After you have created all the segments, the next step is to choose suitable joint types and link the segments together to form a hierarchical skeletal structure. A joint can have up to six degrees of freedom (DOF), which indicates how freely the joint is able to move in relation to its parent segment.
When you link segments, start with the root segment and link from that until you have a continuous link chain to the leaf segment. Nexus lets you link in any order you like, but it is easier to keep track if you follow the chain.
To display the segment bounding boxes, which enables you to select segments easily in the view pane, in the Options dialog box (F7), ensure the Subjects display is selected and in the Properties pane for Subjects, ensure that Draw Boxes is selected.
To link segments:
On the Subject Preparation tab of the Tools pane, in the Labeling Template Builder section, from the Link Segments drop-down box, choose the appropriate joint type for the two segments you intend to link and click Link.
When linking segments, Vicon recommends that you use the Free Joint for two segments that do not share any marker and the Ball Joint for segments that share markers. For information on the joint types, see About joint types. Joint types as defined by the Labeling Template Builder are for the labeling skeleton, meaning that the true biomechanical joint type may not necessarily be the best choice for the skeleton. For the majority of skeletons, the Nexus 2 labeler works best when the Ball Joint and Free Joint joint types are used. The advanced joint types are only required for very specific labeling needs.
The mouse cursor changes, and the Select the Parent Segment tool tip is displayed.
In the 3D Perspective view, click on the bounding box of the segment you want to be the parent segment.
If you have trouble selecting segments by clicking on the bounding boxes in the 3D Perspective view, you can also link segments by clicking on their names in the list under the Joints node in the Subjects resources tree and then clicking Link.
The segment bounding box turns red and the mouse cursor label changes to Select the Child Segment.
- Click on the bounding box of the required segment (typically the distal segment).
In the 3D Perspective view, the child segment changes color, and a cylinder appears between the parent and child to represent the link.
The cursor changes back to Select the Parent Segment, which means that you can immediately select next parent/child pair, if you intend to use the same joint type.
- Continue selecting parent and child segments until either you have linked all the segments or you need to change the joint type.
- To change to a different joint type:
- Click Link, which turns off the cursor tool tip and activates the Link Segment drop-down box.
- Select the required joint type from the drop-down list.
- To resume linking parent/child segments with the new joint type, click Link again.
When all segments are defined, click the Link button again to exit the segment linking utility.
When you have finished linking all the segments, the name of the subject in the Subjects resources tree, which was red when you first created it, is now displayed in black. The red color indicates that the subject is incomplete; for example, it has unlinked segments. The black color indicates that the subject is properly defined (markers, segments, and joints).
On the Subjects tab, expand the Joints node and check that the joints are all correctly connected, and that their icons display the expected Degrees of Freedom (DOFs).
You can now assign the marker and segment properties as required.
About joint types
To link segments, you can use the following joint types:
- Free Joint This joint type is used to link two segments that do not share any markers, and which can therefore move freely relative to each other. It is a 6 DOF joint with full translational as well as rotational freedom. It is used for the root segment to allow it to move freely with respect to the global origin; you cannot change the root's joint type.
Ball Joint This 3 DOF joint with full rotational (but not translational) freedom, used to link two segments that share one marker, and which can therefore rotate freely around all three axes with respect to each other, but cannot translate. This joint type has the position of the child segment defined from the position of the parent joint, but its orientation can vary freely.
Use Ball Joints (3 DOF) for segments that share one marker and Free Joints (6 DOF) for segments that share none.
- Hardy Spicer Joint A 2 DOF joint with two rotational degrees around two axes, namely the Y and Z axes of the parent joint. This joint type has two perpendicular vectors defining the directions of the two axes around which the joint can rotate.
- Hinge Joint This 1 DOF joint allows rotation around a single axis only, and should be used to link two segments that share two markers. The hinge axis is aligned with the two markers that are shared.