Calibration

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Back to OptiWiki --->  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 in the 3D space as well as the amount of distortion in the image. Using the information, Motive constructs a 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 a change in the camera setup over the course of capture, the whole system will need to be recalibrated in order to accommodate for the change. Moreover, even if the system setup is not altered, the accuracy of a system will naturally deteriorate over time due to ambient factors, such as fluctuation in 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 Camera Settings  ->  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

System settings used for calibration need should be kept unchanged for capture as well. If camera settings are altered after the calibration, the system would potentially need to be recalibrated as a result. 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. Each camera must be mounted securely so that they do not move during capture. 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. Therefore confirming software settings and hardware setup is preferred before calibration.

Masking

masking tools from left to right: Auto mask, remove masks, rectangular masks,circular masks, and drawing tool

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 interferences could not be removed from the setup. In this case, these irrelevant reflections can be filtered out via using the Masking Tool. 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. The Mask Visible feature automatically detects all of the existing reflections present in the 2D view and masks over them. Furthermore, masks can also be created manually by selecting pixels, rectangular regions, or circular regions in the image via the Draw Mask features. Use the masking tool to remove any extraneous reflections before proceeding to wanding.

However, you should be careful when using the masking features because it totally ignores all the pixels from 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 data loss or frequent marker occlusions. Therefore, all extraneous illuminations must be removed or covered before the using the masking tool. After all reflections are removed or masked from the view, proceed onto the wanding process.


Extraneous reflections Masking applied Tracked markers in the view Markers occluded by masking
Calib masking.png Calib masking2.png Calib masking3.png Calib masking4.png


Wanding

The wanding process is the core pipeline that samples 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. There are a number of calibration wands suited for different capture applications. 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 the setup, press Start Wanding in the Calibration pane to initiate the wanding process. Bring your calibration wand into the capture volume and start waving the wand slowly across the entire capture volume. If you wish to start calibrating inside the volume, cover one of the markers and expose it wherever you wish to start calibrating. Motive will recognize the calibration marker and begin sampling. When at least two cameras detect all three markers on the wand while no other reflections are present in the volume, the wand will be recognized and it will be shown in colors on the 2D view. The cameras will capture the sample data required for calibration. Cover as much spaces as possible for sufficient sampling. A table displaying the status of the wanding process will show up in the Calibration pane, and this table 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 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.

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 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.

Prime Calib.jpg Prime CalibWand.jpg



Calibration Results

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.

Calibration Result Report

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.


Calib Result.pngCalib Calc.png


Overall Reprojection Displays the overall resulting reprojection error values from the calibration.
Worst Camera Displays the highest reprojection error value from the calibration.
Triangulation Recommended maximum residual setting for point cloud reconstruction.
Overall Wand Error Displays a mean error value of the detected wand length throughout the wanding process.
Ray Length Displays a suggested maximum distance, or a ray length, from a camera.


Calibration Summary

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 acceptable.


Calib PreGround.png Calib CameraSum.png


Overall Result Grades the quality of the calibration result.
Maximum Error (px) Displays the maximum reprojection error from the calibration.
Minimum Error (px) Displays the minimum reprojection error from the calibration.
Average Error (px) Displays the average reprojection error from the calibration.
Wand Error (mm) Displays a mean error value of the detected wand length throughout the wanding process.
Calculation Time Displays the total calculation time.
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Ground Plane and Origin

The final step of the calibration process is setting the ground plane and the origin. This step is performed 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:
Calib verticaloff.png

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

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.


Calibration Files

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.

Active LED Calibration

The OptiTrack motion capture system is designed to track retro-reflective markers. However, 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.