Quick Start Guide: Precision Capture

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Quick Start Guide: Precision Capture is under development. It will be available soon. Thank you for your patience!


This quick start guide details key instructions for capturing in a small to medium sized volume for extremely accurate tracking using an OptiTrack motion capture system. For more general information not included in this page, please refer to the general Quick Start Guide:Getting Started or corresponding workflow pages.


Residual Value

When tracking with sub-millimeter accuracy, the key value to monitor is average residual values for each marker. The residual value is a distance offset between the converging rays when reconstructing a marker; hence indicating preciseness of the reconstruction. The tolerable residual distance is defined from the Reconstruction pane. If you select one or more markers in the Live mode or from a recorded 2D data, then the mean residual value will be displayed in the status bar located at the bottom of Motive. With a smaller residual value, tracked rays converge more precisely and more accurate 3D reconstruction is achieved. A well-tracked marker will have a sub-millimeter average residual value.


Hardware Setup

Proper camera placement techniques can greatly improve tracking results. The following guide highlights important setup instructions for small volume tracking. For more details on the system hardware setup, read through the Hardware Setup pages.


Setup Environment

Avoid populated area when setting up the system and recording a capture. Clear any obstacles or trip hazards around the capture volume. Physical impart on the setup will distort the calibration quality which could be critical especially when tracking at sub-millimeter accuracy. For best results, routinely recalibrate the capture volume.


Mounting Locations

Place the cameras in a random pattern or circumnavigating arrangement around the target volume. For more precise tracking, it is beneficial to place the cameras closer, but avoid placing them right next to each other. Cameras in close vicinity will capture only similar vantages which will not contribute to the reconstruction.

For more information, read through the Camera Placements page.

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Captured 2D images from each camera are used to precisely locate the 2D positions from the perspective of the camera, but the data on the remaining dimension, in and out of the screen, can only be obtained through reconstructed 3D data from multiple cameras in the setup. Accordingly, for accurate results in all directions, cameras should be mounted so that each camera gets unqiue vantage of the target volume. A spherical arrangement will be more beneficial for tracking all three axis.


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Mount Securely

For most accurate results, cameras should be securely mounted onto a truss system or an extremely rigid object. For precision tracking applications, any slight deformation or fluctuation to the mount structures may affect the tracking quality. 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.

For more information on mounting structures, read through the Camera Mount Structures page.


Camera Focus Settings

F-stop

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.

Aim and Focus

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, please read through the Aiming and Focusing workflow page.


Infrared Black Background Objects

The motion capture cameras detect reflected infrared light. Thus, having other reflective objects in the volume will alter the results negatively. 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.

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Environmental Factors

Environmental factors can play a surprisingly large role when measuring at a sub-millimeter accuracy.

Temperature

In a mocap system, camera mount structures and other hardware setup 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 iron has relatively high thermal expansion coefficient, and therefore, you have to be aware that a temperature change might have changed the system setup.

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