Figure 3-3 Determining A Marker Position - Northern Digital Polaris Vicra User Manual

How the Polaris Vicra System Works
To determine the position of an IR source, the Position Sensor calculates a line between the source
of IR and each sensor (displayed as dotted lines in Figure 3-3). Where the lines cross each other, the
Polaris Vicra System calculates the line separation. (The distance between the lines.) (In a
theoretical case the lines will intersect exactly, but in the modelled "real world" case they will be
apart.) If the line separation at this point is less than a predefined limit, the Polaris Vicra System
considers the point to be a possible marker position. Otherwise, the point is discarded.
Marker

Figure 3-3 Determining a Marker Position

Once the system has measured the positions of all the markers, it calculates the segment length (the
distance between two markers) for each pair of markers, and the angle between each segment. It
compares this data with the marker geometry data in each tool definition file (described on page 31),
to determine which markers belong to which tool. The unique geometry requirements, described in
the "Polaris Tool Design Guide", allow the Polaris Vicra System to distinguish between tools. Any
markers that are not part of a tool are considered stray markers (this may include phantom markers).
See "Stray Marker Reporting" on page 37 for more details on stray markers and phantom markers.
The Polaris Vicra System has "acquired" a tool once it has matched the minimum number of
markers (a parameter in the tool definition file) for the tool and can calculate a transformation for
the tool. Once a tool has been acquired, the Position Sensor tracks it using a predictive algorithm.
Three Marker Lock On
If the "3 marker lock on" option is enabled in the tool definition file, the Polaris Vicra System will
acquire and track the tool as long as it can detect at least three markers. The system will not report
the transformations unless the minimum number of markers is used to calculate the transformation.
For example, consider a four-marker tool with the "3 marker lock on" option enabled. If the system
can only detect three of the markers on the tool, it will continue to track that tool but will only report
transformations if the minimum number of markers is set to 3. If the minimum number of markers is
set to 4, the system will continue to track the tool in the background, but will report the tool as
MISSING. Selecting 3 marker lock on in this case will result in the tool transformations being
reported faster, once the minimum number of markers becomes visible, because the system does not
have to spend time re-acquiring the tool.
Note
For more details on the "3 marker lock on" option, see the "Polaris Tool Design Guide".
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