Scale calibration and set a fixed origin

After you’ve calibrated the system in the usual way, including setting the volume origin (see Set the volume origin), for improved scaling across calibrations and a permanently fixed origin point, you can create a large custom calibration object from markers permanently placed across the volume. You can then use this calibration object, coupled with using the Perform Rescale option, for subsequent setting of 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.

The following topics in this section explain how to do this.

Set up the custom calibration object

To initially set up the custom calibration object:

1. In the volume, place permanently static active markers to create a custom calibration object. Note the following points:
   • For best performance, fix the markers to the floor (provides both stability and the ability to spread across the volume).
     If this is not possible, position them on a wall or truss.
   • To guarantee a good measurement, locate 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 points at two furthest edges of the object, and record the results. This can be a single measurement (eg, if the object’s sides are equal), or two measurements across two different axes (eg, 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 Evoke, create an object for the markers of the custom calibration object (for details, see Create basic objects).
   Do not save this object: it is important that the object that is used for this scaling is created from the current marker observations.

3. In the Tracking panel, ensure the custom calibration object is selected, right-click and then click Use for Camera Calibration.

   

   The Calibration Scaling dialog box is displayed and you are alerted that the origin of the selected object will be set to the global origin (0,0,0) when you click Save.

4. In the Calibration Scaling dialog box:

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

      

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

      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.

   3. In the Measured Distance column, enter the distance that you measured in millimeters.

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

      In the New Settings section, the scale factor that will be applied is displayed, together with the output names for the scaled wand and calibration objects.

      

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

      A scaled wand and a calibration object 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).

If you do not add measurements, the object is saved as a calibration object with no scaling modification, and no scaled wand is saved. You can still use the new calibration object for set origin operations, including rescaling. This will provide consistency of scale across set origin operations in different calibrations, but the volume will not be scaled to verified external measurements.

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 both a correctly scaled volume according to the measurements that you recorded 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 Evoke now closely align with the real world measurements of the fixed objects, as recorded in Step 2 of Set up the custom calibration object.