OptiTrack Unreal Engine 4 Plugin

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Plugin Overview

The OptiTrack Unreal Engine 4 Plugin allows real-time streaming of live rigid body tracking data from Motive into Unreal Engine 4 (abbreviated as UE4). The plugin also features HMD tracking integration for virtual reality applications. Once the plugins are added to the Unreal Engine directory, OptiTrack – NatNet Streaming Client and OptiTrack – Oculus Rift plugins will be available within the UE4 project. This article focuses on the streaming workflow as well as setting up the HMD in UE4 using the plugin. For general instructions on setting up the system, please refer to the Getting Started guide.

The NatNet Streaming Client plugin will allow you to stream rigid body tracking data from Motive onto Unreal Engine, and the Oculus Rift plugin will allow you to implement HMD tracking onto your scene. Once the plugins are enabled, OptiTrack Client Origin object can be called, and it connects the project to the server application (i.e. Motive) via NatNet protocol. Position and orientation of the Client Origin object reconciles the global origin in Motive, and all streamed tracking data will be referenced against to it. This origin can be placed anywhere in the scene and scaled to different sizes. Once the project is connected to the server application, imported tracking data can be used to instantiate the HMD and animate the objects in virtual reality applications.

Virtual Reality Experience

OptiTrack motion capture systems provide motion tracking with outstanding accuracy, and all of which can be accomplished with extremely low latency. Utilizing its tracking capability, an OptiTrack system can be used to provide fully immersive virtual reality experience by integrating head-mounted display (HMD) device. From this combined setup, even tiny movements can be precisely captured and reflected in the virtual scene.

Version Requirements

This plugin is supported for the following versions of Unreal Engine, and it also requires the corresponding version of Oculus runtime as indicated in the table.

Plugin Version	UE Version	Oculus SDK
1.0	UE 4.10	0.8
1.1	UE 4.11	1.3
1.2	UE 4.12	Latest
1.3	UE 4.13	Latest
1.4	UE 4.14	Latest
1.5	UE 4.15	Latest
1.6	UE 4.16	Latest
1.7	UE 4.17	Latest
1.8	UE 4.18.3	Latest

As of OptiTrack Unreal Engine 4 Plugin 1.3.0, Oculus DK2, CV1, and Gear VR HMD models are supported

Motive Data Streaming Setup (Server)

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Broadcast Frame Data set to true for streaming.

To stream the rigid body data, follow the general data streaming workflow provided from the Data Streaming page. Make sure appropriate network interface is defined and the Stream Rigid Bodies is enabled under the streaming settings.

Streaming in Motive

• Open the Data Streaming pane in Motive
• Select the network interface address for streaming data.
• Select desired data types to stream under streaming options.
• When streaming skeletons, set the appropriate bone naming convention for the client application. If the character uses the FBX naming convention, this will need to be set to FBX.
• Check Broadcast Frame Data at the top.
• Configure streaming settings and designate the corresponding IP address from client applications
• Stream live or playback captures

When streaming the data over a wifi network, Unicast transmission must be used.

Unreal Engine Setup (Client)

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This section includes instructions on installing and configuring the Unreal Engine plugin to receive tracking data from Motive.

Setting Up the Plugin

1. Download the plugin ZIP file.
2. Extract the contents from the ZIP file.
3. Open the OptiTrack folder, transfer the OptiTrackNatNet and OptiTrackOculusRift folders into the Unreal Engine plugin directory. By default, this directory is installed in the C:\Program Files\Epic Games\4.##\Engine\Plugins folder. There will be other plugins installed in the folder already. Copy and paste the OptiTrack plugin folders into this directory.
4. Open/Create a UE4 project, and under the plugin devices, enable both of the OptiTrack plugin devices.
5. Under the Edit menu on the main command bar, open the plugin panel.
6. Browse to Virtual Reality section and enable the OptiTrack plugins (OptiTrack - NatNet Streaming Client and OptiTrack - OculusVR) that are added to the folder.

UE4 plugins panel listing the enabled OptiTrack plugins.

Note: When using the Oculus Plugin, make sure to disable the built-in Oculus Rift (or the Gear VR) and Stream VR HMD plugins under virtual reality plugins category. Otherwise, the editor may crash, or they may end up having a higher priority than the OptiTrack Oculus Plugin, and if enabled, will be used in preference to it.

Unreal Engine Client Setup

OptiTrack Client Origin

Once the plugin is properly added, the Client Origin Object can be found under Modes panel in UE4.

The OptiTrack Client Origin enables the Unreal Engine (client) to communicate with the NatNet server where Motive’s rigid body and HMD tracking data is obtained. In the client object, both the communication address and the rigid body ID for the HMD are configured. Add this object simply dragging and dropping the OptiTrack Client Origin from the Modes panel into the scene.

Once the OptiTrack Client Origin is placed, position and orientation of this object will reconcile the global origin of Motive in Unreal Engine. In other words, the tracking data will be represented according to where this Client Origin object is placed and how it is oriented.

Global Origin: Both position and orientation of the OptiTrackClientOrigin will represent the global origin of the tracking volume within Motive.

Connecting Unreal Engine to Motive

1. Once the plugin is added and enabled in the project, OptiTrack Client Origin class will be available from the Modes panel.
2. Drag and drop the OptiTrack Client Origin into the scene.
3. Place the OptiTrack Client Origin to the desired tracking origin within the scene.
4. Select the instantiated OptiTrackClientOrigin object from the World Outliner panel.
5. Under the Details panel, expand the properties of the OptiTrack Client Origin.
6. Enter matching server and client network address information that was defined in the Data Streaming Pane in Motive.
7. In Motive, make sure the Broadcast Frame Data entry is checked.
8. Now the UE4 project is connected with Motive, and the tracking data will be available in UE4 when the scene is played.

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  Data streaming settings in Motive
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  OptiTrack Client Origin properties defined with corresponding server and client address.

Version 1.8: Advance settings: Auto-initialize In UE4 plugin version 1.8 and above, you can set up the project so at the client origin gets initialized at a user-defined event. This setting is enabled by default. But when there is a need to provide the server IP address dynamically within the game, you can disable the ‘’auto-initialize’’ feature and configure using blueprint how and when the client origin gets initialized.

Animating Rigid Body

OptiTrack Rigid Body Actor

Actor objects in Unreal Engine can be animated using rigid body tracking data from Motive. Once the OptiTrack - NatNet Streaming Client plugin is enabled, OptiTrack Rigid Body component will be available to use, and by attaching this component to an actor, you can animate its child components according to the movement of the associated rigid body. Each parent component is given a Trackable ID value, and rigid bodies with the matching Streaming ID will be used to translate and rotate the target actor in Unreal Engine. Please note that this component must be attached to the parent node to animate its child actors.

You can also use the OptiTrack Rigid Body Actor, which will be available on the Modes panel. This actor is basically a blank actor with just the OptiTrack Rigid Body component attached to it. You can use this to easily associate the objects within the scene:

Steps

1. [UE4] From the Modes panel, search for OptiTrack Rigid Body Actor. Then drag and drop the actor into the scene.
2. [UE4] Under this rigid body actor, attach the target actor that you wish to animate.
3. [UE4] Set the relative location and rotation to zero on this target actor, which should be a child of the rigid body actor at this point.
4. [Motive] In Motive, assign a value to Streaming ID property for the target rigid body.
5. [UE4] OptiTrack Rigid Body Actor component properties, match the Tracking ID with the Streaming ID of the rigid body asset.
6. Make sure both Motive and OptiTrack Client Origin is set up for streaming. Then hit Play, and the attached actor object will be animated according to the live-streamed rigid body tracking data.

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  Streaming ID of a selected rigid body asset in Motive.
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  Once the OptiTrack NatNet streaming client plugin is properly installed, the OptiTrack Rigid Body component will be available within the components list in UE4.
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  Within the OptiTrack Rigid Body Component, input the Streaming ID for the corresponding rigid body asset in Motive.

Animating Skeleton

Integrating skeleton tracking in Unreal Engine requires a 3D retargeting process. This can be accomplished through a third party software that retargets the 3D data to produce realistic motion in a scene. For more information, contact us.

Notes for IKinema LiveAction for UE 4.18. When using IKinema LiveAction for UE 4.18 along with OptiTrack UE plugin 4.18.3, you will need to replace NatNet library file (NatNetLIb.dll / NatNetLib.lib) included in the IKinema LiveAction plugin directory with the library files included within the Optitrack UE plugin package. This is required only for this particular version of the LiveAction because it uses a slightly different version of the NatNet in its package. Copy both the LIB and DLL files for the corresponding platform, 32-bit or 64-bit, from the OptiTrack plugin and replace the library files included on the LiveAction plugin package: The OptiTrack UE plugin library files exists under the following folder: C:\Program Files\Epic Games\UE_4.18\Engine\Plugins\UE_4.18\OptiTrack\OptitrackNatNet\ThirdParty\NatNetSDK\lib The LiveAction plugin library files exists under the following folder: C:\Program Files\Epic Games\UE_4.18\Engine\Plugins\LiveAction\Source\ThirdParty\NatNet\NatNetSDK\lib\

Integrating HMD (Oculus DK2, CV1)

First of all, make sure the OptiTrack – OculusVR is enabled in your project in the plugins panel in order to integrate the HMD. Once the Client Origin object is called into the project, input the streaming ID for the HMD rigid body to the HMD Rigid Body ID section under the OptiTrack Client Origin. Then within the project World Settings, set the Default Pawn Class to be the OptiTrackHMDPawn class. Once this is set up, follow the HMD setup steps in the next section to prepare your HMD object in Motive.

Linking the HMD Tracking Data

1. [UE4] Enable the OptiTrack – Oculus Rift plugin from the plugins panel.
2. [UE4] Open the World Settings panel under the Window tab.
3. [UE4] If not done already, create a new GameMode by adding a new blueprint under the Game Mode section.
4. [UE4] Change the Default Pawn Class to OptiTrackHmdPawn. You may need to restart the editor before this change takes effect.
5. [UE4] Select the OptitrackClientOrigin and access the Details panel.
6. [UE4] Expand the OptiTrack Client Origin section.
7. [UE4] In the HMD Rigid Body ID entry, input the respective Streaming ID value configured from the rigid body properties in Motive.
8. [UE4] Now the tracking data is linked to the Unreal Engine project, and the HMD will display the scene when it is played.

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  Defining the default pawn class from World Setttings: GameMode in UE4.
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  Inputting the Streaming ID information for tracking the HMD.

Integrating HMD (Android: GearVR)

Client origin configuration for GearVR

Gear VR devices can also be integrated and the tracking information can be streamed via wifi connections using a router with sufficient bandwidth. The required bandwidth will vary depending on many factors (e.g. router type, the number of tracked object, etc.). The following settings must be configured in addition to the above HMD settings for developing Gear VR experiences using the plugin. For more information on developing Gear VR applications in Unreal Engine, refer to UE 4 documentations.

Settings

• [UE 4] For developing Android applications in Unreal Engine, make sure the project environment is set up for Android development: Android Quick Start Guide .
• [UE4] Follow the GearVR Quick Start Guide and configure the project for GearVR development.
• [UE4] Generate and include the OSIG file (oculus signature) in the directory. See: Oculus Signature File Generator.
• [Motive] Under the Data Streaming pane, set the Network Type setting under the Advanced Settings to Unicast. Note that the plugin currently only supports the Unicast broadcasting for streaming onto Android, multicasting will be supported in the future releases.

Connecting via Wifi

1. [Android] Connect the smartphone to the internet router which the host PC (server) is connected to.

2. [UE4] Configure the Client object.

- Use Connection Type: Unicast.

- Set the Local IP address to (0.0.0.0). The local address is not required for unicast connections.

- For the Server Address, connect to the IP address that the host PC (server) is connected to. This could also be found from the Android OS when the smartphone is connected to the wifi router.

3. [UE4] Connect a smartphone into the PC, build and run the project.

4. Make sure Motive is streaming, and the Gear VR HMD tracking data will be streamed into the UE4 application.

HMD Tracking Setup

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First of all, setup and optimize the motion capture volume as explained in the Getting Started guide or the Hardware Setup documentation. If you plan to install any obstacles (e.g. walls) within the capture volume, make sure they are non-reflective, and place and orient the cameras so that every corner is thoroughly captured by multiple cameras. For typical rigid body tracking, attach the rigid body markers as instructed in the Rigid Body Tracking page.

General Setup Steps

1. Attach the markers on the HMD
2. Create a rigid body asset
3. Calibrate the Pivot Point of the rigid body
4. Configure the rigid body settings in Motive

Marker Attachment

There are two different types of markers that can be used to track an object with OptiTrack systems: Passive markers and Active markers. Passive markers are retroreflective markers that reflect infrared light emitted from the IR LEDs on the camera. On the other hand, the active markers are LED markers that emit the IR light. Either type of markers can be used to track HMDs.

Use appropriate adhesives and marker posts to securely attach the markers to the HMD. For active markers, the active Tag must be attached also in order to synchronize and power the LEDs also. Oculus CV1 HMDs can use the active CV1 clip enclosure to easily place 8 active markers over the HMD. Depending on which marker you are using to track the HMD, the setup instructions will vary slightly.

Oculus CV1 HMD ClipPassive Markers

Active Markers

Attachment & Placement

With the HMD bracket, simply attach the active marker bracket onto an CV1 HMD and use the active markers on it to define a rigid body.

When using the active markers, the markers are actively labeled individually from each of their own unique illumination patterns, and only the matching active markers get contributed to the rigid body solve. The biggest benefit from the individual labeling is that markers can be placed in perfectly symmetrical placements among multiple rigid body assets.

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  HMD markers with passive markers
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  Putting the HMD bracket on
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  When detaching the bracket from the HMD, first pull up the two latches behind the bracket and slowly pull it from the top.

Passive Markers

Attachment

HMD markers with passive markers

When attaching retroreflective markers, make sure markers are securely attached and readily captured by the cameras. For attaching the markers, we recommend using our 20 mm wide and 30 mm tall M4 threaded plastic marker bases with Acrylic adhesives, available at the webstore, to attach the markers onto the HMD.

Placement

A markered HMD will be defined as a rigid body in Motive. When placing markers, make sure the placement asymmetry is respected in the arrangement within the HMD. Also, the marker arrangements between multiple HMDs must be incongruent. For more details, read about marker placement from the Rigid Body Tracking page.

Create an HMD Rigid Body

HMD with Active ClipHMD with Attached Markers

Creating Active Clip HMD

Creating an HMD rigid body in the Builder pane.

This feature can be used only with HMDs that have the OptiTrack Active HMD clips mounted.

For using OptiTrack system for VR applications, it is important that the pivot point of HMD rigid body gets placed at the appropriate location, which is at the root of the nose in between the eyes. When using the HMD clips, you can utilize the HMD creation tools in the Builder pane to have Motive estimate this spot and place the pivot point accordingly. It utilizes known marker configurations on the clip to precisely positions the pivot point and sets the desired orientation.

HMDs with passive markers can utilize the External Pivot Alignment tool to calibrate the pivot point.

Steps

1. First of all, make sure Motive is configured for tracking active markers.
2. Open the Builder pane under View tab and click Rigid Bodies.
3. Under the Type drop-down menu, select HMD. This will bring up the options for defining an HMD rigid body.
4. If the selected marker matches one of the Active clips, it will indicate which type of Active Clip is being used.
5. Under the Orientation drop-down menu, select the desired orientation of the HMD. The orientation used for streaming to Unity is +Z forward and Unreal Engine is +X forward, or you can also specify the expected orientation axis on the client plugin side.
6. Hold the HMD at the center of the tracking volume where all of the active markers are tracked well.
7. Select the 8 active markers in the 3D viewport.
8. Click Create. An HMD rigid body will be created from the selected markers and it will initiate the calibration process.
9. During calibration, slowly rotate the HMD to collect data samples in different orientations.
10. Once all necessary samples are collected, the calibrated HMD rigid body will be created.

Creating HMD from the markers

In Motive, select the markers that are attached to the HMD and create a rigid body (Hotkey: CTRL + T) as you would do for other rigid bodies. Once this is set up, use the External Pivot Alignment tool in the Builder pane (rigid body → Edit) to calibrate the pivot point of the created rigid body:

For using OptiTrack system for VR applications, it is important that the pivot point of HMD rigid body gets placed at the appropriate location, which is at the root of the nose in between the eyes. External Pivot Alignment tool utilizes both the OptiTrack mocap system and the HMD's own tracking system (Oculus Tracker / Vive Base Stations) for precisely calculating the location of the pivot point of an HMD. When using this feature, both systems must be tracking the HMD simultaneously.

HMD Compatibility Notes: For Motive versions 2.1 Final and later: Oculus and Vive HMDs are supported. For older versions of Motive: Only Oculus HMDs are supported. The corresponding native HMD tracker must be tracking the HMD along with the motion capture system simultaneously. For CV1 active HMD clips, the rigid body definition will be provided separately.

External Pivot Calibration Tips: Do not move around the HMD during calibration. You will want to keep it at one position and slowly rotate the HMD so that it can calibration from different orientations. When rotating, please rotate it slowly. Camera Exposure: Lower down the camera exposure setting to where the exposure is minimized but the HMD is still tracked well in Motive. This will help with preventing IR interference between the VR HMD's own tracking system and the motion capture system during the pivot calibration process. Set this to around 30us ~ 100us. Frame rate: Lower down the system frame rate down to 30 FPS for better sampling. Calibration Result: For best VR experience, please aim for the error result below ~1mm/marker.

Steps

1. First of all, connect the HMD Tracker (Oculus Tracker / Vive Base Stations) and have its corresponding software running in the background.
2. Launch Motive.
3. [Motive] Create a rigid body asset from the markers on the HMD.
4. [Motive] Open the Builder pane and access editing options for rigid bodies.
5. [Motive] Select the HMD rigid body in Motive.
6. [Motive → Builder pane] Under Detected Type, it should indicate the type of HMD you will be using.
7. [Motive → Builder pane] Set the desired number of sample counts (Default: 750) and the desired orientation.
8. [Motive → Builder pane] Press start to initiate the sampling process.
9. [Motive → Builder pane] During calibration, make sure both the mocap system and the HMD tracker (Oculus Tracker / Vive Base Stations) is tracking the HMD.
10. [Motive → Builder pane] Once it starts collecting the samples, point the HMD towards the tracker and slowly rotate it and sample different orientations.
11. [Motive → Builder pane] When it finishes collecting samples, it will display the offset distance, in mm, between positions of the pivot point detected by the HMD tracking system and the motion capture system.
12. [Motive → Builder pane] If the results are satisfying, press Apply to calibrate the HMD rigid body.

Rigid Body Properties

Once the rigid body is created, select the asset and open the Properties pane. The following settings need to be configured for HMD tracking.

Rigid body properties for the HMD asset.

Deflection

Deflection setting is the tolerable distance, in millimeters, that a rigid body marker may deviate from its expected position before it is unlabeled and unassociated with the rigid body. The deflection is set to 4 mm by default. For HMD tracking applications, we recommended lowering this value to 3 or 2. This will reduce the amount of computation required for labeling, and overall latency may be reduced.

Tracking Algorithm

The tracking algorithm setting determines which protocol to use for solving the rigid body in Motive.

Passive Markers

Use the ray-based tracking algorithm for HMDs with passive markers. This algorithm is recommended because it allows tracking to be less susceptible to marker occlusions and solves rigid bodies more robustly without introducing trajectory gaps. Using ray-based solving will increase processing load, but since it can stabilize the tracking a lot, this is more suitable for tracking the passive marker HMDs where even minor occlusions can acutely affect the experience.

Active Markers

Use the marker-based tracking algorithm for HMDs using active marker clips. Ray-based tracking is not supported for active markers since active markers themselves already provide stable tracking from active labeling using unique illumination patterns. Thus, marker-based tracking will provide robust tracking for HMDs with active markers.

Streaming ID

The User Data value for each rigid body is used as an identification number to reference the rigid body in external applications. Log this value for each rigid body (including the HMD(s)) in the scene. This number will be used to associate the assets in the client applications using the plugin devices.

Min Marker Count (Passive Markers only)

This setting may need to be modified when tracking HMDs with passive markers. To prevent the swapping of the rigid body definition, set this setting to 4. When tracking multiple HMDs, there could be limitations to a variety of unique marker arrangements that could be achieved. If this value is set to a lower value, a set of three markers on an HMD may be congruent to another set in a different HMD, and the rigid body definitions may be switched in Motive.

Recalibrating Pivot Point

After the rigid body definition have been created for the HMD, the position and orientation of the rigid body pivot point should get placed precisely on the root of the player's nose with the respective orientation axis pointing the forward direction. If needed, you can use the Builder pane edit tools to recalibrate the HMD pivot point positions.

• The pivot point is placed on the bridge of the nose; specifically, right at the midpoint between two eyes.
• Orientation axes of the rigid body should precisely align with the physical HMD component.

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  Uncalibrated HMD rigid body definition. The pivot point is not positioned nor oriented properly.
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  Calibrated HMD rigid body definition. x-axis forward for UE4 integration.

Manually Calibrating the HMD Pivot Point

In most cases, you will be utilizing the previously described calibration tool for precisely positioning and orienting the HMD rigid body. However, if, for some reason, you want to manually position the pivot point without using the HMD Calibration feature, follow the instruction on this page: Manually Calibrating the HMD Pivot Point.

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  Trackables pane HMD calibration tab.
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  HMD calibration results

Export the Rigid Body

Tip: Once you have the rigid body asset for the HMD configured, you can export the asset into a TRA file for future uses. Importing the TRA file (e.g. CV1.tra) will load the rigid body (HMD) asset and make it available for use; however, the marker placement must remain unchanged in order to re-load previously created rigid bodies.

Live Streaming

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Once you have connected the client object and configured the script components on the assets from the scene. Play the scene, and if the properties are correctly configured, the assets in the game scene will be animated according to the position and orientation of the rigid bodies and the HMD in Motive. When playing the scene, the HMD and the assigned actor objects will be placed in respect to their location in Motive’s coordinate system disregards to their position in the editor.

Wireless Multiplayer Setup

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When setting up multiplayer games on wireless clients, it is more beneficial for each client to make direct connection to both the tracking-server (Motive) and the game-server, rather than rebroadcasting the streamed tracking data through the game-server. Then, any of the game related actions that interacts with the tracking data can be processed on the game-server, and this server can send out the corresponding updates to the wireless clients. This allows the wireless clients to only receive both the tracking data or updates without having to send back any information; in other words, minimizing the number of data transfers needed. If wireless clients are sending data there will be a minimum of two transfers on the wireless network, and each transfer of data through wireless network is at risk of latency or lost packets.

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