This algorithm computes transformation between the tip of a tracked pointer and the coordinate system of tracking marker attached to the object.

Usually the marker's coordinate system is named as Stylus, the coordinate system that has its origin at the tooltip is named as StylusTip. The StylusTip coordinate system can be chosen to have the same orientation as the Stylus coordinate system, however this may not be aligned with the principal axes of the stylus object. This misalignment may be confusing for the users. The StylusTip coordinate system could be aligned with the principal axes of the stylus, by asking the user to translate or sping the stylus along its axes or to perform a landmark registration. However, these extra steps increase the complexity of the procedure and generally the accuracy of the stylus orientation is not critical, as it is used for visual representation only. Therefore, in Plus the approximate stylus orientation is computed from the data acquired for the pivot calibration. The marker's origin (Stylus coordinate system origin) is assumed to be close to the long axis of the stylus, therefore the -Z axis of the StylusTip coordinate system is chosen as the unit vector pointing from the StylusTip origin to the Stylus origin and the other two axes are aligned with the X and Y axes of the Stylus coordinate system.

Configuration settings

vtkPlusPivotCalibrationAlgo
- ObjectMarkerCoordinateFrame
- ReferenceCoordinateFrame
- ObjectPivotPointCoordinateFrame

Example configuration file PlusDeviceSet_fCal_Ultrasonix_L14-5_Ascension3DG_2.0.xml

<PlusConfiguration version="2.3">
  <DataCollection StartupDelaySec="1.0" >
    <DeviceSet 
      Name="fCal: Ultrasonix US (L14-5 probe) + Ascension3DG tracker (Probe, Reference, Stylus) - fCal Phantom 2.0"
      Description="Ascension3DG sensors should be plugged in to the Ascension3DG DriveBay mounted on Ultrasonix US in the following order from to leftmost slot (Transducer 1) to the right: 1 Probe, 2 Reference, 3 Stylus." />
    <Device
      Id="TrackerDevice" 
      Type="Ascension3DG" 
      LocalTimeOffsetSec="0.0" 
      FilterAcWideNotch="1"
      ToolReferenceFrame="Tracker" >
      <DataSources>
        <DataSource Type="Tool" Id="Probe" PortName="0"  />
        <DataSource Type="Tool" Id="Reference" PortName="1"  />
        <DataSource Type="Tool" Id="Stylus" PortName="2"  />
      </DataSources>
      <OutputChannels>
        <OutputChannel Id="TrackerStream" >
          <DataSource Id="Probe"/>
          <DataSource Id="Reference"/>
          <DataSource Id="Stylus"/>
        </OutputChannel>
      </OutputChannels>
    </Device>
    <Device
      Id="VideoDevice"
      Type="SonixVideo" 
      AcquisitionRate="30" 
      LocalTimeOffsetSec="-0.2976"
      IP="127.0.0.1"
      AutoClipEnabled="TRUE" >
      <DataSources>
        <DataSource Type="Video" Id="Video" PortName="B" PortUsImageOrientation="UF"  />
      </DataSources>
      <OutputChannels>
        <OutputChannel Id="VideoStream" VideoDataSourceId="Video"/>
      </OutputChannels>
    </Device>
    <Device 
      Id="TrackedVideoDevice" 
      Type="VirtualMixer" >
      <InputChannels>
        <InputChannel Id="TrackerStream" />
        <InputChannel Id="VideoStream" />
      </InputChannels>
      <OutputChannels>
        <OutputChannel Id="TrackedVideoStream"/>
      </OutputChannels>
    </Device>
  </DataCollection>
  <CoordinateDefinitions>
    <Transform From="Image" To="TransducerOriginPixel"
      Matrix="1 0 0 -410
              0 1 0 5
              0 0 1 0
              0 0 0 1"
      Date="2011.12.06 17:57:00" Error="0.0" />    
  </CoordinateDefinitions> 
  <Rendering WorldCoordinateFrame="Reference">
    <DisplayableObject Type="Model" ObjectCoordinateFrame="TransducerOrigin" Id="ProbeModel"
      File="Probe_L14-5_38.stl"
      ModelToObjectTransform="
        -1 0 0 29.7
        0 -1 0 1.5
        0 0 1 -14
        0 0 0 1" />
    <DisplayableObject Type="Model" ObjectCoordinateFrame="Reference" Id="Volume"/>
    <DisplayableObject Type="Model" ObjectCoordinateFrame="StylusTip" Id="StylusModel" File="Stylus_Example.stl" />
    <DisplayableObject Id="PhantomModel" Type="Model" ObjectCoordinateFrame="Phantom"
      Opacity="0.6"
      File="fCal_2.0.stl"
      ModelToObjectTransform="
        1 0 0 -35.0
        0 1 0 -10.0
        0 0 1 -5.0
        0 0 0 1" />
    <DisplayableObject Type="Image" ObjectCoordinateFrame="Image" Id="LiveImage"/>
  </Rendering>
<Segmentation
    ApproximateSpacingMmPerPixel="0.078"
    MorphologicalOpeningCircleRadiusMm="0.27"
    MorphologicalOpeningBarSizeMm="2.0"
    ClipRectangleOrigin="27 27" 
    ClipRectangleSize="766 562" 
    MaxLinePairDistanceErrorPercent="10"
    AngleToleranceDegrees="10"
    MaxAngleDifferenceDegrees="10"
    MinThetaDegrees="-70"
    MaxThetaDegrees="70"
    MaxLineShiftMm="10.0"
    ThresholdImagePercent="10"
    CollinearPointsMaxDistanceFromLineMm="0.6"
    UseOriginalImageIntensityForDotIntensityScore="FALSE"
    NumberOfMaximumFiducialPointCandidates="20"
  />  <PhantomDefinition>
    <Description
      Name="fCAL"
      Type="Multi-N"
      Version="2.0"
      WiringVersion="2.0"
      Institution="Queen's University PerkLab"
    />
    <Geometry>
      <Pattern Type="NWire">
        <Wire Name="7:G1_g1" EndPointFront="30.0 0.0 20.0" EndPointBack="30.0 40.0 20.0" />
        <Wire Name="8:L1_h1" EndPointFront="55.0 0.0 20.0" EndPointBack="35.0 40.0 20.0" />
        <Wire Name="9:M1_m1" EndPointFront="60.0 0.0 20.0" EndPointBack="60.0 40.0 20.0" />
      </Pattern>
      <Pattern Type="NWire">
        <Wire Name="4:G3_g3" EndPointFront="30.0 0.0 10.0" EndPointBack="30.0 40.0 10.0" />
        <Wire Name="5:H3_l3" EndPointFront="35.0 0.0 10.0" EndPointBack="55.0 40.0 10.0" />
        <Wire Name="6:M3_m3" EndPointFront="60.0 0.0 10.0" EndPointBack="60.0 40.0 10.0" />
      </Pattern>
      <Pattern Type="NWire">
        <Wire Name="1:H5_h5" EndPointFront="35.0 0.0 0.0" EndPointBack="35.0 40.0 0.0" />
        <Wire Name="2:L5_i5" EndPointFront="55.0 0.0 0.0" EndPointBack="40.0 40.0 0.0" />
        <Wire Name="3:M5_m5" EndPointFront="60.0 0.0 0.0" EndPointBack="60.0 40.0 0.0" />
      </Pattern>
      <Landmarks>
        <Landmark Name="#1" Position="104.3 5.0 20.0" />
        <Landmark Name="#2" Position="104.3 45.0 20.0" />
        <Landmark Name="#3" Position="104.3 45.0 0.0" />
        <Landmark Name="#4" Position="104.3 -5.0 0.0" />
        
        <Landmark Name="#5" Position="-34.3 45.0 15.0" />
        <Landmark Name="#6" Position="-34.3 -5.0 20.0" />
        <Landmark Name="#7" Position="-34.3 -5.0 0.0" />
        <Landmark Name="#8" Position="-34.3 45.0 0.0" />
      </Landmarks>      
      <Planes>
        <Plane Name="Superior" BasePoint="5.5 3.0 35.0" EndPoint1="135.0 3.0 35.0" EndPoint2="135.0 55.0 35.0" />
        <Plane Name="Inferior" BasePoint="5.5 55.0 0.0" EndPoint1="135.0 55.0 0.0" EndPoint2="135.0 5.5 0.0" />
        <Plane Name="Outside Unmarked" BasePoint="140.0 5.0 30.0" EndPoint1="140.0 50.0 30.0" EndPoint2="140.0 50.0 5.0" />
        <Plane Name="Inside Unmarked" BasePoint="125.0 40.0 30.0" EndPoint1="125.0 20.0 10.0" EndPoint2="125.0 40.0 10.0" />
        <Plane Name="Outside Marked" BasePoint="0.0 40.0 30.0" EndPoint1="0.0 40.0 5.5" EndPoint2="0.0 15.5 5.5" />
        <Plane Name="Outside Posterior" BasePoint="130.0 0.0 30.0" EndPoint1="20.0 0.0 30.0" EndPoint2="20.0 0.0 15.0" />
        <Plane Name="Inside Posterior" BasePoint="110.0 10.0 30.0" EndPoint1="40.0 10.0 30.0" EndPoint2="30.0 10.0 10.0" />
        <Plane Name="Outside Anterior" BasePoint="130.0 60.0 30.0" EndPoint1="20.0 60.0 30.0" EndPoint2="20.0 60.0 5.0" />
      </Planes>      
      <References>
        <Reference Name="#1" Position="104.3 5.0 20.0" />
        <Reference Name="#2" Position="104.3 45.0 20.0" />
        <Reference Name="#3" Position="104.3 45.0 0.0" />
        <Reference Name="#4" Position="104.3 -5.0 0.0" />
        <Reference Name="#5" Position="-34.3 45.0 15.0" />
        <Reference Name="#6" Position="-34.3 -5.0 20.0" />
        <Reference Name="#7" Position="-34.3 -5.0 0.0" />
        <Reference Name="#8" Position="-34.3 45.0 0.0" />
      </References>
    </Geometry>
  </PhantomDefinition>
<VolumeReconstruction OutputSpacing="0.5 0.5 0.5"
    ClipRectangleOrigin="0 0" ClipRectangleSize="820 616"
    Interpolation="LINEAR" Optimization="FULL" CompoundingMode="MEAN" FillHoles="OFF" />
  <fCal
    PhantomModelId="PhantomModel"
    ReconstructedVolumeId="Volume"
    TransducerModelId="ProbeModel"
    StylusModelId="StylusModel"
    ImageDisplayableObjectId="LiveImage"
    NumberOfCalibrationImagesToAcquire="200"
    NumberOfValidationImagesToAcquire="100"
    NumberOfStylusCalibrationPointsToAcquire="200"
    RecordingIntervalMs="100"
    MaxTimeSpentWithProcessingMs="70"
    ImageCoordinateFrame="Image"
    ProbeCoordinateFrame="Probe"
    ReferenceCoordinateFrame="Reference"
    TransducerOriginCoordinateFrame="TransducerOrigin"
    TransducerOriginPixelCoordinateFrame="TransducerOriginPixel"
    TemporalCalibrationDurationSec="10" 
    FixedChannelId="VideoStream" 
    FixedSourceId="Video"
    MovingChannelId="TrackerStream"
    MovingSourceId="ProbeToTracker"
    DefaultSelectedChannelId="TrackedVideoStream" 
    FreeHandStartupDelaySec="5" />
  <vtkPlusPivotCalibrationAlgo
    ObjectMarkerCoordinateFrame="Stylus"
    ReferenceCoordinateFrame="Reference"
    ObjectPivotPointCoordinateFrame="StylusTip" />
  <vtkPlusPhantomLandmarkRegistrationAlgo
    PhantomCoordinateFrame="Phantom"
    ReferenceCoordinateFrame="Reference"
    StylusTipCoordinateFrame="StylusTip" 
    DetectionTimeSec="1.0"
    StylusTipMaximumDisplacementThresholdMm="1.8" />
  <vtkPlusProbeCalibrationAlgo
    ImageCoordinateFrame="Image"
    ProbeCoordinateFrame="Probe"
    PhantomCoordinateFrame="Phantom"
    ReferenceCoordinateFrame="Reference" />
</PlusConfiguration>