Main Page → Motive Documentation → Capture Setup → Calibration
Page Scope
Calibration is essential for high quality optical motion capture systems. During calibration, the system computes position and orientation of each camera and amounts of distortions in captured images, and they are used constructs a 3D capture volume in Motive. This is done by observing 2D images from multiple synchronized cameras and associating the position of known calibration markers from each camera through triangulation.
Please note that if there is any change in a camera setup over the course of capture, the system must be recalibrated to accommodate for changes. Moreover, even if setups are not altered, calibration accuracy may naturally deteriorate over time due to ambient factors, such as more or less light entering the capture volume as the day progresses and fluctuation in temperature. Thus, for accurate results, it is recommended to periodically calibrate the system.
By default, Motive will start up on the calibration layout containing necessary panes for the calibration process. This layout can also be accessed by clicking on a calibration layout from the top-right corner , or by using the Ctrl+1 hotkey.
The Calibration pane will guide you through the calibration process. This pane can be accessed by clicking on the icon on the toolbar or by entering the calibration layout from the top-right corner . For a new system calibration, click the New Calibration button and it will take you to the next step.
Before performing system calibration, all extraneous reflections or unnecessary markers should ideally be removed or covered so that they are not seen by the cameras. If this is not possible, extraneous reflections can be ignored by applying masks over them in Motive.
When the cameras detect reflections in their view, it will be indicated with a warning sign to alert which cameras are seeing reflections; for Prime series cameras, the indicator LED ring will also light up in white.
Masks can be applied by clicking Mask in the calibration pane, and it will apply red masks over all of the reflections detected in the 2D camera view. Once masked, the pixels in the masked regions will entirely be filtered out from the data. Please note that Masks get applied additively, so if there are already masks applied in the camera view, clear them out first before applying a new one.
Active Wanding:
Masking from the Cameras Viewport
You should be careful when using the masking features because masked pixels are completely filtered from the 2D data. In other words, the data in masked regions will not be collected for computing the 3D data, and excessive use of masking may result in data loss or frequent marker occlusions. For this reason, all removable reflective objects must be taken out or covered before the using the masking tool so the masking can be minimized. After all reflections are removed or masked from the view, proceed onto the wanding process.
The wanding process is the core pipeline for collecting calibration sample into Motive. A calibration wand is waved in front of the cameras repeatedly throughout the volume, allowing all cameras to see the calibration markers. Through this process, each camera captures sample data points in order to compute their respective position and orientation in the 3D space.
It is important to understand the requirements of good wanding samples. For a streamline process, the following requirements must be met:
There are different types of calibration wands suited for different capture applications.
Calibration Wands
After wanding throughout all areas of the volume, consult the each 2D view from the Camera Preview Pane to evaluate individual camera coverage. Each camera should be thoroughly covered with wand samples. If there are any large gaps, attempt to focus wanding on those to increase coverage. When sufficient amounts of calibration samples are collected by each camera, press Calculate in the Calibration Pane, and Motive will start calculating the calibration for the capture volume. Generally, 2,500 - 6,000 samples are enough. ↑
Wanding Tips
Marker Labeling Mode
For Prime series cameras, the LED indicator ring displays the status of the wanding process. As soon as the wanding is initiated, the LED ring will turn dark. When a camera is detecting all three markers on the calibration wand, a part of its LED ring will glow blue to indicate that the camera is collecting samples, and the clock-position of the blue light will indicate the wand position in the respective camera view. As calibration samples are collected by each camera, green lights will fill up around the ring to provide feedback on whether enough samples have been collected. Eventually, we want all of the cameras to be filled with a bright green light to make sure enough samples covering all areas of the camera view are collected. Also, starting from Motive 3.0, any cameras that do not have enough samples collected towards the end of the wanding process, the ring light will start glow in white.
After sufficient marker samples have been collected, press Continue to calibrate using collected samples. The time needed for the calculation varies depending on the number of cameras included in the setup as well as the number of collected samples. As Motive starts calculating, blue wanding paths will be displayed on the view panes, and Calibration pane will provide visual feedback on calibration result of each camera. If you click Show list, you can check amount of error on each camera also.
Tip: Calibration details for recorded Takes can also be reviewed. Select a Take in the Data pane, and related calibration results will be displayed under the Properties pane. This information is available only for Takes recorded in Motive 1.10 and above.
After the calculation, a calibration result will be reported in the Calibration pane. The result is directly related to the mean error and the calibration result tiers are (on order from worst to best): Poor, Fair, Good, Great, Excellent, and Exceptional. If the results are acceptable, press Continue to apply the calibration. If not, press cancel and repeat the wanding process. In general, if it reports anything below excellent, you might want to adjust camera settings, wanding techniques, and try again.
The final step of the calibration process is setting the ground plane and the origin. This is accomplished by placing the calibration square in your volume and telling Motive where the calibration square is. Place the calibration square inside the volume where you want the origin to be located and the ground plane to be leveled to. The position and orientation of the calibration square will be referenced for setting the coordinate system in Motive. Align the calibration square so that it references the desired axis orientation.
The longer leg on the calibration square will indicate the positive z axis, and shorter leg will indicate the direction of the positive x axis. Accordingly, the positive y axis will automatically be directed upward in a right-hand coordinate system. Next step is to use the level indicator on the calibration square to ensure the orientation is horizontal to the ground. If any adjustment is needed, rotate the nob beneath the markers to adjust the balance of the calibration square.
After confirming that the calibration square is properly placed and detected by the Calibration pane, press Set Ground Plane. You may need to manually select the markers on the ground plane if Motive fails to auto-detect the ground plane. If needed, the ground plane can be adjusted later.
Duo/Trio Tracking Bars: The global origin of the tracking bars can be adjusted from the Calibration pane also. Read through the Adjusting Global Origin for Tracking Bars page for more details on this.
Custom calibration square can also be used to define the ground plane. A set of three markers will be needed, and for accurate ground plane, these markers need to form a right-angle with one arm longer than the other, just like the shape of the calibration square. When using a custom calibration square, select Custom in the drop-down menu, manually input the correct vertical offset and select the markers before setting the ground plane.
The Vertical Offset is the offset distance between the center of markers on the calibration square and the actual ground. For custom calibration square, you will need to define this in order to take account of the offset distance and sets the global origin slightly below the markers. Accordingly, this value should correspond to the actual distance between the center of the marker and the lowest tip at the vertex of the calibration square. This setting can also be used when you want to place the ground plane at a specific elevation. A positive offset value will place the plane below the markers, and a negative value will place the plane above the markers.
Ground Plane Refinement feature is used to improve the leveling of the coordinate plane. To refine the ground plane, use the bottom page selector to access the refine page. Then, place several markers with a known radius on the ground, and adjust the vertical offset value to the corresponding radius. You can then select these markers in Motive and press Refine Ground Plane, and it will refine the leveling of the plane using the position data from each marker. This feature is especially useful when establishing a ground plane for a large volume, because the surface may not be perfectly uniform throughout the plane.
If you wish to adjust position and orientation of the global origin after the capture has been taken, you can apply the capture volume translation and rotation from the Calibration pane. For applying changes to recorded Takes, Anew set of 3D data must be reconstructed from the recorded 2D data after the modification has been applied, .
Calibration files can be used to preserve calibration results. The information from the calibration is exported or imported via the CAL file format. Calibration files reduce the effort of calibrating the system every time you open Motive. Calibration files will be automatically saved into the default folders after each calibration but in general, it is suggested to export calibration before each capture session. By default, Motive loads the last calibration file that was created, this can be changed via the Application Settings.
Note: Whenever there is a change to the system setup (e.g. cameras moved) these calibration files will no longer be relevant and the system will need to be recalibrated.
The continuous calibration feature continuously monitors and refines the camera calibration to its best quality. When enabled, minor distortions to the camera system setup can be adjusted automatically without wanding the volume again. In other words, you can calibrate a camera system once and you will no longer have to worry about external distortions such as vibrations, thermal expansion on camera mounts, or small displacements on the cameras. For detailed information, read through the Continuous Calibration page.
Continuous calibration can be enabled, or disabled, from the Calibration Pane once a system has been calibrated. It will also show when the continue calibration has updated last time.
When capturing throughout a whole day, temperature fluctuations may degrade calibration quality and you will want to recalibrate the capture volume at different times of the day. However, repeating entire calibration process could be tedious and time-consuming especially with a high camera count setup. In this case, instead of repeating the entire calibration process, you can just record Takes with the wand waves and the calibration square, and use the take to re-calibrate the volume in the post-processing. This offline calibration can save calibration calculation time on the capture day because you can process the recorded wanding take in the post-processing instead. Also, the users can inspect the collected capture data and decide to re-calibrate the recorded Take only when any signs of degraded calibration quality is seen from the captures.
Offline Calibration Steps
1) Capture wanding/ground plane takes. At different times of the day, record wanding Takes that closely resembles the calibration wanding process. Also record corresponding ground plane Takes with calibration square set in the volume for defining the ground plane.
Whenever a system is calibrated, a Calibration Wanding file gets saved and it could be used to reproduce the calibration file through the offline calibration process
2) Load the recorded Wanding Take. If you wish to re-calibrate the cameras for captured Takes during playback, load the wanding take that was recorded around the same time.
3) Motive: Calibration pane. In the Edit mode, press Start Wanding. The wanding samples from recorded 2D data will be loaded.
4) Motive: Calibration pane. Press Calculate, and wait until the calculation process is complete.
5) Motive: Calibration pane. Apply Result and export the calibration file. File tab → Export Camera Calibration.
6) Load the recorded Ground Plane Take.
7) Open the saved calibration file. With the Ground Plane Take loaded in Motive, open the exported calibration file, and the saved camera calibration will be applied to the ground plane take.
8) Motive: Perspective View. From 2D data of the Ground Plane Take, select the calibration square markers.
9) Motive: Calibration pane: Ground Plane. Set the Ground plane.
10) Motive: Perspective View. Switch back to the Live mode. The recorded Take is now re-calibrated.
The partial calibration feature allows you to update the calibration for some selection of cameras in a system. The way this feature works is by updating the position of the selected cameras relative to the already calibrated cameras. This means that you only need to wand in front of the selected cameras as long as there is at least one unselected camera that can also see the wand samples.
This feature is especially helpful for high camera count systems where you only need to adjust a few cameras instead of re-calibrating the whole system. One common way to get into this situation is by bumping into a single camera. Partial calibrations allow you to quickly re-calibrate the single bumped camera that is now out of place. This feature is also useful for those who need to do a calibration without changing the location of the ground plane. The reason the ground plane does not need to be reset is because as long as there is at least one unselected camera Motive can use that camera to retain the position of the ground plane relative to the cameras.
Partial Calibration Steps
Notes:
Cameras can be modified using the gizmo tool if the Settings Window > General > Calibration > "Editable in 3D View" property is enabled. Without this property turned on the gizmo tool will not activate when a camera is selected to avoid accidentally changing a calibration. The process for using the gizmo tool to fix a misaligned camera is as follows:
The OptiTrack motion capture system is designed to track retro-reflective markers. However, active LED markers can also be tracked with appropriate customization. If you wish to use Active LED markers for capture, the system will ideally need to be calibrated using an active LED wand. Please contact us for more details regarding Active LED tracking.
› Q : It seems like the tracking measurements are scaled incorrectly. For example, I am seeing a 2m distance between two markers where the actual distance is only 1m.
A: Tracking measurement scaling incorrectly is likely because the capture volume was calibrated with a mismatching calibration wand. In such cases, make sure the correct wand dimension is selected under the OptiWand setting.
› Q : The calibration wand does not get detected during the wanding process.
A: This could happen for two main reasons. It could be happening either because the markers on the calibration wand have been displaced from the intended positions or because the marker surface has been scratched or damaged. In such cases, Motive will not be able to detect the calibration wand, and for this reason, always try to keep the calibration wand markers untouched and undistorted.
If markers on the calibration wand have been damaged, please contact us to have them replaced.
› Q : Calibration result returns bad results
A: * Make sure you are not wanding too fast. If the wand moves too fast, the centroid of the captured markers would not be good enough for calibration.
› Q : Is it possible to reconstruct a new set of 3D data using a different camera calibration file?
A: Yes, this is possible but note that this will be reasonable only when the camera setup has remained unchanged. You can do this by the following steps:
› Q : Which calibration wand is a good size?
A: In general, smaller calibration wands are suitable for calibrating smaller volumes, and larger calibration wands are suitable for calibrating larger volumes.
Back: Capture Setup
Next: Marker Setup
