Scale calibration and set a fixed origin

You can then use this calibration object (coupled with the Perform Rescale option) to set the system origin. This improves calibration stability and consistency over time. Calibration scaling may be particularly useful for large volumes if the standard Vicon wand does not provide sufficient accuracy when used as an L-Frame.

If it is impractical to use permanently fixed markers in your volume, you can still benefit from following this procedure. However, when recalibrating, swap a scaled wand object for the custom calibration object. While your results will not be as accurate as they would be with a large calibration object, you will still benefit from the scaling involved. For details, see Step 2 of Recalibrate with scaled wand & custom calibration object.

The following topics explain how to scale calibration and set a fixed origin.

Set up the custom calibration object

To initially set up the custom calibration object:

1. In the volume, place the permanently static active markers to create a custom calibration object. When placing the markers, note the following points:
   • For best performance, fix the markers to the floor (this provides both stability and the ability to spread across the volume). If this is not possible, position the markers on a wall or truss.
   • To guarantee a good measurement, place the custom calibration object in an area of good camera coverage.
   • For best scaling performance, position the custom calibration object across the central two-thirds of the volume (to avoid variability introduced by gaps in coverage at the volume edges). If this is unfeasible, try to make the custom calibration object as large as possible, and position it towards the center of the volume.
2. As accurately as possible, measure the distances between the points at the two furthest edges of the calibration object and record the results. This can be a single measurement (for example, if the object's sides are equal, or take two measurements across two different axes if the volume is not square).

Scale the Active Wand and the custom calibration object

Each time you change the custom calibration object:

1. Calibrate using the normal process and standard .vsk files and set the origin using a standard calibration object (see Calibrate cameras)

2. In Shogun Live, create a prop for the markers of the custom calibration object (for more information on how to create a prop, see Create props).

   Do not save this as a new prop: The new object to use for this scaling must be created from the current marker observations.

3. In the Tracking panel, right-click on the custom calibration object and click Use for Camera Calibration.

   

4. In the Calibration Scaling dialog box, complete the following steps:

   Note: You must track this calibration object for the process to work.

   1. In the Current Settings section, check that the Current Wand is the correct wand for your calibration and that the Selected Prop is also correct.

      

   2. Ensure the selected prop is enabled, then, in the 3D View or Subject view, select a pair of markers that correspond to a measurement that you took earlier, and click Add Measurement.

      Tip: Hold the shift key while selecting the markers for which you want to provide a measured distance.

      

   3. A line is added to the table containing the Model Distance, which is the distance between the markers as measured by the current Vicon calibration. In the Measured Distance column, enter the measured distance in millimeters.

      

   4. For each set of measurements you have taken for the object, repeat steps b and c.

      

   5. In the New Settings section, the scale factor that will be applied is displayed, together with the output names for the scaled wand and L-Frame.

   6. If you are happy with this measurement, click Save to exit the dialog box.

      

   7. A scaled wand and an L-frame file are saved by default to C:\Users\Public\Documents\Vicon\CalibrationObjects and are displayed in the Wand Wave and Set Volume Origin lists in the Camera Calibration panel.

      

      The current calibration is also scaled by the same factor, so you do not need to recalibrate after this operation.

5. In future, to obtain an accurately scaled volume, use the new scaled wand and calibration object when performing the calibration and set object operations (see Recalibrate with scaled wand & custom calibration object).

Recalibrate with scaled wand & custom calibration object

1. Calibrate using the normal process, but in the Wand Wave section, select the new scaled Active Wand .vsk file.

   

2. For the best results, set the origin using the normal process, but:

   • In the Set Volume Origin section, select the scaled calibration object.

   • Ensure Perform Rescale is selected.

     

   This ensures a correctly scaled volume according to the recorded measurements of the custom calibration object and also a permanently fixed origin, based on the static objects.

   Tip: If you can't use permanently fixed objects in your volume, in Step 2 above, substitute the scaled wand object in place of the custom calibration object. Your results will not be as accurate as they would with the custom calibration object, but you will still benefit from the scaling of the wand.

3. Validate the new calibration by repeating the measurements from Step 5 of Scale the Active Wand and the custom calibration object.

The measurements in Shogun now closely align with the real-world measurements of the fixed objects, as recorded in Step 2 of Set up the custom calibration object.