Setting the volume origin

Setting the volume origin 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 Tracker workspace and so that you can change the way data is visualized in the workspace. You set the global coordinate system immediately after you calibrate your Vicon cameras.

To set the global coordinate system:

1. Display a 3D Perspective view.

2. On the Calibrate tab, in the Wand drop-down list, ensure that the calibration object that you used when you calibrated the cameras is selected.

   Tip: Tracker determines the unit of length for calculating the volume based on the length of the calibration wand. If you specify a wand that is a different length from the one you used during the Vicon camera calibration, the volume will have the wrong unit of length, so Tracker will be unable to locate the L-frame calibration object.

3. In the Set Volume Origin section, from the L-Frame drop-down list, select the type of static calibration object you are using to set the volume origin.
4. 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 coordinate system.

5. In the Set Volume Origin section, click Start.

   The calibration object tracking process starts, Tracker identifies the calibration object in the capture volume, displays a 3D representation of it in the 3D Perspective view, and switches the Start button to its Set Origin setting.

6. Click Set Origin to complete the calibration object tracking process.

   Tracker 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 pane, the floor grid is displayed aligned with the capture volume floor and the representations of the cameras are distributed in the position and orientation in which the physical cameras are located around the capture volume. When the global coordinate system has been successfully set, it is automatically saved to an .xcp file.

7. Verify that the global coordinate system was set successfully by checking that the system tracks the static calibration object.

   If it does not, check the following:

   • Was the correct dynamic calibration object selected from the Wand drop-down list at the top of the tools pane?

     If not, repeat this procedure from Step 2, ensuring that you select the correct entry for the calibration wand you used for calibrating the Vicon cameras.

   • Was the correct static calibration object selected from the L-Frame drop-down list at the top of the Calibrate tab?

     If not, repeat this procedure from Step 3, ensuring that you select the correct entry for the L-frame you are using.

After you have set the global coordinate system, you can display the volume axis marker in the lower-left corner of the 3D Perspective view pane.

You turn the display on or off in the Options dialog box by selecting or deselecting Volume Axis under General View Options.

From Tracker 3.2 and later, you can also ensure that your coordinate system is accurately aligned with the floor of the capture volume by automatically detecting the floor plane, using markers in the volume to define it. For more information, see Refine the position of the floor plane.

From Tracker 3.7 and later, after you have set the volume origin (see Setting the volume origin), to improve calibration stability and consistency over time (particularly in large volumes), you can create a large custom calibration object from markers permanently placed across the volume. You can then use this object, coupled with using the Auto Scale option, for subsequent setting of the system origin. For more information, see Improve volume scaling and set a fixed origin.

Refine the position of the floor plane

To ensure that your coordinate system is accurately aligned with the floor of the capture volume, you can adjust the position of the floor plane, using markers in the volume to automatically define it.

To calibrate the floor plane:

1. Scatter a quantity of the same size Vicon markers across the floor of your capture volume. If you want Tracker to automatically recognize these markers as floor plane markers, ensure that they outnumber any other groups of markers (e.g, markers on a wand or markers that were used to focus cameras, etc).
2. Complete the usual system calibration procedure to set the volume origin.
3. On the Calibrate tab, expand Set Volume Origin and display the Advanced properties.

4. Next to Set Floor Plane, ensure that the options are as required:

   • Auto detect: Markers are automatically detected, based on the marker group that defines the plane with the most markers. If, when you click Start, the wrong markers are selected in the 3D Perspective view, you can add or remove markers in the volume until selection is as required. To ensure that the tolerance setting (in mm), which specifies the height of the group of markers that define the floor plane, is as required, click the drop-down arrow next to Auto Detect.
   • Offset: Set the amount (in mm) by which to adjust the floor plane (default is zero) in X Y and Z planes. The offset is applied after you have set the origin using the Set Floor Plane setting.

5. Click the Start button in the Set Floor Plane line.

   The button toggles to its Stop setting.

6. After a few seconds, click the Stop button.

   In the 3D Perspective view the cameras shift as a group slightly along one or more rotation axes to better reflect an average of the markers scattered across the floor.

   Your Vicon cameras are now calibrated and ready to capture data.