During Calibration process, a calibration square is used to define global coordinate axes as well as the ground plane for the capture volume. Each calibration square has different vertical offset value. When defining the ground plane, Motive will recognize the square and ask user whether to change the value to the matching offset.
| Square Type | Descriptions |
|---|---|
When creating a custom ground plane, you can use Motive to help you move the markers to create approximately 90 degree between the 3 markers. This is of course contingent on how good your calibration is, however, this will still give you a fairly accurate starting point when setting your ground plane.
For Motive 1.7 or higher, Right-Handed Coordinate System is used as the standard, across internal and exported formats and data streams. As a result, Motive 1.7 now interprets the L-Frame differently than previous releases:
CS-200:
Long arm: Positive z
Short arm: Positive x
Vertical offset: 19 mm
Marker size: 14 mm (diameter)
CS-400: Used for general for common mocap applications. Contains knobs for adjusting the balance as well as slots for aligning with a force plate.
Long arm: Positive z
Short arm: Positive x
Vertical offset: 45 mm
Marker size: 19 mm (diameter)
Legacy L-frame square: Legacy calibration square designed before changing to the Right-hand coordinate system.
Long arm: Positive z
Short arm: Negative x
Custom Calibration square: Position three markers in your volume in the shape of a typical calibration square (creating a ~90 degree angle with one arm longer than the other). Then select the markers to set the ground plane.
Long arm: Positive z
Short arm: Negative x
CS-100: Used to define a ground plane in a small, precise motion capture volumes.
Long arm: Positive z
Short arm: Positive x
Vertical offset: 11.5 mm
Marker size: 9.5 mm (diameter)
This page provides detailed instructions on camera system calibration and information about the Calibration pane.
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.
Prepare and optimize the capture volume for setting up a motion capture system.
Apply masks to ignore existing reflections in the camera view.
Collect calibration samples through the wanding process.
Review the wanding result and apply calibration.
Set the ground plane to complete the system calibration.
Cameras need to be appropriately placed and configured to fully cover the capture volume.
Each camera must be mounted securely so that they remain stationary during capture.
Motive's camera settings used for calibration should ideally remain unchanged throughout the capture. Re-calibration may be required if there is any significant modifications to the settings that influence the data acquisition, such as camera settings, gain settings, and Filter Switcher settings.
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.
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:
Applying masks to camera views only applies to calibration wands with passive markers. Active calibration wands are capable of calibrating the capture volume while the LEDs of all the cameras are turned off. If the capture has a large amount reflective material that cannot be moved, this method highly recommended.
Check the calibration pane to see if any of the cameras are seeing extraneous reflections or noise in their view.
Check the corresponding camera view to identify where the extraneous reflection is coming from, and if possible, remove them from the capture volume or cover them so that the cameras do not see them.
In the Calibration pane, click Mask to apply masks over all of the existing reflections in the view.
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:
At least two, or more, cameras must see all of the three calibration markers simultaneously.
Cameras should only see calibration markers. If any other reflection or noise is detected during the wanding process, the sample will not be collected and may affect the calibration result negatively. For this reason, person who is wanding should not be wearing anything reflective.
The markers on the calibration wand must be in good quality. If the marker surface is damaged or scuffed, the system may struggle to collect wanding samples.
There are different types of calibration wands suited for different capture applications.\
Calibration Wands
CW-500: The CW-500 calibration wand has a wand-width of 500mm when the markers are placed in the configuration A. This wand is suitable for calibrating a large size capture volume because the markers are spaced out further apart, allowing the cameras to easily capture individual markers even at long distances.
CW-500 Active:Hosting the same dimensions as the CW-500, the active version is recommended for capture volumes that have a large amount of reflective material that cannot be removed. This wand calibrates the volume while the LEDs of all mounted cameras are turned off.
CW-250: The CW-250 calibration wand has a wand-width of 250mm. This wand is suitable for calibrating small to medium size volumes. With narrower wand-width, it allows cameras, that are set up in a smaller volume, to be able to easily capture all three calibration markers within the same frame. CW-500 wand can also be used like CW-250 wand if the markers are positioned at configuration B.
CWM-125 / CWM-250: Both CWM-125 and CWM-250 wands are designed for calibrating the system for precision capture applications. The accuracy of the calibrated wand width is most precise and reliable on these wands, and they are most suitable for doing precision capture in a small volume capture applications.
Before starting the wanding process, if any of the cameras are detecting extraneous reflections, return to the masking steps and make sure they are either masked or removed.
Set the Calibration Type. If you are calibrating a new capture volume, choose Full Calibration.
Under the Wand settings, specify the wand that you will be using to calibrate the volume. It is very important to input the matching wand size here. When an incorrect dimension is given to Motive, the calibrated 3D volume will be scaled incorrectly.
Double check the calibration setting. Once confirmed, press Start Wanding to start collecting the wanding sample. Here, do not have any specific camera selected if you wish to perform calibration for the entire camera system.
Start wanding. Bring your calibration wand into the capture volume and start waving the wand gently across the entire capture volume. Gently draw figure-eight repetitively with the wand to collect samples at varying orientations and cover as much space as possible for sufficient sampling. Wanding trails will be shown in colors on the 2D view. A table displaying the status of the wanding process will show up in the Calibration pane to monitor the progress. For best results, wand the volume evenly and comprehensively throughout the volume, covering both low and high elevations. If you wish to start calibrating inside the volume, cover one of the markers and expose it wherever you wish to start wanding. When at least two cameras detect all the three markers while no other reflections are present in the volume, the wand will be recognized, and Motive will start collecting samples.
You'll want to wand until the camera squares in the Calibration pane turn from dark green (insufficient amount of samples) to light green (sufficient amount of samples). Once all the squares have turned light green the Start Calculating button will now be active.
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, 1,000-4,000 samples are enough. Samples above this threshold are unnecessary and can actually be detrimental to a calibration's accuracy.
Wanding Tips
Avoid waving the wand too fast. This may introduce bad samples.
Avoid wearing reflective clothing or accessories while wanding. This can introduce extraneous samples which can negatively affect the calibration result.
Try not to collect samples beyond 10,000. Extra samples could negatively affect the calibration.
Try to collect wanding samples covering different areas of each camera view. The status indicator on Prime cameras can be used to monitor the sample coverage on individual cameras.
Although it is beneficial to collect samples all over the volume, it is sometimes useful to collect more samples in the vicinity of the target regions where more tracking is needed. By doing so, calibration results will have a better accuracy in the specific region.
Marker Labeling Mode
When performing calibration wanding, please make sure the Marker Labeling Mode is set to the default Passive Markers Only setting. This setting can be found under Application Settings: Application Settings → Live-Reconstruction tab → Marker Labeling Mode. There are known problems with wanding in one of the active marker labeling modes. This applies for both passive marker calibration wands and IR LED wands.
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.
For more information on camera status indicators, please visit our wiki page here.
Calibration Type
You can selected different calibration types before wanding: Full and Refine
Full: Calibrate cameras from scratch, discarding any prior known position of the camera group or lens distortion information. A Full calibration will also take the longest time to run.
Refine: Adjusts slight changes on the calibration of the cameras based on prior calibrations. This will solve faster than a Full calibration. Only use this if your previous calibration closely reflects the placement of cameras. In other words, Refine calibration only works if you do not move the cameras significantly from when you last calibrated them. Only slight modifications can be allowed in camera position and orientation, which often occurs naturally from the environment such as mount expansion.
Refinement results will be poor if a full calibration has not been completed previously on the selected cameras.
After sufficient marker samples have been collected, press Start Calculating 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.