Advanced calibration

The Advanced calibration can be used to simultaneously optimize both the camera linearizations (intrinsic calibration) and the capture volume calibration (extrinsic calibration). In many cases, this will lead to decreased 3D residuals of the measured trajectories and improved 3D tracking.

The advanced calibration can be specially beneficial in the following cases:

  • Systems with wide-angle lenses

  • Large capture volumes

Requirements for the advanced calibration

For the advanced calibration, it is important that all cameras are sufficiently covered by the movements of the calibration wand. Mostly, this can be achieved by optimizing the camera setup for the used capture volume according to the following guidelines.

  • Make sure that the cameras are pointed in a way so that the central part of the sensor is used.

  • Make sure that for each camera a significant part of the sensor is covered by the movements of the wand. The recommended sensor coverage is at least 75%.

  • Make sure that there is sufficient depth coverage for all the cameras.

How to perform an advanced calibration

The advanced calibration uses the same calibration settings as the standard wand calibration, and is performed in the same way.

Follow these steps to perform an advanced calibration:

  1. Make sure that the calibration options are correct under Project Options > Input Devices > Camera System > Calibration.

    It is important to use a sufficiently large number of calibration points. The exact length depends on the duration of the calibration. The recommended number of points is 3000 or more.

  2. Place the L-frame in the desired location

  3. Start a preview.

  4. Open the Advanced calibration dialog via the Capture menu: Capture > Advanced calibration....

  5. Set the duration of the calibration and press OK to start.

  6. During the calibration, move the wand through the whole capture volume, varying the orientation of the wand (see chapter How to move the wand).

  7. When done, inspect the calibration results.

If the calibration is passed, it will be automatically used as the current calibration of the project, and the linearization files on the Linearization page in the project will be replaced by the new ones resulting from the advanced calibration.

Evaluation of the calibration quality

Before using the Advanced calibration, it is recommended to perform one or more standard calibrations first, which can be used as a baseline when comparing the calibration results. The calibration results appear directly after performing a calibration, or can be viewed for the currently loaded calibration on the Current Calibration page in the Project Options. When comparing the results, pay special attention to the following values:

  • Average residual per camera (Av Res (mm)).

    The average residual camera should be as low as possible, and the values should be as equal as possible between cameras.

  • Points per camera (Points).

    The number of points used for the calculation per camera should be as high as possible, and should be as equal as possible between cameras.

  • Standard deviation of the wand length (in mm).

    The standard deviation of the wand should be as low as possible.

The exact values will depend on several factors, so it is not possible to give any absolute figures. If the results of the advanced calibration are significantly better than those of the standard calibration using factory linearizations, it is recommended to use it.

A good way to compare the advanced linearization with a standard calibration is by first doing a standard calibration based on factory linearizations, and then recalibrate the same file using the advanced calibration method. The comparison will then be based on exactly the same wand movements. To recalibrate a calibration with the advanced method:

  1. Open the original calibration file from the Calibrations folder in the project.

  2. In the Capture menu, click on the Advanced calibration... button to open the recalibration dialog.

  3. Click OK to start the recalibration.

  4. When done, inspect the calibration results in the results dialog.

  5. It is recommended to save the file with a different name, for example by adding a suffix _advanced to the file name. This way it will be easier to revisit the calibration results for comparison.

For more information about calibration results, see chapter Calibration results.

The linearizations resulting from the advanced calibration are specific for the used camera setup. Using these linearizations in a different setup may degrade the tracking performance.

Choice of linearizations

After a successful advanced calibration, the linearization files on the Linearization page in the project will be replaced with new files, which are added to the linearization files folder (see Folder options). The advanced calibration will not replace the factory linearizations that are stored on the cameras.

In the calibration options, you can choose which set of linearization files is used for the next calibration. This is done by the Linearization setting under Calibration type in Project Options > Cameras > Calibration.

The behavior for these choices in combination with the calibration method is as follows:

  • Use linearizations loaded in project

    • Standard calibration

      Perform a standard calibration based on optimized linearization files that are currently loaded in the project. For more information, see chapter Using optimized linearizations for standard calibrations.

    • Advanced calibration

      Perform an advanced calibration, using the optimized linearization files that are currently loaded in the project as starting point.

  • Use factory linearizations

    • Standard calibration

      Perform a "traditional" calibration using factory linearization files from the cameras.

    • Advanced calibration

      Perform an advanced calibration, using the factory linearization files from the cameras as starting point.

By default, the choice is set to Use factory linearizations. This way, the standard calibration behaves as a traditional calibration in QTM, and the advanced calibration starts with the factory linearizations as a reference starting point for the calculation of new, optimized linearization files.

The Linearization choice is grayed out when the factory linearizations are loaded in the project. This is for example the case after performing a standard calibration using factory linearizations.

Use scenarios

Using optimized linearizations for standard calibrations

After performing an advanced calibration, the linearization files on the Linearization page in the project will be replaced by optimized linearization files. By setting the Linearization choice on the Calibration page to Use linearizations loaded in project, these will then also be used for subsequent standard and refine calibrations. This brings the advantage that standard and refine calibrations will benefit from the optimized linearizations with similar decreased residuals as the advanced calibration, without requiring the same level of coverage as an advanced calibration, so that the calibration can be performed more quickly.

Using projects with different camera setups

When using the same cameras in different camera setups, it is recommended to use separate projects. Calibrations performed within one project will not transfer to another when they contain different linearization files. As a result advanced calibrations will not be mixed between projects.