The objective for using the pointer is to identify anatomical landmarks without having to place motion capture markers at the location. There are many reasons for using a pointer instead of a marker.
Some anatomical landmarks are obscured by fat (e.g. the Pelvis ASIS markers for overweight subjects
Some anatomical landmarks are obscured from the cameras (e.g. some medial landmarks)
Some anatomical landmarks are close together (e.g. on the foot) and tracking the markers can be challenging
In order to attach markers, the subject must expose that part of the body, which can be uncomfortable for some subjects
If a tracking marker falls off, or is moved inadvertently, after the anatomical markers have been removed. The standing trial must be recorded again, which requires all anatomical landmarks to be replaced.
Each landmark is identified relative to 3 markers that are attached rigidly to the segment that the landmark is associated with.
Computing the location relative to 3 markers that are tracked by the cameras means that the subject can move during the data capture and the landmark will move relatively.
This can be especially useful for defining the static trial because you can orient the subject so that there is a better view of the markers for the motion capture system.
An example of the landmark definition in Visual3D is:
Landmark Name:
_RL.WRIST (e.g. Right Lateral Wrist)
Starting Point: RWR1
Ending Point: RWR2
Lateral Object: RWR4
Offset using the Following ML/AP/AXIAL Offsets
ML: -0.00809
AP: 0.0238
AXIAL: -0.0209
This landmark is created relative to three tracking markers (RWR1, RWR2, RWR4) with a fixed offset. The location of this landmark can be computed for all movement trials assigned to this model.
A new C3D file can be exported that includes these landmarks as if they were Motion Capture markers. This file can now be used by other software that doesn't recognize Visual3D landmarks. The C3D Parameter description for these virtual markers indicates that it was created using Visual3D's Pointer module.