You create segments from the markers you reconstructed previously (see Capture and reconstruct the marker set) using the Labeling Template Builder in the Subject Preparation tab of the Tools pane. Typical segments you might create for a human subject include the pelvis, head, femur, etc.
You create the segments for your template by grouping markers that move in roughly the same way. For example, if you have three markers on the shank, one at the knee joint, one half-way down the shank and one at the ankle joint, these three markers can be considered to be attached to the same segment (known as the TIBIA segment). For areas of the body that have multiple segments and joints, such as the ribs and spine, it may still make sense to group them together (eg, as a thorax) as these segments can be considered to move together, although this is dependent on the exact dynamic movements to be captured.
Ideally, a segment should be defined by three or more markers in the following order: one to define the origin, one to define the primary axis of the segment and one to define the secondary axis. In addition, any number of additional markers that do not specifically define the segment's axes can be added.
To create segments:
- Go to the Subject Preparation tab of the Tools pane .
- In the Labeling Template Builder section, click in the Create Segments field. Enter a name for the first segment, for example Pelvis, and then click Create.
Segment names can contain spaces, and if they are preceded by Left or Right (followed by a space), they are automatically colored appropriately if you are using the default color scheme.
The mouse pointer changes to a cross-hair with the label Select the Origin Marker.
- In the 3D Perspective view, click on the marker that defines the origin of the segment you are creating. For the Pelvis, this could be the LASI or the RASI marker.Tip
Choose the origin marker for distal segments to be on a more proximal segment. However, remember that the purpose of the labeling skeleton template is to define a labeling skeleton that is suitable for labeling. The segments are related to marker positions, not to joint centers as in a biomechanical model.
Nexus highlights the selected marker. The mouse pointer label changes to Select the Primary Axis Marker.
- Click on the marker that defines the primary or long axis of the segment you are creating. For the Pelvis, this could be the unused LASI or RASI marker.
The marker is highlighted and the mouse cursor label changes to Select the Secondary Axis Marker.
- Click on the marker that defines the secondary axis. The marker is highlighted and the mouse cursor label changes to Select an Additional Marker.
This marker will fully define the segment by requiring the primary axis and the secondary axis marker to be in the same plane. For the Pelvis, this could be the SACR marker.
- Click on the chosen marker.
- In the same way, select any other markers that you want to associate with the same segment.Tip
If you have problems selecting a marker, it may be because the selection operation selects something else, like the bounding box of a segment. To turn off the visualization of the segment, clear the Subjects option in the Options dialog box.
- When you have finished creating the segment, click again on the Create button.
In the 3D Perspective view, the selected markers are now displayed in gold. The segment's axes are displayed as X=red, Y=green and Z=blue. By convention, the Z axis is the primary axis and the secondary axis marker defines the YZ plane.Tip
If, at any point in the steps above, you make a mistake (for example, you click on the wrong marker), you must finish creating the segment first and then use the Undo operation to remove the segment. You can do this by either using the CTRL+Z shortcut or clicking the Undo button on the Nexus toolbar. You can then recreate the segment.
Repeat the above steps to define all segments.
All markers must be part of at least one segment.Important
When you create segments, they are only valid for that particular frame. For this reason, you must create all segments on the same frame. To verify that all segments exist in a frame, ensure that all segments are in black (and not gray) in your subject's segments list. If some segments are black, and some are gray, delete the gray segments (and associated markers) and re-create those segments until all segments are in black. This is vital for the next step of linking segments.
Tips for creating segments
When you select the markers to define a segment, keep the following basics in mind:
- One marker defines the location of one point, but not a whole segment
- Two markers define a line, with length and direction.
- Three non co-linear markers define a complete segment as well as a plane
- Four markers provide extra information which enables occluded markers to be re-created based on visible markers
- One or more of the points used to determine the location and attitude (orientation) of a segment may be derived from a neighboring segment. In fact, if the positions and attitudes of the neighboring segments are fully determined, and the degrees of freedom of the joints are defined, it may be possible to determine the location and attitude of a segment without any markers being attached to it.
You can choose any marker within the segment to be the origin of that segment, and any set of mutually perpendicular directions as the axes. However, there are a few guidelines which simplify these choices.
- The majority of models in biomechanics involve linked segments which form a chain, such as Pelvis-Femur-Tibia-Foot, and such models describe the movements of each segment relative to the more proximal segment.
- The auto-labeler will perform better if the skeletal topology defined in the template file roughly matches a simplified version of the real anatomical structure. Therefore, Vicon strongly recommends that you define the labeling skeleton template using segments that are as close to the real anatomy as possible.
The following are additional tips and recommendations to help you in defining segments:
- Start by defining the root segment (the most proximal) and work your way towards the leaf segments (the most distal ones). Often, the root segment will be the pelvis or the thorax segment when defining a human labeling skeleton template, but it could be any segment.
- Where possible, align a segment's primary axis with the biomechanical long axis. Where there is no obvious long axis (as in the pelvis or trunk segment), the long axis can be defined using any two markers.
- If two markers are assigned to the same segment and the distance between the two markers will change significantly during the trial, we suggest that you optimize the VST with a Range-Of-Motion (ROM) trial (see Calibrate a labeling skeleton using a ROM trial in the Vicon Nexus User Guide. Alternatively, you may want to look at creating a separate segment altogether.
- If you are creating a segment with restricted movement, for example, finger markers, it may make sense for your segment to consist of only two markers.
Remember that the purpose of the labeling skeleton template is to define a skeleton that is suitable for labeling. The segments are related to marker positions, not to joint centers as in a biomechanical model.