User guide

Charnwood Dynamics Ltd. Coda cx1 User Guide – Advanced Topics III - 5
CX1 USER GUIDE - COMPLETE.doc 26/04/04 113/162
ENHANCING SEGMENT REPRESENTATION
In the field of motion analysis it is now common practice to derive representation of a rigid
body segment from the set of markers assembled upon it. Too often the nature of the
motion causes some or all of these to drop out of view for short periods. We present here
some ideas for dealing with this problem. Ultimately, over-determination using many
markers provides for very high resolution.
The scope for enhancing marker representations varies from low-pass frequency filtering,
through interpolation for a single out-of-view marker, to multiple-marker virtual-model
reconstruction. Digital filtering and basic cubic interpolation algorithms are already
incorporated into Codamotion Analysis software. The other analytical techniques
described herein are not yet fully integrated into Codamotion Analysis software but are
readily applied, post-acquisition, to movement data files resaved as tabulated text for
spreadsheet processing.
Coda’s capabilities now extend to (real-time) tracking of up to 56 markers, allowing for
multiple representations of single body segments, perhaps as a contingency for markers
becoming obscured or otherwise going out of view. In some cases we might seek to
obtain the best rigid representation of a non-rigid body where markers attached to the
outer surface are subject to relative surface movement or oscillations.
Although a minimum of 3 markers are required to fully specify all the rotations and
translations of a rigid segment we begin by considering just one marker.
Single marker representation
A single marker can represent no more than a single point on a body segment as its
motion is tracked. As such, its movement must be regarded as entirely translational,
revealing nothing about rotational movement. If this marker disappears from Coda’s view
for a period during the motion capture we can do little more than interpolate its position by
temporal dependence throughout that period: there is no relative spatial context for its
position.
Two-marker representation
A pair of markers mounted upon a rigid segment present sufficient information (while they
both remain in view) to describe both translational and rotational movement, though not
fully, as rotations about the ‘axis’ joining the two markers remains undefined. This
arrangement is typical of a simple ‘stick-figure’ description of the human form where limb
segments are indicated as straight lines between markers placed over joints. Though not
very sophisticated, it is obviously far better than representing each limb segment by a
single marker bearing no spatial relationship to markers on adjacent segments.
There is, however, an important additional advantage over single marker representation
when dealing with out-of-view periods: provided both markers are simultaneously visible
at some time during the motion capture we have the option to apply certain deductions
about the relative positions of the two markers to the interpolation procedure. (Of course,
if both markers are never simultaneously in view it would not be possible to derive any
such spatial context information.)