User guide

Charnwood Dynamics Ltd. Coda cx1 User Guide – Advanced Topics III - 1
CX1 USER GUIDE - COMPLETE.doc 26/04/04 91/162
In the second category are those applications requiring analytical, ‘de-constructive’
methods, for example, where a virtual marker is the only means for tracking a point of
interest located in a position where it would not be feasible to place a real marker, or
where a real marker would be too often out of view. In such cases the ‘unobtainable’
location may be referred to as a point whose position is fixed relative to visible markers
which provide a rigid context frame. If the relative position can be determined (possibly
from a static situation where markers are guaranteed to be in view) then appropriate
weights and offsets for the context frame markers may be solved and applied to dynamic
tracking.
Some users may wish to apply this method where a rigid segment is determined with
some redundancy in the number of markers; they are referred to the application note on
the related topic: Enhancing Segment Representation.
Assigning weights by model design (first category)
The simplest virtual marker is a point mid-way between two markers:
The Virtual Marker is defined from Markers M
1
and M
2
with the (default) weighting factors
1.0.
The weighting factors are automatically normalized (dividing each by the total weight) in
this case obtaining 0.5 for each marker. Thus, the position of M
V
will be calculated as
x
v
= 0.5 x
1
+ 0.5 x
2
y
v
= 0.5 y
1
+ 0.5 y
2
z
v
= 0.5 z
1
+ 0.5 z
2
or
P
v
= 0.5 P
1
+ 0.5 P
2
= (P
1
+ P
2
) / 2 where P is the position vector of each marker.
This might be used to define a virtual neck marker from two shoulder markers, for
example.
If the weighting factors are different (and positive), the virtual marker can be placed
anywhere on the line between M
1
& M
2
. For example, if w
1
= 0.9 and w
2
= 0.1, the virtual
marker position is
P
v
= 0.9 P
1
+ 0.1 P
2
Note that the virtual marker is closer to the marker with the larger weighting factor. This
corresponds to a centre of mass calculation.
M
1
M
2
M
V
M
1
M
2
w
1
w
2
M
V

balance point