Main Page → OptiTrack Unreal Engine 4 Plugin → Unity: Oculus HMD Tracking
This page provides instructions on how to use the OptiTrack Unreal Engine 4 Plugin for tracking of the Oculus head-mounted display (HMD) within Unreal Engine.
Oculus HMDs can be tracked by OptiTrack motion capture system and integrated into Unreal Engine 4 using the OptiTrack Unreal Engine 4 Plugin. More specifically, the OptiTrack - OculusVR component of the plugin is used to reflect HMD tracking data into the scene. Once this is set up, the motion capture system will override the tracking of Oculus Tracker to track the HMD from the attached markers for unique VR experiences. For tracking of objects other than the HMDs, please read through the OptiTrack Unreal Engine 4 Plugin page for details.
The following section provides instructions on setting up the HMD for tracking in VR applications. This includes placing markers on the HMD for tracking and creating a corresponding rigid body definition in Motive.
General Setup Steps
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. 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.
For best tracking results, using the HMD clip is strongly recommended.
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.
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.
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.
Oculus Rift Sensor
Creating Active Clip HMD
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.
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:
External Pivot Calibration Tips:
Once the rigid body is created, select the asset and open the Properties pane. The following settings need to be configured for HMD tracking.
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.
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.
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.
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.
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.
Tip: Once you have the rigid body asset for the HMD configured, you can export the asset into a MOTIVE file (*.motive) for future uses. Importing the MOTIVE 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.
The plugin also features HMD tracking integration for virtual reality experiences. For general instructions on developing VR application in Unity, refer to the Unity documentation: Unity Virtual Reality tutorial.
Oculus DK2, CV1, and Gear VR HMD models are supported
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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.). For more specific information on this setup, contact us. 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 Unity, refer to Unity documentation.