Setting the volume origin (global coordinate system) tells the Vicon system where the center of your capture volume is and what its orientation is (x, y, and z axes), so that subjects are displayed the right way up in the Nexus view pane and so that you can change the way data is visualized. You normally set the volume origin immediately after the previous step, Calibrate Vicon cameras.
To set the volume origin, you need a calibration device, normally an Active Wand, which is supplied with your Vicon system.
To set the global coordinate system:
- Remove from the capture volume all markers and the sources of any unwanted reflections that have not been accounted for by camera masks previously created in a Camera view (see Mask unwanted reflections).
- In the Resources pane, ensure Nexus is in Live mode.
- Display a 3D Perspective view.
In the System Preparation tools pane, expand the Set Volume Origin section and from the L-Frame list, select the type of calibration device you are using to set the volume origin. (This is normally the calibration device that was supplied with your Vicon system.)
If it is impractical to place the wand in the required position, you can instead use the One Marker or Three Markers options to set the origin. (The Three Markers option creates two vectors that determine the orientation of the volume.) To use these options, position the marker(s) in the volume and then click Set next to the chosen option. You can then select the required marker(s) in the 3D Perspective view. If you selected Three Markers, you are prompted to select an origin marker, a primary (X-axis) marker, and secondary (Z-axis) marker.
- In the capture volume, place the calibration object flat on the floor in the position and orientation that you would like to be the origin of the global coordinates system (often a force plate corner or another clearly marked area of the volume).
- In the Set Volume Origin section, click Start.
The calibration object tracking process starts, Nexus identifies the calibration object in the capture volume, displays a 3D representation of it in the 3D Perspective view, and changes the Start button to Set Origin.
- Click Set Origin to complete the calibration object tracking process. Nexus sets the global origin and axes to correspond to the position and orientation of the calibration object in the capture volume. In the 3D Perspective view, the floor grid is aligned with the capture volume floor and the cameras are distributed in the position and orientation in which the physical cameras are located. When the global coordinate system has been successfully set, it is automatically saved to an .xcp file. The latest calibration (.xcp) file is saved to the default location. This file is automatically copied to every subsequent trial captured.
Verify that the global coordinate system was successfully set by checking that the system tracks the calibration object.
If it does not, check that you selected the correct calibration object from the L-Frame drop-down list at the top of the Tools pane. If not, repeat this procedure from step 4, ensuring that you select the correct entry for your calibration device. After you have set the global coordinate system, you can display the volume axes marker in the bottom corner of the 3D Perspective view.
- Turn the display of the volume axes on or off in the Options dialog box (F7) by selecting or deselecting Volume Axes under General View Options.
Improve the calibration volume reproducibility
After you have set the volume origin (see Set the volume origin), to achieve maximum positional reproducibility of the Vicon coordinate system, you can create a large custom L-Frame object from markers permanently placed around the edge of the volume. You can then use this L-Frame object for subsequent setting of the system origin.
This provides a high degree of precision of the coordinate system across the camera calibration, which is particularly useful in larger volumes.
To help with this, select the Auto Scale option when you set the origin, as described below.
This improves the consistency of the volume size, further improving the positional reproducibility.
The following procedure describes how to create a custom L-frame in Nexus. You can instead create an L-frame object in Tracker and export it for use in Nexus, but remember to first copy it from your Tracker CalibrationObjects folder:
to your Nexus CalibrationObjects folder:
To perform a camera calibration with rescale:
- With Nexus in Live mode, mask and calibrate the cameras (see Mask unwanted reflections and Calibrate Vicon cameras).
- Using an Active Wand, set the volume origin (see Set the volume origin).
- Place the markers that you want to use to create the L-Frame subject around the edges of the volume. Use at least four markers that span the whole volume.
- In the Capture Tools pane, click Start to capture a short (eg, 5-second) trial.
- With Nexus in Offline mode, on the Data Management tab at the bottom of the Nexus window, load the trial that you just captured.
- Run the Reconstruct pipeline (see Reconstruct and label movement trials).
- In the Subject Preparation Tools pane, go to the Create L-Frame Subject section and in the Create L-Frame field, enter a suitable name (eg,
My L-Frame), and click Create.
- In the view pane, select the markers that you want to use to create the L-Frame subject.
- When you have selected all markers, click Create again.
- In the Subjects Resources pane, right-click on the subject and then click Save Subject.
- Right-click again on the subject, click Export to L-Frame List and click Yes when prompted.
- With Nexus back in Live mode, in the System Preparation Tools pane, in the Set Volume Origin section, ensure Show Advanced is selected.
Click on the L-Frame menu to select the new L-frame and ensure Auto Scale is selected.
This ensures that the marker distances in the new L-frame are used for volume scaling.
The system scale is adjusted to provide improved positional reproducibility.