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With an optimized system setup, motion capture systems can be used to obtain extremely accurate tracking data in a small to medium sized capture volume. This quick start guide covers instructions for optimizing the system setup and precision verification methods, as well as some important cautions to keep in mind. For more general instructions, please refer to the Quick Start Guide: Getting Started or corresponding workflow pages. Before going into details on precision tracking with an OptiTrack system, let's start with a brief explanation on the residual value, which is the key reconstruction output for monitoring the tracking precision.
The residual value is a average offset distance between the converging rays when reconstructing a marker; hence indicating preciseness of the reconstruction. The tolerable residual distance is defined from the Reconstruction pane. When one or more markers are selected in the Live mode or from a recorded 2D data, the corresponding mean residual value is displayed in the status bar located at the bottom of Motive It can also be displayed on the 3D view port if Show Selected Residual is set to true in Application Settings. A smaller residual value means that the tracked rays converge more precisely and more accurate 3D reconstruction is achieved. A well-tracked marker will have a sub-millimeter average residual value.
First of all, optimize the capture volume for most precise and accurate tracking results:
The motion capture cameras detect reflected infrared light. Thus, having other reflective objects in the volume will alter the results negatively, which could be critical especially for precise tracking applications. If possible, have background objects that are IR black and non-reflective. Capturing in a dark background provides better contrast between bright and dark pixels, which could be less distinguishable in a white background. The following images show a clear image of a marker with good contrast (left) and a less clear marker (right) whose centroid calculation may have been compromised by an extraneous bright pixels from the background.
Proper camera placement techniques can greatly improve the tracking result and the measurement accuracy. The following guide highlights important setup instructions for the small volume tracking. For more details on general system setup, read through the Hardware Setup pages.
For precise tracking, better results will be obtained by placing the cameras closer to the target object (adjusting focus will be required) in a sphere or dome shaped camera arrangement as shown in the images to the right. Good positional data in all dimensions (X, Y, and Z axis) will only happen if there are cameras contributing to the calculation from a variety of different locations; each unique perspective adds additional data that wasn't there before.
For most accurate results, cameras should be securely fastened onto a truss system or an extremely rigid object. In sub-millimeter tracking applications, any slight deformation or fluctuation to the mount structures may affect the result. Small size truss system is ideal for the setup. Take extreme caution when using speed rails mounted onto a wall, because building may fluctuate greatly especially on hot days.
Increase the f-stop to a higher number (smaller aperture) to gain a larger depth of field. Increased depth of field will make greater portion of the capture volume in-focus and will make measurements more consistent throughout the volume.
Especiallly for precise and close-up captures, cameras aim and focus should be adjusted as perfectly as possible. Aim the cameras towards the center of the capture volume. Optimize the camera focus by zooming into a marker in Motive, and rotating the focus knob on the camera until the smallest marker is captured as clearly as possible with the best image contrast.
For more information on adjusting the camera aiming and focus, please read through the Aiming and Focusing workflow page.
The following sections cover key configuration settings which may need to be optimized for the micron volume tracking.
To open the cameras pane, click the camera icon at the main tool bar in Motive.
To open the cameras pane click View > Reconstruction Settings the top of Motive. Read through the Reconstruction page for specific details. For the micron volume tracking, important reconstruction settings and the appropriate values are listed below:
The following calibration instructions are specific to precision tracking. For more general information, refer to the Calibration page.
For calibrating small capture volumes for precision tracking, we recommend using a Micron Series wand, either the CWM-250 or CWM-125. These wands are made of invar alloy, very rigid and insensitive to temperature, and they are designed for providing a precise and constant reference dimension for the calibration process. At the bottom of the wand head, there is a label tag which shows a factory-calibrated wand length with sub-millimeter accuracy. For best results, input this information into Motive. In the Calibration Pane, select Micron Series under the OptiWand dropdown menu, and define the exact length in the wand length section.
The CW-500 wand is designed for medium to large capture volumes, and it is not suited for calibrating the micron volume. It does not have a sticker stating the exact dimension. Also, the wand is made with aluminum, which makes it more vulnerable to thermal expansions. During the wanding process, Motive references the defined wand length for calibrating the capture volume. Any distortions in the wand length would cause the calibrated capture volume to be scaled slightly differently, which can be significant when capturing precise measurements. For this reason, a micron series wand should be used for precision tracking applications.
Note: Never touch the marker on the CWM-250 or CWM-125 since any changes can affect the calibration and overall data.
Precision Capture Calibration Tips
Calibration reports and analyzing the reported error is a complicated subject because the calibration can only use its own samples for validation. For example, sampling near the edge of the volume may improve the accuracy of the system, but cause Motive to report slightly worse calibration results. This is because the samples near the edge will have more errors to be corrected. Acceptable mean error varies based on your volume size, number of cameras, and desired accuracy. The key metrics to keep an eye on are the Mean 3D Error for the Overall Reprojection and the Wand Error. For several of our tests we only used calibrations with the Mean 3D Error less than 0.80 mm and the Wand Error less than 0.030 mm. These numbers may be hard to reproduce in regular volumes. Again, the acceptable numbers are subjective, but lower numbers are better in general.
Note: Calibrations may fail because there are not enough unique samples, such as only wanding in a plane or never rotating your wand. This happens more commonly when wanding small volumes because it is hard to reach everywhere.
Retroreflective markers are often placed closer together than usual during close-up captures. When markers are placed too close to each other, their reflections may merge in the camera’s imager. Merged reflections will have an inaccurate centroid locations, or they may even be completely discarded by the circularity filter. There are editing methods to discard or modify the missing data. However, for most reliable results, such marker intrusions should be prevented before the capture by separating the marker placements or by optimizing the camera placements.
In a mocap system, camera mount structures and other hardware components may be affected by temperature fluctuations. Refer to linear thermal expansion coefficient tables to find out what materials are susceptible to temperature changes. For example, aluminum has relatively high thermal expansion coefficient, and therefore, you have to be cautious when mounting cameras onto aluminum mounting structures. For best accuracy, routinely recalibrate the capture volume, and take the temperature fluctuation into an account both when selecting the mount structures and before collecting data.
An ideal way to avoid the ambient temperature influence is to install in a temperature controlled volume. If such option is not available, there are few considerations to keep in mind. First of all, routinely calibrate the volume before capture, and recalibrate the volume in between sessions when capturing for a long period of time. Place the truss or tripods on a rigid surface such as concrete; carpets or rugs should be avoided. Wall mounts can also be affected by temperature changes because many buildings fluctuate slightly with changing outside temperature. The changes are especially noticeable on hot days and will significantly affect your results. If these situations are unavoidable, consistently monitor the average residual value for how well your rays converge to individual markers.
The cameras will heat up with extended use, and change in internal hardware temperature may also affect the capture data. For this reason, avoid capturing and/or calibrating right after powering the system. Tests have found that the cameras need to be warmed up in Live mode for about an hour until it reaches a stable temperature. For Ethernet camera models, camera temperatures can be monitored from the Camera Preview (2D) pane in Motive (Camera Preview (2D) pane > Eye Icon > Camera Info).
Avoid any vibrations or movements of the setup while taking precision measurements. Especially for measuring at sub-millimeters, even a minimal shift can affect the recordings. Re-calibrate the capture volume if your average residual values start to deviate. In particular, watch out for the following:
The following methods can be used to check the tracking accuracy and to better optimize the reconstructions settings in Motive.
First, go into the perspective pane > select a marker, then go to the Camera Preview Pane > Eye Button > Marker Centroids = True. Make sure your cameras are in object mode, then zoom into the selected marker. The marker will have two crosshairs on it; one white and one yellow. The amount of offset between the crosshairs will give you an idea of how closely the view of that camera (white) aligns with the reconstructed position (yellow). Switching between the grayscale mode and the object mode will make the errors more distinguishable. The below image is an example of a poor calibration. A good calibration should have the yellow and white lines closely aligning over each other.
The calibration quality can also be analyzed through checking the convergence of the tracked rays into a marker. This is not as precise as the first method, but the tracked rays can be used to check calibration quality of multiple cameras at once. First of all, make sure tracked rays are visible; Perspective View pane > Eye button > Tracked Rays. Then, open the perspective view pane and select a marker. Zoom all the way into the marker (you may need to zoom into the sphere), and you will be able to see the tracking rays (green) converging into the center of the marker. A good calibration should have all the rays converging into approximately one point, as shown in the following image.