Difference between revisions of "Calibration"

(Calibration Results)
(Calibration Results)
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Revision as of 23:03, 23 September 2015


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(Overall workflow block diagrams)

Like in many other measurement systems, calibration is an essential step for the optical mocap system as well. During the calibration, the mocap system computes the position and orientation of each camera in the 3D space and constructs the capture volume in Motive. Specifically, this is done by observing 2D images from multiple synchronized cameras and associating the position of known calibration markers from each camera through a triangulation algorithm.

If there is a change in the setup, the whole system will need to be recalibrated in order to accommodate for the difference. Moreover, even if the system setup is not altered, the accuracy of a system will naturally deviate over time due to ambient factors, such as fluctuation in the temperature and other environmental conditions. Thus for accurate results, the system needs to be calibrated periodically for accuracy of the captured data.

Steps in Calibration
Optimizing the Setting  ->  Masking  ->  Wanding  -> Calculating -> Setting the Ground Plane

Tip: 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 Icon CalibLayout.png.

Preparing and Optimizing the Setup

The system settings for the calibration needs to be kept constant for the actual capture as well. If the setting is altered after the calibration, the system needs to be recalibrated in order to accommodate for the change, otherwise the data will be inaccurate. In order to avoid such inconveniences of changing the setup and calibrating multiple times, it is important to prepare both hardware and software setup before the calibration.

Camera must be mounted securely so that they do not move during the capture. Before calibration, cameras need to be appropriately placed to cover the capture volume. Also, they must be aimed and focused to optimize tracking. See also camera mounts and camera placements.

The software settings, in Motive, used for calibration should also remain unchanged throughout the capture. Specifically, a recalibration will be required if there is any significant modifications to the settings that influences the data acquisition, such as camera settings, reconstruction settings, gain settings, and Filter Switcher settings. If these settings are modified, it is recommended to calibrate the system again. Therefore, confirming software settings along with hardware settings is necessary before calibration.


For the calibration process, it is important to remove any extraneous reflections or markers from the volume. In fact, the system will refuse to calibrate if reflections other than the calibration wand are present in the camera view. However, in some cases, unwanted reflections or ambient interference could be inevitable due to the nature of the system setup. Conveniently, these irrelevant reflections can be filtered out via using Masking Tool.

Masking is the first step in calibration of the optical mocap system. The masking feature in Motive ignores pixels from selected, or masked, regions in the 2D camera view. This is very useful when blocking unwanted reflections that could not be removed from the setup. Mask Visible feature automatically detects all of the existing reflections present in the 2D view and places masks over them. Furthermore, masks can also be created manually by selecting pixels, rectangular regions, or circular regions in the image through the Draw Mask features. Use the masking tool to remove any extraneous reflection before proceeding to wanding.

(Masking Tool bar)

However, you should be careful when using the masking feature because it totally ignores selected regions from the 2D image of the camera. In other words, the data in masked regions will not be acquired by the system for computing the 3D data, and excessive use of it may result in inadequate data.

(Masking Screenshots)

After all of the extraneous reflections have been removed from the view, proceed on to the wanding process.


Wanding process is the core pipeline that transfers the calibration data into Motive. A calibration wand is waved in front of the cameras repeatedly, allowing all cameras to see the markers. Through this process, each camera captures sample frames of the markers in order to compute their respective position and orientation in the 3D space.

Calibration Wands

There are a number of calibration wands suited for different purposes of the capture, and you can also create a custom calibration wand to use with Motive. Select the corresponding wand type under the OptiWand section from the Calibration Options. After confirming all of the setup is prepared and ready to go, press Start Wanding in the Calibration pane to start the wanding process.


Bring your calibration wand out in the capture volume and start waving the wand slowly across the entire capture volume, covering as much space as possible for sufficient sampling. The cameras will start capturing the sample data. A chart displaying the status of the wanding process will show up in the Calibration pane, and this chart can be used to monitor the progress. For best results, wand the volume evenly and comprehensively throughout the space, covering both low and high elevations.

TIP: Although it is beneficial to collect samples all over the volume, it is sometimes useful to collect more samples around the target regions where more tracking is needed. By doing so, calibration results will have a better accuracy in the specific region.

Calib Wanding.png
Wanding process from the 2D camera view.

After wanding through all areas of the volume, consult the each 2D views 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, 2,000 - 10,000 samples are enough.

Prime Series Indicator ring for wanding

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, and then green light will fill up around the ring as the camera collects the sample data from the calibration wand. Eventually, the ring will be filled with green light when sufficient amount of samples are collected. The LED will glow blue if the calibration wand is detected by the camera, and the clock position of the blue light will indicate the respective wand location in the Camera Preview pane.
Prime Calib.jpg Prime CalibWand.jpg

Calibration Results

After sufficient marker samples have been collected, press Calculate to compute the calibration using collected samples.The time needed for the calibration calculation varies depending on the number of cameras included in the setup as well as the amount of collected samples. Immediately after clicking calculate, the samples window will turn into the solver window. It will display the solver stage at the top, followed by the overall result rating and the overall quality selection. The overall result rating is the lowest rating of any one camera in the volume. The overall quality selection shows the current solver quality.

After going through the calculation, a Calibration Result Report will pop up, and detailed information regarding the calibration will be displayed. Calibration result is directly related to the mean error, and will update, and there calibration results are diagnosed rated (on order from worst to best): Poor, Fair, Good, Great, Excellent, Exceptional. If the results are satisfying, press Apply to use the setting. If not, press cancel and repeat the wanding process. It is recommended to save your calibration file, for later use.

Calib Result.pngCalib Calc.png

After the calculation has completed, you will see cameras displayed in the 3D view pane of Motive. However, the constructed capture volume in Motive will not be aligned with the coordinate plane yet. This is because the ground plane is not set. Proceed to setting the ground plane if the calibration results are satisfying.

Calib PreGround.png Calib CameraSum.png

Ground Plane and Origin

The final step of the calibration process is setting the ground plane and the origin. This step is simply done by placing the calibration square in your volume and telling Motive where the calibration square is. Place the calibration square inside the volume at a preferred location where you want the origin to be placed and the ground plane to be leveled to. The position and orientation of the calibration square will be referenced to set 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.

(Note that the coordinate system convention of the calibration square has been updated since Motive 1.7, please refer to calibration square page for changes.)

Calib GroundOrigin.png

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, go to Motive and select the three markers from the 3D view. After selecting the markers, press Set Ground Plane to reorient the coordinate axis and the ground plane in respect to the calibration square. After setting the ground plane, Motive will ask to save the calibration data, CAL.

Vertical offset:

The Vertical Offset setting in the Calibration pane is used to compensate for the distance between the center of markers on the calibration square and the actual ground. Defining this value takes account of the offset distance and sets the global origin slightly below the markers.

The default value for Vertical Offset is 45 mm, which corresponds to the actual distance between the center of marker and the lowest tip at the vertex of the CS-400 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.

(Screenshot of vertical offset)

Ground Plane Refinement

Ground Plane Refinement feature is used to improve the leveling of the coordinate plane. To refine the ground plane, 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 markers. This feature is especially useful when establishing a ground plane for a large volume, because the surface may not be perfectly uniform.

Calibration Files

Calibration files can be used to preserve calibration results. The information from the calibration is exported or imported as the CAL file format, and it reduces the effort of calibrating the system every time you open up Motive. The calibration file can also be stored within the project so that it can be loaded whenever the same project is accessed. By default, Motive loads the last calibration file that was created, this can be turned off in the Application Settings.

Note that whenever there are changes to the setup, these calibration files will no longer be relevant and the system needs to be recalibrated.

Active LED Calibration

OptiTrack motion capture system is designed to track retroreflective markers. However, in case if you wish to used Active LED markers for the mocap, the system needs to be calibrated using an Active LED wands. Please contact us for more details with tracking with Active LED markers.