
The first step in body modeling is to define the points. Points can be classified into two groups: primary points and secondary points. Primary points are those to be digitized while secondary points are those to be computed based on the coordinates of the primary points. The joint centers are the typical secondary points. Secondary points are necessary when markers are attached to the body surface in a 3D analysis since it is impossible to locate markers at the joint centers. If the image plane coordinates of the joint centers are to be obtained directly from manual digitizing, they must be defined as primary points. One question here is how to compute the coordinates of the secondary points defined. There are two possible ways: (1) using the joint center computation algorithm described in the Computation of the Joint Center in the Joint Kinematics section, and (2) adding additional programs that specifically compute the coordinates of the secondary points. Method 2 is more complex because the computation algorithm may be studyspecific so that one will need different addon programs for different studies. Method 1 can be incorporated in a standard motion analysis program thus discarding the need to add the addon programs. In Kwon3D, a point can have the following properties:
The stickfigure graphics requires the information about how to connect the points. The connectedpoint list will sufficiently provide this information. Several points can be connected to the current point in the stickfigure graphics. The stickfigure graphics is very useful at least in the digitizing process. More sophisticated graphics can be added in the later stages for more effective feedback and presentation. 
© YoungHoo Kwon, 1998 