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After you’ve calibrated the system in the usual way, including Setting the volume origin, for maximum real-world accuracy (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. This improves calibration stability and consistency over time.

Note
If it is impractical to use permanently fixed markers in your volume, you can still benefit from following this procedure, but when you come to recalibrate, substitute a 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 involved. For details, see Step 2 of Recalibrate with the scaled wand and custom calibration object.


These topics explain how to do this:

Choose LEDs or markers for your custom calibration object

To create your custom calibration object, you can use either of the following:

  • An asymmetrical pattern of LEDs (emitting 850 nm wavelength light)
    If you use LEDs to create your custom calibration object, ensure the LEDs are turned on only during setting the origin, and that they are turned off during the calibration wand wave and while the volume is in use.
    or
  • Reflective markers
    If you use reflective markers to create your custom calibration object, remember that as passive markers are continually visible, you may need to cover them while the volume is in use.

Set up the custom calibration object

To initially set up the custom calibration object:

  1. In the volume, place permanently static LEDs or markers to create a custom calibration object. Note the following points:
    • For best performance, fix the LEDs or 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 (see Calibrate Vicon cameras).
  2. Set the origin using a standard calibration object (see Setting the volume origin).
  3. In Tracker, create an object for the markers of the custom calibration object (for details, see Creating an object).
    Do not save this object: it is important that the object that is used for this scaling is created from the current marker observations.
  4. On the Objects tab, ensure the custom calibration object is selected, right-click and then click Use for Camera Calibration.



  5. In the Calibration Scaling dialog box:
    1. In the Current Settings section, check that the Selected Wand is the correct wand for your calibration, and that the Selected Object is correct.


    2. Ensure the custom calibration object is enabled, then 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\Tracker3.x\CalibrationObjects and are displayed in the Wand and L-Frame lists on the Calibrate tab.



      The current calibration is also scaled by the same factor, so you do not need to recalibrate after this operation.
  6. 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 the scaled wand and 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 auto-scale. This will provide consistency of scale across set origin operations, but the volume will not be scaled to external measurements.

Recalibrate with the scaled wand and custom calibration object

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



  2. For the best results, set the origin using the normal process, but:
    • In the L-Frame field, select the scaled custom calibration object.
    • Ensure Auto Scale 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 markers.

    Tip
    If you can't use permanently fixed markers 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 Tracker should now closely align to the real world measurements of the fixed markers, as recorded in Step 2 of Set up the custom calibration object.