THIS PAGE IS UNDER CONSTRUCTION THIS PAGE IS UNDER CONSTRUCTION THIS PAGE IS UNDER CONSTRUCTION THIS PAGE IS UNDER CONSTRUCTION THIS PAGE IS UNDER CONSTRUCTION
Calibration is one of the core pipelines in the system setup process. The purpose of calibration is to adjust the measurement standard of a system, ensuring the reliability and accuracy of the setup. This process is accomplished by comparing the observed readings with the known standards or previously measured readings. This comparison is then used to establish a specific standard for the particular setup.
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 reference markers, calibration markers, from each camera through the triangulation algorithm.
If there is a change in the setup, the whole system needs 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, 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.
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 installation must be robust and stable in order to remain stationary throughout the capture. In advance to the calibration, cameras need to be appropriately placed to cover the capture volume, and they also need to be aimed and focused at their optimal setting in advance to calibration.
Moreover, 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 allows ignoring 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 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.
After all of the extraneous reflections have been removed from the view, proceed on to the wanding process.
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 it around. The cameras will then 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. Wave the wand across the volume, covering up both low and high elevations. It is beneficial to sample the data from everywhere in the volume. When a sufficient number of samples is collected by each camera, press Calculate in the Calibration pane, and Motive will start calculating the calibration for the capture volume.
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.
Prime Series: LED Indicator ring
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 2D view
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. After going through the calculation, a Calibration Result Report will pop up, and it will display detailed information about the calibration.
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.
The final step of the calibration process is setting the ground plane and the origin. This step is simply done by using the calibration square 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 negative x axis. Accordingly, the positive y axis will automatically be directed upward.
(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.)
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.
When 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.
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 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
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
This feature is especially useful when establishing a ground plane on a slanted surface (e.g. force plates on slope), because the level indicators won’t be reliable in those settings. It helps ensuring
that the coordinate plane is horizontal to the surface.
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 changed from Application Settings.
Note that whenever there is changes to the system setup, these calibration files will no longer be relevant and the system needs to be recalibrated. ↑
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.