Back to the Main Page → Back to Motive Documentation → Back to Capture Setup
Like many other measurement systems, calibration is essential for optical motion capture systems as well. During camera calibration, the system computes position and orientation of each camera as well as amounts of distortions in captured images. Using calibration data, Motive constructs a 3D capture volume. Specifically, this is done by observing 2D images from multiple synchronized cameras and associating the position of known calibration markers from each camera through triangulation. Note that if there is any change in a camera setup over the course of capture, whole system will need to be recalibrated in order to accommodate for changes. Moreover, even if setups are not altered, calibration accuracy will naturally deteriorate over time due to ambient factors, such as fluctuation in temperature and other environmental conditions. Thus for accurate results, a system needs to be calibrated periodically for accuracy of captured data.
See also: Calibration Pane, Reconstruction
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 , or by using the Ctrl+1 hotkey.
System settings used for calibration should be kept unchanged. If camera settings are altered after the calibration, the system would potentially need to be recalibrated. To avoid such inconveniences, it is important to optimize both hardware and software setup before the calibration. First, cameras need to be appropriately placed and configured to fully cover the capture volume. Secondly, each camera must be mounted securely so that they remain stationary during capture. Lastly, Motive's camera settings used for calibration should ideally 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, gain settings, and Filter Switcher settings. If these settings are modified, it is recommended the system be recalibrated. ↑
All extraneous reflections or unnecessary markers are ideally removed from the capture volume before calibration. In fact, the system will refuse to calibrate if there are too many reflections other than the calibration wand present in the camera views. However, in certain situations, unwanted reflections or ambient interference could not be removed from the setup. In this case, these irrelevant reflections can be ignored via using the Masking Tool. This tool applies red masks over the extraneous reflections seen from the 2D camera view, and each pixel in the masked regions is entirely filtered out. This is very useful when blocking unwanted reflections that could not be removed from the setup. Use the masking tool to remove any extraneous reflections before proceeding to wanding.
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. Therefore, all removable reflective objects must be taken out or covered before the using the masking tool. After all reflections are removed or masked from the view, proceed onto the wanding process. ↑
The wanding process is the core pipeline that samples 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 in order to compute their respective position and orientation in the 3D space. There are a number of calibration wands suited for different capture applications.
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, 2,000 - 10,000 samples are enough. ↑
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.
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 lights 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. A single 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.
After sufficient marker samples have been collected, press Calculate to calibrate 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. The Calibration Result is directly related to the mean error, and will update, 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 Apply to use the result. If not, press cancel and repeat the wanding process. It is recommended to save your calibration file, for later use.
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. If calibration results are acceptable, proceed to setting the ground plane. ↑
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.
Note: The coordinate system convention of the calibration square has been updated since Motive 1.7, please refer to Calibration Squares page for changes.
After confirming that the calibration square is properly placed, open the Ground Plane tab from the Calibration Pane. Select the three calibration square markers in the 3D Perspective View. When the markers are selected, press Set Ground Plane to reorient the global coordinate axis 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 vertical offset value corresponds to the actual distance between the center of the marker and the lowest tip at the vertex of the calibration square. Motive will recognize the calibration square and set the default offset value for the detected 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, 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.↑
Camera calibration refinement feature is used in order to adjust the calibration for only selected cameras in a system. Especially for a high camera count setup, it could be overwhelming to calibrate the whole system again each time a single camera position is distorted. In this case, the existing calibration can be refined in Motive to accommodate the minor changes in a setup. It allow the wanding process to be simplified for only selected cameras within the system. To use this feature, existing calibration on other cameras must be reliable.
Since this feature refers to other cameras to calibrate the selected cameras, and it is important that the referenced cameras have reliable calibration qualities. When wanding, few of the other cameras (depending on the minimum rays setting) also need to capture the wand along with the selected cameras.
Be aware that the calibration refinement is used to modify the calibration only for minor changes. For long-term maintenance of the calibration quality, the entire capture volume needs to be routinely calibrated.
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. The Calibration File can also be stored within the project so that it can be loaded whenever a project is accessed. By default, Motive loads the last calibration file that was created, this can be changed via the Application Settings.
Note that whenever there is a change to the system setup, these calibration files will no longer be relevant and the system will need to be recalibrated. ↑
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. ↑
Back: Capture Setup
Next: Marker Setup
