OptiTrack Unreal Engine 4 Plugin

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Plugin Overview

The OptiTrack Unreal Engine Plugin allows you to stream real-time tracking data from Motive into Unreal Engine 4 (UE4). Using this plugin, you can stream rigid body tracking data, skeleton tracking data, and/or HMD tracking data into a UE4 project. This article focuses on how to set up for those different streaming workflows. For basic instructions on how to set up a motion capture system, please refer to the Getting Started guide instead.

The OptiTrack Unreal Engine Plugin is divided into two parts:

• The OptiTrack - NatNet Streaming Client which allows you to stream rigid body and skeleton data from Motive onto Unreal Engine.
• The OptiTrack - OculusVR plugin which allows you to implement tracking of Oculus HMDs onto your scenes.

Once the plugins are enabled, they can be used to connect and receive streaming data from Motive by setting up an OptiTrack Client Origin object in the Unreal Engine level.

HMD Compatibility

• As of version 1.2.1, the OptiTrack - OculusVR plugin supports Oculus DK2, CV1, and Gear VR HMD models.
• Tracking of Vive HMDs can be integrated using the OptiTrack OpenVR Driver.

Version Requirements

This plugin is supported for the following versions of the 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
1.9	UE 4.19.2	Latest
1.20	UE 4.20.3	Latest
1.21	UE 4.21.2	Latest

Motive Data Streaming Setup (Server)

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First of all, follow the below instructions to set up the data streaming settings in Motive. Once this is configured, Motive will be broadcasting tracking data onto a designated network interface where client applications can receive them.

Streaming Settings

Open the Data Streaming pane in Motive and set the following settings:

• Enable the Broadcast Frame Data setting at the top
• From the Local Interface drop-down menu, select a network address that you wish to stream the data over to.
• Select desired data types to stream under streaming options:

- Streaming of rigid bodies is required.

- Streaming of skeleton is optional for skeleton tracking.

- For tracking HMDs, disabling the Marker streaming is adviced.

• Set the Skeleton Coordinates to Local.
• 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.
• For streaming over a wifi network, setting the Transmission Type to Unicast is strongly adviced.

Additional Tips For best results, it is advised to run Motive and Unreal Engine separately on different computers, so that they are not competing for processing resources. When streaming the data over a wifi network, Unicast transmission must be used. In order to stream data from the Edit mode, a capture-recording must be playing back in Motive. For additional information on data streaming in general, read through the Data Streaming page.

Unreal Engine Setup (Client)

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Next step is to configure the client plugin inside UE4. Please follow below instructions to install and configure the OptiTrack Unreal Engine plugin to receive the streamed tracking data.

Enable the Plugin

1. Download the plugin ZIP file: download page
2. Extract the contents from the ZIP file.
3. Open the extracted OptiTrack folder, transfer the OptiTrackNatNet and OptiTrackOculusRift folders into the Unreal Engine's plugin directory. By default, this directory is located in the C:\Program Files\Epic Games\4.##\Engine\Plugins folder and there will be other plugins in that folder already. Simply copy and paste the OptiTrack plugin folders into this directory.
4. Open/Create a UE4 project.
5. Under the Edit menu, click Plugin to open up the panel where all of the available plugins are listed.
6. Browse to OptiTrack section and enable the OptiTrack - NatNet Streaming Client.
7. (optional) For Oculus HMD users, browse to OptiTrack section and enable OptiTrack - OculusVR plugin for Oculus HMD tracking. After this is enabled, please make sure to disable the built-in Oculus Rift and Stream VR HMD plugins under virtual reality plugins category. Otherwise, the editor may crash.
8. Click Apply to submit the changes. It may require the UE4 project to be restarted.

UE4 plugins panel listing the enabled OptiTrack plugins.

Set up the Client Origin

Once the OptiTrack - NatNet Streaming Client plugin is enabled, OptiTrack Client Origin will be available in the Modes panel within UE4. This is the baseline node for making the connection to Motive.

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 class enables the Unreal Engine (client) to communicate with Motive where the rigid body, skeleton, and HMD tracking data are streamed from.

This can be added simply by drag-and-dropping the OptiTrack Client Origin from the Modes panel into the level in UE4. Once the client origin is placed within the level, its position and orientation 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 positioned 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. [Motive] First of all, make sure the Broadcast Frame Data entry is checked in the Data Streaming pane of Motive.
2. [UE4] Once the plugin is added and enabled in the project, OptiTrack Client Origin class will be available from the Modes panel.
3. [UE4] Drag and drop the OptiTrack Client Origin into the scene.
4. [UE4] Place the OptiTrack Client Origin to the desired tracking origin within the scene.
5. [UE4] Select the instantiated OptiTrackClientOrigin object from the World Outliner panel.
6. [UE4] In the Details panel, make sure its Auto Connect setting is checked. This configures the client origin to automatically search the network and connect to Motive.
7. Now that the client origin is set, the client origin will attempt to connect to Motive and start receiving the tracking data whenever 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.

Connecting to a designated IP address If you wish to connect to a server on a specific network address, you can uncheck the Auto Connect setting and enter the IP address of the client where the instance of UE is running and also the IP address of the server which was designated in the Data Streaming pane in Motive. You may need to run the ipconfig command in the command prompt to obtain an appropriate IP address of the client.

Advance settings: Auto-initialize By default, the auto-initialize feature is enabled and the client origin will get auto-initialized whenever the scene is played. But when needed, you can disable this and set up the project so the client origin gets initialized when a user-defined event is triggered. For example, if there is a need to provide the server IP address dynamically within a game, you can disable this and configure using blueprint how and when the client origin gets initialized. This is available in plugin version 1.8 and above.

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 in the project, OptiTrack Rigid Body component will be available to use. By attaching this component onto an actor, you can animate its child actors according to the movement of a rigid body in Motive. Each rigid body component is given a Trackable ID value, and tracking information of rigid body assets, in Motive, with the matching Streaming ID will be used to update the transform of the target actor in Unreal Engine.

You can also use the OptiTrack Rigid Body Actor prefab under the Modes panel. This is simply a blank actor with just the OptiTrack Rigid Body component attached to it, and you can use this to easily associate the objects within the scene:

Set Up 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. Please make sure the target actor's transformation is set to movable.
3. [UE4] Set the relative locations and rotations to all zeroes on this target actor. This actor should be listed as a child of the rigid body actor.
4. [Motive] In Motive, assign a value to Streaming ID property for the target rigid body.
5. [UE4] In the properties of the OptiTrack Rigid Body Actor component, match the Tracking ID with the Streaming ID of the rigid body asset in Motive.
6. Make sure both Motive and OptiTrack Client Origin is set up for streaming, 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.

RigidBodyComponent Properties

Tracking ID

ID of the rigid body used to derive the position and orientatation transform of the attached actor. This ID must match with the Streaming ID of the respective rigid body in Motive.

Hide on Invalid Definition

When this is checked, the corresponding rigid body actor will be hidden from the level until the associated rigid body data is streamed out from Motive and received by the plugin.

Disable Low Latency Update

Low latency update feature allows rigid body position and orientation transform to be updated immediately before rendering minimizing the latency. This is enabled by default. For debugging, you can check this setting to disable this behavior.

Tracking Origin

This sets a specific client origin to use for receiving tracking data. When this is unset, the plugin will default to the first client origin that it finds in the scene.

Respect Parent Transform

When this is set to true, the rigid body transform data from Motive will be applied in respect to the parent actor's pivot coordinates. By default, this is set to false, and all of the tracking data will be applied in respect to the pivot axis of the client origin.

Animating Skeleton

Characters in Unreal Engine can be animated using skeleton tracking data from Motive. There are two different approaches to this: skeleton bone mapping and skeleton retargetting.

Bone Mapping

For plugin versions above 1.9.2, the skeleton component can be used to allow streaming of the skeleton via bone mapping. This means that the motion of each skeleton bone in Motive can be projected to the bones of the character model. Please note that this is different from skeleton retargeting, the bone segments in Motive get mapped directly to the character model (bone mapping) instead of being translated into something that is usable by a more abstract biped model (retargeting). The instructions on setting up the skeleton component are documented under the following page:

Go to: Unreal Engine Skeleton Streaming

Skeleton Retargetting

Skeleton tracking data can also be integrated into Unreal Engine via a 3D retargeting process. This can be accomplished through a third party software, such as IKinema, that retargets captured 3D data onto a character 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 exist 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\

Drawing Markers

When needed, you can also draw streamed labeled marker data from Motive into the scene in UE. In most applications, you do not have to draw the markers into the scene as rigid body data and the skeleton data will be mainly used; however, getting markers generated in the scene may be helpful for debugging purposes. To enable drawing of the markers, expand the OptiTrackClientOrigin component properties, and set the Draw Markers to true, and also, the Labeled Markers in the Motive's data streaming pane must be set to true as well.

Labeled markers shown in UE.

Intergrating HMDs

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OptiTrack motion capture systems can be used to track head mounted displays (HMD) and integrate the tracking data into Unreal Engine for VR applications. For instructions on integrating HMD tracking data into Unreal Engine, please refer to the corresponding pages:

• Unreal Engine: Oculus HMD Tracking
• Unreal Engine: Vive HMD Tracking

Supported HMDs At the time of writing, the following HMDs are supported: HTC Vive HTC Vive Pro Oculus DK2 Oculus CV1 Oculus GearVR

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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