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Table of Contents
To track a segment, a minimum of three noncolinear targets are required.
As long as the three targets are not in a straight line, the segment can be tracked. However, the distance between the targets will affect your tracking.
For example, assume you have a segment as shown below:
ClusterSize_Segment.png{{/images/thumb/c/c8/ClusterSize_Segment.png/250px-ClusterSize_Segment.png
There are two options, you can either:
- put the targets spread out over the entire segment
- place the targets close together
Spread Out Tracking Targets
If the targets are spread out over the segment:
ClusterSize_Large_Segment.png{{/images/thumb/5/5a/ClusterSize_Large_Segment.png/250px-ClusterSize_Large_Segment.png
It takes a large amount of target movement to produce a large change in the position and orientation of the segment:
ClusterSize_Large_Segment_Rotation.png{{/images/thumb/e/e1/ClusterSize_Large_Segment_Rotation.png/250px-ClusterSize_Large_Segment_Rotation.png
Close Together Tracking Targets
If the targets are spread close together:
ClusterSize_Small_Segment.png{{/images/thumb/f/fd/ClusterSize_Small_Segment.png/250px-ClusterSize_Small_Segment.png
It takes a small amount of target movement to produce a large change in the position and orientation of the segment:
ClusterSize_Small_Segment_Rotation.png{{/images/thumb/2/2d/ClusterSize_Small_Segment_Rotation.png/250px-ClusterSize_Small_Segment_Rotation.png
Application
The closer the targets are together, the more susceptible the tracking will be to noise. So try to use best judgement when placing markers/defining clusters.
This is true for tracking any segment, but it tends to be most relevant when tracking the pelvis (using a small cluster on the lower back vs using the ASIS and PSIS) or tracking the foot (using a heel cluster vs heel and met targets).