This task page enables you to calibrate a tracked pointer and use it to record the location of points in 3D space (which we call fiducial landmarks).

Check that a tracked pointer is available and is being tracked by Stradwin. It should be visible in the 3D window (when the 3D window is running live). If you don't yet have a tracked pointer, press the button on the pointer task page labelled 'Configure position source', which will bring up the 'Position source' dialog from the configuration menu. Use this to set up your position sensor appropriately to track the pointer.

Note that if you connect a pointer to Port 3 of a Polaris or trackSTAR position sensor, it will be assumed to be a Cambridge Pointer used in probe calibration. For this type of pointer, the calibrated point is offset from the actual pointer tip by 22 mm.

Pointer calibration

If you wish to calibrate the pointer, you need to find a location into which the tip of the pointer can be held such that tip does not move when the orientation and position of the rest of the pointer is moved. If the tip of the pointer is a small sphere then a hemispherical dimple of the same diameter is ideal.

Hold the tip of the pointer into its dimple, click on 'Start Pointer Calibration', move the body of the pointer around and twist it about its axis. Make sure the tip of the pointer does not move and that the position sensor attached to the pointer is always correctly tracked. The calibration will be invalid if inaccurate positions are recorded or if the centre of the pointer tip moves. When you have moved the pointer about for about 10 seconds and exercised as many degrees of freedom as you can without disturbing the tip, click on the 'Stop Pointer Calibration' button.

When you quit Stradwin, the pointer calibration is saved in the stradwin.ini file. By default, each user has a separate copy of this file which is saved in their home folder and reloaded when the program starts up. Details of how to change this behaviour are given on the installing Stradwin page.

Recording landmark locations

Once the pointer is calibrated, it is easy to record the locations of points in 3D space and relate them to 3D scan data. In order for this to work you must not move the fixed part of the position sensor between recording the separate things that you wish to locate in the same coordinate system.

To record a location, move the pointer to it and click on the 'Grab Pointer Location' button. You can give the location a particular name by editing the string in the window. If you grab the wrong location, you can delete the landmark using the button provided. There is also a button to delete all the landmarks you have recorded in one go.

Once a fiducial landmark has been recorded using the pointer it is available for measuring lengths and angles using the landmark task page.