server application (hl2ss.exe) has access to:

• Camera (Settings -> Privacy -> Camera).
• Eye tracker (Settings -> Privacy -> Eye tracker).
• Microphone (Settings -> Privacy -> Microphone).
• User movements (Settings -> Privacy -> User movements).

Python client

The Python scripts in the viewer directory demonstrate how to connect to the server, receive the data, unpack it, and decode it in real time. Additional samples show how to associate data from multiple streams. Run the server on your HoloLens and set the host variable of the Python scripts to your HoloLens IP address.

Interfaces

• RM VLC: viewer/client_stream_rm_vlc.py
• RM Depth AHAT: viewer/client_stream_rm_depth_ahat.py
• RM Depth Long Throw: viewer/client_stream_rm_depth_longthrow.py
• RM IMU: viewer/client_stream_rm_imu.py
• Front Camera: viewer/client_stream_pv.py
• Microphone: viewer/client_stream_microphone.py
• Spatial Input: viewer/client_stream_si.py
• Remote Configuration: viewer/client_ipc_rc.py
• Spatial Mapping: viewer/client_ipc_sm.py
• Scene Understanding: viewer/client_ipc_su.py
• Voice Input: viewer/client_ipc_vi.py
• Unity Message Queue: viewer/client_ipc_umq.py
• Extended Eye Tracking: viewer/client_stream_eet.py
• Extended Audio: viewer/client_stream_extended_audio.py

Required packages

• OpenCV pip install opencv-python
• PyAV pip install av
• NumPy pip install numpy

Optional packages

• pynput pip install pynput
• Open3D pip install open3d
• PyAudio pip install PyAudio
• MMDetection

Unity plugin

For streaming sensor data from a Unity application. All interfaces are supported. A sample Unity project (2020.3.42f1) can be found in the unity directory. If you wish to create a new project you can start here.

Build and run the sample project

1. Open the project in Unity. If the MRTK Project Configurator window pops up just close it.
2. Go to Build Settings (File -> Build Settings).
3. Switch to Universal Windows Platform.
4. Set Target Device to HoloLens.
5. Set Architecture to ARM64.
6. Set Build and Run on Remote Device (via Device Portal).
7. Set Device Portal Address to your HoloLens IP address (e.g., https://192.168.1.7) and set your Device Portal Username and Password.
8. Click Build and Run. Unity may ask for a Build folder. You can create a new one named Build.

Adding the plugin to an existing Unity Project

1. Download the latest plugin zip file and extract the Assets folder into your Unity project folder.
2. In the Unity Editor configure the hl2ss, Eye Tracking, and Scene Understanding DLLs as UWP ARM64.
   1. In the Project window navigate to Assets/Plugins/WSA, select the DLL, and then go to the Inspector window.
   2. Set SDK to UWP.
   3. Set CPU to ARM64.
   4. Click Apply.
3. Add the Hololens2SensorStreaming.cs script to the Main Camera.
4. Enable the following capabilities (Edit -> Project Settings -> Player -> Publishing Settings):
   • InternetClientServer
   • InternetClient
   • PrivateNetworkClientServer
   • Webcam
   • Microphone
   • Spatial Perception
   • Gaze Input
5. The plugin also requires the perceptionSensorsExperimental and backgroundSpatialPerception capabilities, which are not available in the Publishing Settings capabilities list. You can extract the Editor folder (plugin zip file) into the Assets folder (in your Unity project) which contains a script (BuildPostProcessor.cs) that adds the capabilities automatically after building the project. Alternatively, you can manually edit the Package.appxmanifest after building. See here for an example.

Remote Unity Scene

The plugin has basic support for creating and controlling 3D primitives and text objects via TCP for the purpose of sending feedback to the HoloLens user. See the unity_sample Python scripts in the viewer directory for some examples. Some of the supported features include:

• Create primitive: sphere, capsule, cylinder, cube, plane, and quad.
• Set active: enable or disable game object.
• Set world transform: position, rotation, and scale.
• Set local transform: position, rotation, and scale w.r.t. Main Camera.
• Set color: rgba with support for semi-transparency.
• Set texture: upload png or jpg file.
• Create text: creates a TextMeshPro object.
• Set text: sets the text, font size and color of a TextMeshPro object.
• Text to speech: upload text.
• Remove: destroy game object.
• Remove all: destroy all game objects created by the plugin.

To enable this functionality add the RemoteUnityScene.cs script to the Main Camera and set the Material field to BasicMaterial.

Build from source and deploy

Building the server application and the Unity plugin requires a Windows 10 machine.

1. Install the tools.
2. Open the Visual Studio solution (sln file in the hl2ss folder) in Visual Studio 2022.
3. Set build configuration to Release ARM64. Building for x86 and x64 (HoloLens emulator), and ARM is not supported.
4. Right click the hl2ss project and select Properties. Navigate to Configuration Properties -> Debugging and set Machine Name to your HoloLens IP address.
5. Build (Build -> Build Solution). If you get an error saying that hl2ss.winmd does not exist, copy the hl2ss.winmd file from etc into the hl2ss\ARM64\Release\hl2ss folder.
6. Run (Remote Machine). You may need to pair your HoloLens first.

The server application will remain installed on the HoloLens even after power off. The Unity plugin is in the hl2ss\ARM64\Release\plugin folder. If you wish to create the server application appxbundle, right click the hl2ss project and select Publish -> Create App Packages.

Known issues and limitations

• Multiple streams can be active at the same time but only one client per stream is allowed.
• Occasionally, the server might crash when accessing the Front Camera and RM Depth Long Throw streams simultaneously. See microsoft/HoloLens2ForCV#142.
• The RM Depth AHAT and RM Depth Long Throw streams cannot be accessed simultaneously.

References

This project uses the HoloLens 2 Research Mode API and the Cannon library, both available at the HoloLens2ForCV repository. Lossless* depth compression enabled by the Zdepth library.