Difference between revisions of "Quick Start Guide: Precision Capture"

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[[Category:Quick Start Guides]]
 
[[Category:Quick Start Guides]]
  [[OptiTrack Documentation Wiki|Back to the Main Page]] → [[Quick Start Guides|Back to the Quick Start Guides List]]
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  [[OptiTrack Documentation Wiki|Main Page]] → [[Quick Start Guides|Quick Start Guides]] → [[Quick Start Guide: Precision Capture| Precision Capture]]
 
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With an optimized system setup, motion capture systems can be used to obtain extremely accurate tracking data in a small to medium sized capture volume. This quick start guide covers instructions for optimizing the system setup for precision captures and the corresponding verification methods. This page also includes some important cautions to keep in mind. For more general instructions, please refer to the [[Quick Start Guide: Getting Started]] or corresponding workflow pages. Before going into details on precision tracking with an OptiTrack system, let's start with a brief explanation on the residual value, which is the key [[Data Types#Reconstruction|reconstruction]] output for monitoring precision.
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With an optimized system setup, motion capture systems are capable of obtaining extremely accurate tracking data from a small to medium sized capture volume. This quick start guide includes general tips and suggestions on precision capture system setups and important cautions to keep in mind. This page also covers some of the precision verification methods in Motive. For more general instructions, please refer to the [[Quick Start Guide: Getting Started]] or corresponding workflow pages.
  
  
==Residual Value==
+
=Residual Value=
 +
----
 +
<div class="padded">
 +
Before going into details on precision tracking with an OptiTrack system, let's start with a brief explanation of the residual value, which is the key [[Data Types#Reconstruction|reconstruction]] output for monitoring the system precision.The [[Reconstruction Settings#Residual_.28mm.29|residual]] value is an average offset distance between the converging rays when reconstructing a marker; hence indicating preciseness of the reconstruction. A smaller residual value means that the tracked rays converge more precisely and achieve more accurate 3D reconstruction. A well-tracked marker will have a sub-millimeter average residual value. In Motive, the tolerable residual distance is defined from the [[Application Settings: Live Reconstruction|Reconstruction Settings]] under the Application Settings panel.
  
The [[Reconstruction#Options|'''residual''']] value is an average offset distance between the converging rays when reconstructing a marker; hence indicating preciseness of the reconstruction. The tolerable residual distance is defined from the [[Reconstruction]] pane. When one or more markers are selected in the Live mode or from a recorded 2D data, the corresponding mean residual value is displayed over the [[Status Bar]] located at the bottom of Motive. The residual values can also be displayed on the 3D viewport if ''Show Selected Residual'' is set to true in [[Application Settings#Ray Options|Application Settings]]. A smaller residual value means that the tracked rays converge more precisely and more accurate 3D reconstruction is achieved. A well-tracked marker will have a sub-millimeter average residual value.
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When one or more markers are selected in the Live mode or from the [[Reconstruction and 2D Mode#2D Mode|2D Mode]] of capture data, the corresponding mean residual value is displayed over the [[Status Panel]] located at the bottom-right corner of Motive.  
  
<center>[[Image:MVQSG_Residual1.png|300 px]] [[Image:MVQSG_Residual2.png|320 px]]</center>
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<center>
 +
<li style="display: inline-block; vertical-align:top;">[[Image:MVQSG_Residual1.png|thumb|300 px|Multiple tracked rays contributing to a 3D reconstruction.]]</li>
 +
<li style="display: inline-block; vertical-align:top;">[[Image:MVQSG_Residual2.png|thumb|320 px|Residual offsets at the converging point of the multiple rays.]]</li>
 +
</center>
 +
</div>
  
 +
=Capture Volume=
 +
----
  
==Capture Volume==
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{{Indent|
First of all, optimize the capture volume for most precise and accurate tracking results. Avoid populated area when setting up the system and recording a capture. Clear any obstacles or trip hazards around the capture volume. Physical contact on the setup will distort the calibration quality which could be critical especially when tracking at a sub-millimeter accuracy. Lastly, for best results, routinely recalibrate the capture volume.
+
First of all, optimize the capture volume for most precise and accurate tracking results. Avoid populated area when setting up the system and recording a capture. Clear any obstacles or trip hazards around the capture volume. Physical impacts on the setup will distort the calibration quality, and it could be critical especially when tracking at a sub-millimeter accuracy. Lastly, for best results, routinely recalibrate the capture volume.
  
 +
==Infrared Black Background Objects==
  
===Infrared Black Background Objects===
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{{Indent|
 +
Motion capture cameras detect reflected infrared light. Thus, having other reflective objects in the volume will alter the results negatively, which could be critical especially for precise tracking applications. If possible, have background objects that are IR black and non-reflective. Capturing in a dark background provides clear contrast between bright and dark pixels, which could be less distinguishable in a white background.}}
  
Motion capture cameras detect reflected infrared light. Thus, having other reflective objects in the volume will alter the results negatively, which could be critical especially for precise tracking applications. If possible, have background objects that are IR black and non-reflective. Capturing in a dark background provides better contrast between bright and dark pixels, which could be less distinguishable in a white background. The following images show a clear image of a marker with good contrast (left) and a less clear marker (right) whose centroid calculation may have been compromised by an extraneous bright pixels from the background.
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[[Image:MVQSG_IRBlack.png|center|thumb|700 px|The images shows a clear (left) and a less clear(right) image of a marker whose centroid calculation may have been compromised by an extraneous bright pixels from the background.]]
 +
}}
  
<center>[[Image:MVQSG_IRBlack.png|300 px]] [[Image:MVQSG_IRBlack2.png|295 px]]</center>
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=Camera Placement=
 +
----
 +
<div style="float:right"><ul>
 +
<li style="display: inline-block; vertical-align:top;">[[Image:MVQSG_Placement3.png|thumb|280px|Capturing multiple unique vantages measure more accurate positions on all of the coordinate axis]]</li>
 +
<li style="display: inline-block; vertical-align:top;">[[Image:MVQSG_Placement4.png|thumb|250px|Cameras placed in a dome arrangement around a cubic meter volume.]]</li>
 +
</ul></div>
  
 +
<div class="padded">
 +
Optimized camera placement techniques will greatly improve the tracking result and the measurement accuracy. The following guide highlights important setup instructions for the small volume tracking. For more details on general system setup, read through the [[Hardware Setup]] pages.
  
==Camera Placement==
+
====Mounting Locations====
[[Image:MVQSG_Placement4.png|thumb|250px|Cameras placed in a dome arrangement around a cubic meter volume.]]
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:For precise tracking, better results will be obtained by placing cameras closer to the target object (adjusting focus will be required) in a sphere or dome-shaped camera arrangement, as shown in the images on the right. Good positional data in all dimensions (X, Y, and Z axis) will be attained only if there are cameras contributing to the calculation from a variety of different locations; each unique vantage adds additional data.
[[Image:MVQSG_Placement3.png|thumb|250px|Capturing multiple unique vantages measure more accurate positions on all of the coordinate axis]]
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Proper camera placement techniques can greatly improve the tracking result and the measurement accuracy. The following guide highlights important setup instructions for the small volume tracking. For more details on general system setup, read through the [[Hardware Setup]] pages.
+
  
 +
====Mount Securely====
 +
:For most accurate results, cameras should be perfectly stationary, securely fastened onto a truss system or an extremely rigid object. Any slight deformation or fluctuation to the mount structures may affect the result in sub-millimeter tracking applications. A small-sized truss system is ideal for the setup. Take extreme caution when mounting onto speed rails attached to a wall, because the building may fluctuate on hot days.
  
===Mounting Locations===
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<div class="padded">
 +
<li style="display: inline-block; vertical-align:top;">[[Image:CameraMount_Manfrotto.png|thumb||Manfrotto clamp used to mount cameras.]]</li>
 +
<li style="display: inline-block; vertical-align:top;">[[Image:Mount_Clamp2.png|thumb|200 px|Camera mounted onto a truss structure using the clamp.]]</li>
 +
</div></div>
  
For precise tracking, better results will be obtained by placing the cameras closer to the target object (adjusting focus will be required) in a sphere or dome shaped camera arrangement as shown in the images to the right. Good positional data in all dimensions (X, Y, and Z axis) will only happen if there are cameras contributing to the calculation from a variety of different locations; each unique perspective adds additional data that wasn't there before.
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=Focus and Aiming=
 
+
----
 
+
<div class="padded">
===Mount Securely===
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[[Image:FnA_InFocus.png|thumb|500 px|Grayscale images of a marker captured with varying camera focus]]
 
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For most accurate results, cameras should be securely fastened onto a truss system or an extremely rigid object. In sub-millimeter tracking applications, any slight deformation or fluctuation to the mount structures may affect the result. Small size truss system is ideal for the setup. Take extreme caution when using speed rails mounted onto a wall, because building may fluctuate greatly especially on hot days.
+
 
+
<center>[[Image:CameraMount_Manfrotto.png]]  [[Image:Mount_Clamp2.png|250 px]]</center>
+
 
+
 
+
==Focus and Aiming==
+
 
+
[[Image:FnA_InFocus.png|thumb|500 px]]
+
  
 
====F-stop====
 
====F-stop====
Increase the f-stop to a higher number (smaller aperture) to gain a larger depth of field. Increased depth of field will make greater portion of the capture volume in-focus and will make measurements more consistent throughout the volume.
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{{Indent|
 +
Increase the f-stop higher (smaller aperture) to gain a larger depth of field. Increased depth of field will make the greater portion of the capture volume in-focus and will make measurements more consistent throughout the volume.
 +
}}
  
 
====Aim and Focus====
 
====Aim and Focus====
Especiallly for precise and close-up captures, cameras aim and focus should be adjusted as perfectly as possible. Aim the cameras towards the center of the capture volume. Optimize the camera focus by zooming into a marker in Motive, and rotating the focus knob on the camera until the smallest marker is captured as clearly as possible with the best image contrast.
+
{{Indent|
 +
Especially for close-up captures, camera aim and focus should be adjusted precisely. Aim the cameras towards the center of the capture volume. Optimize the camera focus by zooming into a marker in Motive, and rotating the focus knob on the camera until the smallest marker is captured with clearest image contrast.
  
For more information on adjusting the camera aiming and focus, please read through the [[Aiming and Focusing]] workflow page.
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For more information, please read through the [[Aiming and Focusing]] workflow page.
 +
}}
 +
</div>
  
 +
=Motive Settings=
 +
----
  
==Motive Settings==
+
{{Indent|
The following sections cover key configuration settings which may need to be optimized for the precision tracking.
+
The following sections cover key configuration settings which need to be optimized for the precision tracking.
  
 
===Cameras Pane Settings===
 
===Cameras Pane Settings===
To open the cameras pane, click the camera icon at the main tool bar in Motive.
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{{Indent|
 +
To open the cameras pane, click the camera icon on the [[Toolbar|main toolbar]] in Motive.}}}}
  
{| class="wikitable" style = "margin:auto"
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{| class="wikitable" style = "width:80%; margin:auto"
 
!style ="width:10%; padding:5px"|Setting
 
!style ="width:10%; padding:5px"|Setting
 +
!width = 10%|Value
 
!Description
 
!Description
 
|-
 
|-
|'''Gain'''
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|Gain
|Set the Gain setting to 1: Low (Short Range) for all cameras. Higher gain settings will amplify noise in the image.
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|align ="center"|1: Low (Short Range)
 +
|Set the Gain setting to low for all cameras. Higher gain settings will amplify noise in the image.
 
|-
 
|-
|'''Frame Rate'''
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|Frame Rate
 +
|align ="center"|Maximum FPS
 
|Set the system frame rate (FPS) to its maximum value. If you wish to use slower frame rate, use the maximum frame rate during calibration and turn it down for the actual recording.  
 
|Set the system frame rate (FPS) to its maximum value. If you wish to use slower frame rate, use the maximum frame rate during calibration and turn it down for the actual recording.  
 
|-
 
|-
|'''Threshold (THR)'''
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|Threshold (THR)
|rowspan= "2;"|Do not bother changing the Threshold (THR) or LED values. Keep them at their default values THR = 200 and LED = 15. The Values EXP and LED are linked so change only the EXP setting for brighter images. If you turn the EXP higher than 250, make sure to wand extra slow to avoid blurred markers.
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|align ="center"|200
 +
|rowspan= "2;"|Do not bother changing the Threshold (THR) or LED values, keep them at their default settings. The Values EXP and LED are linked so change only the EXP setting for brighter images. If you turn the EXP higher than 250, make sure to wand extra slow to avoid blurred markers.
 
|-
 
|-
|'''IR LED'''
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|IR LED
 +
|align ="center"|15
 
|-
 
|-
|'''Camera Exposure'''
+
|Exposure (EXP)
|For the precision capture, it is not always necessary to set the camera exposure to its lowest value, but instead, the exposure setting should be configured so that the reconstruction is most stable. To test this, zoom into a marker and examine the jitters while changing the exposure setting. Use the exposure value that give the most stable reconstruction. Later sections will cover how to check the reconstruction and tracking quality. For now, set this number as low as possible while maintaining the tracking without losing the contrast. (you want them to look like white circles).
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|align ="center"|Most stable
 +
|For the precision capture, it is not always necessary to set the camera exposure to its lowest value. Instead, the exposure setting should be configured so that the reconstruction is most stable. Zoom into a marker and examine the jitters while changing the exposure setting, and use the exposure value that gives the most stable reconstruction. Later sections will cover how to check the reconstruction and tracking quality. For now, set this number as low as possible while maintaining the tracking without losing the contrast of the reflections.
 
|}
 
|}
  
 +
{{Indent|
 
===Optimizing Reconstruction Settings===
 
===Optimizing Reconstruction Settings===
To open the cameras pane click View > Reconstruction Settings the top of Motive. Read through the [[Reconstruction]] page for specific details. For the precision tracking, important reconstruction settings and the appropriate values are listed below:
+
{{Indent|
 +
To open the cameras pane click [[Command_Bar#View|View]] > Reconstruction Settings on the command bar. Read through the [[Reconstruction Settings]] page for details on each setting. For the precision tracking, important reconstruction settings and the appropriate values are listed below:}}}}
  
{| class="wikitable" style = "margin:auto"
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{| class="wikitable" style = "width:80%; margin:auto"
 
!Setting
 
!Setting
 
!width = 10%|Value
 
!width = 10%|Value
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|-
 
|-
 
|Residual (mm)
 
|Residual (mm)
|<2.00
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|align ="center"|&lt; 2.00
|Allowable residual value should be set smaller for the precision volume tracking, because any offset above 2.00 mm residual value will be considered as inaccurate.
+
|Set the allowable [[Reconstruction Settings#Residual_.28mm.29|residual]] value smaller for the precision volume tracking. Any offset above 2.00 mm will be considered as inaccurate, and the corresponding 2D data will be excluded from reconstruction contribution.
 
|-
 
|-
 
|Minimum Rays
 
|Minimum Rays
|>= 3
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|align ="center"|&ge; 3
|When more rays converge within allowable residual offset, reconstructed marker will have more accurate position.
+
|Set the minimum required number of rays higher. More accurate reconstruction will be achieved when more rays converge within the allowable residual offset.
 
|-
 
|-
|Minimum threshold pixels
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|Minimum Thresholded Pixels
|>= 4
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|align ="center"|&ge; 4
|Since cameras are placed more close to the tracked markers, the minimum number of threshold pixels can be increased to filter out extraneous small reflections if needed.  
+
|Since cameras are placed more close to the tracked markers, each marker will appear bigger in camera views. The minimum number of threshold pixels can be increased to filter out small extraneous reflections if needed.  
 
|-
 
|-
 
|Circularity
 
|Circularity
|>= 0.6
+
|align ="center"|&ge; 0.6
|Increasing the circularity value allows discarding non-marker reflections. Furthermore, it prevents collecting data from [[#Marker Placement|merged reflections]] where the centroid data is no longer reliable.
+
|Increasing the circularity value will filter out non-marker reflections. Furthermore, it prevents collecting data from [[#Marker Placement|merged reflections]] where the calculated centroid is no longer reliable.
 
|}
 
|}
  
 +
=Calibration=
 +
----
  
 
+
{{Indent|
==Calibration==
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The following calibration instructions are specific to precision tracking. For more general information, refer to the [[Calibration]] page.
 
The following calibration instructions are specific to precision tracking. For more general information, refer to the [[Calibration]] page.
  
===Wands===
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==Wands==
For calibrating small capture volumes for precision tracking, we recommend using a Micron Series wand, either the CWM-250 or CWM-125. These wands are made of invar alloy, very rigid and insensitive to temperature, and they are designed for providing a precise and constant reference dimension for the calibration process. At the bottom of the wand head, there is a label tag which shows a factory-calibrated wand length with sub-millimeter accuracy. For best results, input this information into Motive. In the [[Calibration Pane]], select Micron Series under the OptiWand dropdown menu, and define the exact length in the wand length section.
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{{Indent|
 
+
For calibrating small capture volumes for precision tracking, we recommend using a Micron Series wand, either the CWM-250 or CWM-125. These wands are made of invar alloy, very rigid and insensitive to temperature, and they are designed to provide a precise and constant reference dimension during calibration. At the bottom of the wand head, there is a label which shows a factory-calibrated wand length with a sub-millimeter accuracy. In the [[Calibration_Pane#Calibration|Calibration pane]], select Micron Series under the [[Calibration_Pane#Calibration|OptiWand]] dropdown menu, and define the exact length under the ''Wand Length''.}}}}
 
+
<center>[[Image:MVQSG_Wandlabel.png|500 px]] [[Image:MVQSG_WandCalib.png|250 px]]</center>
+
  
 +
<div class="padded"><center><ul>
 +
<li style="display: inline-block; vertical-align: text-top;">[[Image:MVQSG_Wandlabel.png|530 px|thumb|Factory calibrated wand length indicated on the bottom label of the CMW-125]]</li>
 +
<li style="display: inline-block; vertical-align: text-top;">[[Image:MVQSG_WandCalib.png|250 px|thumb|Define the precise wand length in the [[Calibration Pane]]]]</li>
 +
</ul></center></div>
 +
{{Indent|
 +
{{Indent|
  
The CW-500 wand is designed for medium to large capture volumes, and it is not suited for calibrating the micron volume. It does not have a sticker stating the exact dimension. Also, the wand is made with aluminum, which makes it more vulnerable to thermal expansions. During the wanding process, Motive references the defined wand length for calibrating the capture volume. Any distortions in the wand length would cause the calibrated capture volume to be scaled slightly differently, which can be significant when capturing precise measurements. For this reason, a micron series wand should be used for precision tracking applications.
+
The CW-500 wand is designed for capturing medium to large volumes, and it is not suited for calibrating small volumes. Not only it does not have the indication on the factory-calibrated length, but it is also made of aluminum, which makes it more vulnerable to thermal expansions. During the wanding process, Motive references the wand length for calibrating the capture volume, and any distortions in the wand length would cause the calibrated capture volume to be scaled slightly differently, which can be significant when capturing precise measurements. For this reason, a micron series wand is suitable for precision tracking applications.
  
 
Note: '''Never''' touch the marker on the CWM-250 or CWM-125 since any changes can affect the calibration and overall data.
 
Note: '''Never''' touch the marker on the CWM-250 or CWM-125 since any changes can affect the calibration and overall data.
  
 +
{{Info|'''Precision Capture Calibration Tips'''
  
{{Tip|'''Precision Capture Calibration Tips'''
+
*'''Wand slowly'''. Waving the wand around quickly at high exposure settings will blur the markers and distort the centroid calculations, at last, reducing the quality of your calibration.
  
*'''Wand slowly'''. Waving the wand around quickly at high exposures will blur the markers and distort the centroids reducing the quality of your calibrations.
+
*'''Avoid occluding any of the calibration markers while wanding.''' Occluding markers will reduce the quality of the calibration.
  
*'''Try not to occlude any of the calibration markers while wanding.''' Occluding markers will reduce the quality of the calibration.
+
*'''A variety of unique samples is needed to achieve a good calibration.''' Wand in a three-dimensional volume, wave the wand in a variety of orientations and throughout the volume.
  
*'''A variety of unique samples is needed in order to achieve a good calibration.''' For example, if you wand only in horizontal orientation, the calibration may fail. So make sure to wand in a three dimensional volume, put the wand in a variety of orientations, and wand the edges of the volume as well.
+
*Extra wanding in the target area you wish to capture will improve the tracking in the target region.
  
*Extra wanding in the target area you wish to capture will improve the tracking in that region.
+
*Wanding the edges of the volume helps improve the lens distortion calculations. This may cause Motive to report a slightly worse overall calibration report, but will provide better quality calibration; explained below.
  
*Wanding the edges of the volume helps improve the lens distortion calculations. This may cause Motive to report a slightly worse calibration report, but get a better calibration.
+
*Starting/stopping the calibration process with the wand in the volume may help avoid getting rough samples outside your volume when entering and leaving.}}
 
+
*Starting/stopping the calibration process with the wand in the volume may help avoid getting rouge samples outside your volume when entering and leaving.
+
 
}}
 
}}
  
====Calibration Results====
 
Calibration reports and analyzing the reported error is a complicated subject because the calibration can only use its own samples for validation. For example, sampling near the edge of the volume may improve the accuracy of the system, but cause Motive to report slightly worse calibration results. This is because the samples near the edge will have more errors to be corrected. Acceptable mean error varies based on your volume size, number of cameras, and desired accuracy. The key metrics to keep an eye on are the Mean 3D Error for the ''Overall Reprojection'' and the ''Wand Error''. For several of our tests we only used calibrations with the Mean 3D Error less than 0.80 mm and the Wand Error less than 0.030 mm. These numbers may be hard to reproduce in regular volumes. Again, the acceptable numbers are subjective, but lower numbers are better in general.
 
  
Note: Calibrations may fail because there are not enough unique samples, such as only wanding in a plane or never rotating your wand. This happens more commonly when wanding small volumes because it is hard to reach everywhere.
+
==Calibration Results==
 +
{{Indent|
 +
Calibration reports and analyzing the reported error is a complicated subject because the calibration process uses its own samples for validation. For example, sampling near the edge of the volume may improve the accuracy of the system but provide slightly worse calibration results. This is because the samples near the edge will have more errors to be corrected. Acceptable mean error varies based on the size of your volume, the number of cameras, and desired accuracy. The key metrics to keep an eye on are the Mean 3D Error for the ''Overall Reprojection'' and the ''Wand Error''. Generally, use calibrations with the Mean 3D Error less than 0.80 mm and the Wand Error less than 0.030 mm. These numbers may be hard to reproduce in regular volumes. Again, the acceptable numbers are subjective, but lower numbers are better in general.}}
 +
}}
  
  
==Attention: Marker Placement==
+
=Tracking=
[[Image:MVQSG_Optimization4.png|right|300 px|Two markers are placed too close to each other and their reflections are getting merged.]]
+
----
 +
<div class="padded">
 +
==Marker Type==
 +
<div class="padded">
 +
In general, passive retro-reflective markers will provide better tracking accuracy. The boundary of the spherical marker can be more clearly distinguished on passive markers, and the system can identify an accurate position of the marker centroids. The active markers, on the other hand, emit light and the illumination may not appear as spherical on the camera view. Even if a spherical diffuser is used, there can be situations where the light is not evenly distributed. This could provide inaccurate centroid data. Furthermore, the [[Active Marker Tracking|active markers]] that illuminates in pattern may involve some amount of interpolation on the off-frames, which is not preferred for precision tracking applications. For this reason, passive markers are preferred for precision tracking applications.
 +
</div>
  
For close-up capture, it could be inevitable to place the markers close to each other. When markers are placed too close to each other, their reflections may merge in the camera’s imager. Merged reflections will have an inaccurate centroid locations, or they may even be completely discarded by the [[Reconstruction#Circularity|circularity filter]] or the [[Reconstruction##Intrusion_Detection|intrusion detection]] feature. Optimize both the circularity and the intrusion detection settings to make sure only relevant reflections are reconstructed. The optimal balance will depend on the number and arrangement of the cameras in the setup.
+
==Marker Placement==
 +
<div class="padded">
 +
[[Image:MVQSG_Optimization4.png|thumb|280 px|Two markers are placed too close to each other and their reflections are getting merged.]]
 +
 
 +
For close-up capture, it could be inevitable to place markers close to one another, and when markers are placed in close vicinity, their reflections may be merged as seen by the camera’s imager. Merged reflections will have an inaccurate centroid location, or they may even be completely discarded by the [[Device pane#Circularity|circularity filter]] or the [[Device pane#Intrusion Band|intrusion detection]] feature. For best results, keep the circularity filter at a higher setting (>0.6) and decrease the intrusion band in the camera group [[Devices_pane#Device_Properties:_Tracking_Group|filter settings]] to make sure only relevant reflections are reconstructed. The optimal balance will depend on the number and arrangement of the cameras in the setup.
  
 
There are editing methods to discard or modify the missing data. However, for most reliable results, such marker intrusions should be prevented before the capture by separating the marker placements or by optimizing the camera placements.
 
There are editing methods to discard or modify the missing data. However, for most reliable results, such marker intrusions should be prevented before the capture by separating the marker placements or by optimizing the camera placements.
 +
</div>
  
 +
==Refine Rigid Body Definition==
 +
<div class="padded">
 +
Once a rigid body is defined from a set of reconstructed points, utilize the Rigid Body Refinement feature to further refine the rigid body definition for precision tracking. The tool allows Motive to collect additional samples in the live mode for achieving more accurate tracking results.
  
==Attention: Temperature==
+
See: [[Rigid Body Tracking#Rigid Body Refinement|Rigid Body Refinement]]
 +
{{#ev:youtube|WeAbjbSM6_Y|500|center|Using the RigidBody Refinement tool for improving asset definitions.|frame}}
 +
</div>
 +
</div>
 +
 
 +
=Attention: Temperature=
 +
----
 +
<div class="padded">
 
[[Image:Camera_Temp.png|thumb|300px|Temperature of the processor board and the ringlight board displayed in the camera info.]]
 
[[Image:Camera_Temp.png|thumb|300px|Temperature of the processor board and the ringlight board displayed in the camera info.]]
In a mocap system, camera mount structures and other hardware components may be affected by temperature fluctuations. Refer to linear thermal expansion coefficient tables to find out what materials are susceptible to temperature changes. For example, aluminum has relatively high thermal expansion coefficient, and therefore, you have to be cautious when mounting cameras onto aluminum mounting structures. For best accuracy, routinely recalibrate the capture volume, and take the temperature fluctuation into an account both when selecting the mount structures and before collecting data.
+
In a mocap system, camera mount structures and other hardware components may be affected by temperature fluctuations. Refer to linear thermal expansion coefficient tables to examine which materials are susceptible to temperature changes. Avoid using a temperature sensitive material for mounting the cameras. For example, aluminum has relatively high thermal expansion coefficient, and therefore, mounting cameras onto aluminum mounting structures may distort the calibration quality. For best accuracy, routinely recalibrate the capture volume, and take the temperature fluctuation into an account both when selecting the mount structures and before collecting data.
  
====Ambient Temperature====
+
==Ambient Temperature==
An ideal way to avoid the ambient temperature influence is to install in a temperature controlled volume. If such option is not available, there are few considerations to keep in mind. First of all, routinely calibrate the volume before capture, and recalibrate the volume in between sessions when capturing for a long period of time. Place the truss or tripods on a rigid surface such as concrete; carpets or rugs should be avoided. Wall mounts can also be affected by temperature changes because many buildings fluctuate slightly with changing outside temperature. The changes are especially noticeable on hot days and will significantly affect your results. If these situations are unavoidable, consistently monitor the average residual value for how well your rays converge to individual markers.
+
{{Indent|
 +
An ideal method of avoiding influence from environmental temperature is to install the system in a temperature controlled volume. If such option is unavailable, routinely calibrate the volume before capture, and recalibrate the volume in between sessions when capturing for a long period. The effects are especially noticeable on hot days and will significantly affect your results. Thus, consistently monitor the average residual value and how well your rays converge to individual markers.
 +
}}
  
====Camera Heat====
+
==Camera Heat==
The cameras will heat up with extended use, and change in internal hardware temperature may also affect the capture data. For this reason, avoid capturing and/or calibrating right after powering the system. Tests have found that the cameras need to be warmed up in Live mode for about an hour until it reaches a stable temperature. For Ethernet camera models, camera temperatures can be monitored from the Camera Preview (2D) pane in Motive (Camera Preview (2D) pane > Eye Icon > Camera Info).
+
{{Indent|
 +
The cameras will heat up with extended use, and change in internal hardware temperature may also affect the capture data. For this reason, avoid capturing or calibrating right after powering the system. Tests have found that the cameras need to be warmed up in Live mode for about an hour until it reaches a stable temperature. For Ethernet camera models, camera temperatures can be monitored from the [[View Pane#Camera Preview (2D)|Camera Preview (2D)]] pane in Motive (Camera Preview (2D) pane > Eye Icon > Camera Info).
 +
}}
 +
</div>
  
 +
=Attention: Vibrations=
 +
----
  
==Attention: Vibrations==
+
{{Indent|
Avoid any vibrations or movements of the setup while taking precision measurements. Especially for measuring at sub-millimeters, even a minimal shift can affect the recordings. Re-calibrate the capture volume if your average residual values start to deviate. In particular, watch out for the following:
+
Especially for measuring at sub-millimeters, even a minimal shift of the setup can affect the recordings. Re-calibrate the capture volume if your average residual values start to deviate. In particular, watch out for the following:
  
*Avoid touching the truss or mounting device for the cameras.
+
{{Indent|
 +
*Avoid touching the cameras and the camera mounts.
  
 
*Keep the capture area away from heavy foot traffic. People shouldn't be walking around the volume while capture is taking place.
 
*Keep the capture area away from heavy foot traffic. People shouldn't be walking around the volume while capture is taking place.
  
 
*Closing doors, even from the outside, may be noticeable during recording.
 
*Closing doors, even from the outside, may be noticeable during recording.
 +
}}}}
  
==Reconstruction Verification==
 
The following methods can be used to check the tracking accuracy and to better optimize the reconstructions settings in Motive.
 
  
 +
=Reconstruction Verification=
 +
----
 +
 +
{{Indent|
 +
The following methods can be used to check the tracking accuracy and to better optimize the [[Reconstruction Settings|reconstructions settings]] in Motive.
 +
 +
{{Indent|
 
===Verification Method 1===
 
===Verification Method 1===
First, go into the [[View_pane#Perspective_View_(3D)|perspective pane]] > select a marker, then go to the [[View_pane#Camera_Preview_(2D)|Camera Preview pane]] > Eye Button [[Image:Viewport16.png]] > Set Marker Centroids: True. Make sure your cameras are in the object mode, then zoom into the selected marker. The marker will have two crosshairs on it; one white and one yellow. The amount of offset between the crosshairs will give you an idea of how closely the view of that camera (white) aligns with the reconstructed position (yellow). Switching between the grayscale mode and the object mode will make the errors more distinguishable. The below image is an example of a poor calibration. A good calibration should have the yellow and white lines closely aligning over each other.  
+
{{Indent|
 +
First, go into the [[View_pane#Perspective_View_(3D)|perspective view pane]] > select a marker, then go to the [[View_pane#Camera_Preview_(2D)|Camera Preview pane]] > Eye Button [[Image:Viewport16.png]] > Set Marker Centroids: True. Make sure the cameras are in the object mode, then zoom into the selected marker in the 2D view. The marker will have two crosshairs on it; one white and one yellow. The amount of offset between the crosshairs will give you an idea of how closely the calculated 2D centroid location (white) aligns with the reconstructed position (yellow). Switching between the grayscale mode and the object mode will make the errors more distinguishable. The below image is an example of a poor calibration. A good calibration should have the yellow and white lines closely aligning over each other.}}
 +
 
 +
[[Image:MVQSG_Optimization1.png|thumb|700 px|center|Calculated 2D centroid location from the camera's perspective (white) and centroid location derived from reconstructed marker position (yellow).]]
  
<center>[[Image:MVQSG_Optimization1.png|700 px]]</center>
 
  
 
===Verification Method 2===
 
===Verification Method 2===
The calibration quality can also be analyzed through checking the convergence of the tracked rays into a marker. This is not as precise as the first method, but the tracked rays can be used to check calibration quality of multiple cameras at once. First of all, make sure tracked rays are visible; Perspective View pane > Eye button > Tracked Rays. Then, open the perspective view pane and select a marker. Zoom all the way into the marker (you may need to zoom into the sphere), and you will be able to see the tracking rays (green) converging into the center of the marker. A good calibration should have all the rays converging into approximately one point, as shown in the following image.
+
{{Indent|
 +
The calibration quality can also be analyzed through checking the convergence of the tracked rays into a marker. This is not as precise as the first method, but the tracked rays can be used to check calibration quality of multiple cameras at once. First of all, make sure tracked rays are visible; [[View pane#Perspective View (3D)|Perspective View pane]] > Eye button > Tracked Rays. Then, select a marker in the perspective view pane. Zoom all the way into the marker (you may need to zoom into the sphere), and you will be able to see the tracking rays (green) converging into the center of the marker. A good calibration should have all the rays converging into approximately one point, as shown in the following image. Essentially, this is a visual way of examining the average residual offset of the converging rays.}}
  
 
+
[[Image:MVQSG_Optimization2.png|thumb|700 px|center|Monitoring convergence of the tracked rays.]]
<center>[[Image:MVQSG_Optimization2.png|700 px]]</center>
+
}}
 +
}}

Latest revision as of 14:06, 1 June 2018

Main PageQuick Start Guides Precision Capture


With an optimized system setup, motion capture systems are capable of obtaining extremely accurate tracking data from a small to medium sized capture volume. This quick start guide includes general tips and suggestions on precision capture system setups and important cautions to keep in mind. This page also covers some of the precision verification methods in Motive. For more general instructions, please refer to the Quick Start Guide: Getting Started or corresponding workflow pages.


Residual Value[edit]


Before going into details on precision tracking with an OptiTrack system, let's start with a brief explanation of the residual value, which is the key reconstruction output for monitoring the system precision.The residual value is an average offset distance between the converging rays when reconstructing a marker; hence indicating preciseness of the reconstruction. A smaller residual value means that the tracked rays converge more precisely and achieve more accurate 3D reconstruction. A well-tracked marker will have a sub-millimeter average residual value. In Motive, the tolerable residual distance is defined from the Reconstruction Settings under the Application Settings panel.

When one or more markers are selected in the Live mode or from the 2D Mode of capture data, the corresponding mean residual value is displayed over the Status Panel located at the bottom-right corner of Motive.

  • Multiple tracked rays contributing to a 3D reconstruction.
  • Residual offsets at the converging point of the multiple rays.
  • Capture Volume[edit]


    First of all, optimize the capture volume for most precise and accurate tracking results. Avoid populated area when setting up the system and recording a capture. Clear any obstacles or trip hazards around the capture volume. Physical impacts on the setup will distort the calibration quality, and it could be critical especially when tracking at a sub-millimeter accuracy. Lastly, for best results, routinely recalibrate the capture volume.

    Infrared Black Background Objects

    Motion capture cameras detect reflected infrared light. Thus, having other reflective objects in the volume will alter the results negatively, which could be critical especially for precise tracking applications. If possible, have background objects that are IR black and non-reflective. Capturing in a dark background provides clear contrast between bright and dark pixels, which could be less distinguishable in a white background.

    The images shows a clear (left) and a less clear(right) image of a marker whose centroid calculation may have been compromised by an extraneous bright pixels from the background.

    Camera Placement[edit]


    • Capturing multiple unique vantages measure more accurate positions on all of the coordinate axis
    • Cameras placed in a dome arrangement around a cubic meter volume.

    Optimized camera placement techniques will greatly improve the tracking result and the measurement accuracy. The following guide highlights important setup instructions for the small volume tracking. For more details on general system setup, read through the Hardware Setup pages.

    Mounting Locations[edit]

    For precise tracking, better results will be obtained by placing cameras closer to the target object (adjusting focus will be required) in a sphere or dome-shaped camera arrangement, as shown in the images on the right. Good positional data in all dimensions (X, Y, and Z axis) will be attained only if there are cameras contributing to the calculation from a variety of different locations; each unique vantage adds additional data.

    Mount Securely[edit]

    For most accurate results, cameras should be perfectly stationary, securely fastened onto a truss system or an extremely rigid object. Any slight deformation or fluctuation to the mount structures may affect the result in sub-millimeter tracking applications. A small-sized truss system is ideal for the setup. Take extreme caution when mounting onto speed rails attached to a wall, because the building may fluctuate on hot days.
  • Manfrotto clamp used to mount cameras.
  • Camera mounted onto a truss structure using the clamp.
  • Focus and Aiming[edit]


    Grayscale images of a marker captured with varying camera focus

    F-stop[edit]

    Increase the f-stop higher (smaller aperture) to gain a larger depth of field. Increased depth of field will make the greater portion of the capture volume in-focus and will make measurements more consistent throughout the volume.

    Aim and Focus[edit]

    Especially for close-up captures, camera aim and focus should be adjusted precisely. Aim the cameras towards the center of the capture volume. Optimize the camera focus by zooming into a marker in Motive, and rotating the focus knob on the camera until the smallest marker is captured with clearest image contrast.

    For more information, please read through the Aiming and Focusing workflow page.

    Motive Settings[edit]


    The following sections cover key configuration settings which need to be optimized for the precision tracking.

    Cameras Pane Settings

    To open the cameras pane, click the camera icon on the main toolbar in Motive.

    Setting Value Description
    Gain 1: Low (Short Range) Set the Gain setting to low for all cameras. Higher gain settings will amplify noise in the image.
    Frame Rate Maximum FPS Set the system frame rate (FPS) to its maximum value. If you wish to use slower frame rate, use the maximum frame rate during calibration and turn it down for the actual recording.
    Threshold (THR) 200 Do not bother changing the Threshold (THR) or LED values, keep them at their default settings. The Values EXP and LED are linked so change only the EXP setting for brighter images. If you turn the EXP higher than 250, make sure to wand extra slow to avoid blurred markers.
    IR LED 15
    Exposure (EXP) Most stable For the precision capture, it is not always necessary to set the camera exposure to its lowest value. Instead, the exposure setting should be configured so that the reconstruction is most stable. Zoom into a marker and examine the jitters while changing the exposure setting, and use the exposure value that gives the most stable reconstruction. Later sections will cover how to check the reconstruction and tracking quality. For now, set this number as low as possible while maintaining the tracking without losing the contrast of the reflections.

    Optimizing Reconstruction Settings

    To open the cameras pane click View > Reconstruction Settings on the command bar. Read through the Reconstruction Settings page for details on each setting. For the precision tracking, important reconstruction settings and the appropriate values are listed below:

    Setting Value Description
    Residual (mm) < 2.00 Set the allowable residual value smaller for the precision volume tracking. Any offset above 2.00 mm will be considered as inaccurate, and the corresponding 2D data will be excluded from reconstruction contribution.
    Minimum Rays ≥ 3 Set the minimum required number of rays higher. More accurate reconstruction will be achieved when more rays converge within the allowable residual offset.
    Minimum Thresholded Pixels ≥ 4 Since cameras are placed more close to the tracked markers, each marker will appear bigger in camera views. The minimum number of threshold pixels can be increased to filter out small extraneous reflections if needed.
    Circularity ≥ 0.6 Increasing the circularity value will filter out non-marker reflections. Furthermore, it prevents collecting data from merged reflections where the calculated centroid is no longer reliable.

    Calibration[edit]


    The following calibration instructions are specific to precision tracking. For more general information, refer to the Calibration page.

    Wands

    For calibrating small capture volumes for precision tracking, we recommend using a Micron Series wand, either the CWM-250 or CWM-125. These wands are made of invar alloy, very rigid and insensitive to temperature, and they are designed to provide a precise and constant reference dimension during calibration. At the bottom of the wand head, there is a label which shows a factory-calibrated wand length with a sub-millimeter accuracy. In the Calibration pane, select Micron Series under the OptiWand dropdown menu, and define the exact length under the Wand Length.

    • Factory calibrated wand length indicated on the bottom label of the CMW-125
    • Define the precise wand length in the Calibration Pane


    The CW-500 wand is designed for capturing medium to large volumes, and it is not suited for calibrating small volumes. Not only it does not have the indication on the factory-calibrated length, but it is also made of aluminum, which makes it more vulnerable to thermal expansions. During the wanding process, Motive references the wand length for calibrating the capture volume, and any distortions in the wand length would cause the calibrated capture volume to be scaled slightly differently, which can be significant when capturing precise measurements. For this reason, a micron series wand is suitable for precision tracking applications.

    Note: Never touch the marker on the CWM-250 or CWM-125 since any changes can affect the calibration and overall data.

    Info2.png

    Precision Capture Calibration Tips

    • Wand slowly. Waving the wand around quickly at high exposure settings will blur the markers and distort the centroid calculations, at last, reducing the quality of your calibration.
    • Avoid occluding any of the calibration markers while wanding. Occluding markers will reduce the quality of the calibration.
    • A variety of unique samples is needed to achieve a good calibration. Wand in a three-dimensional volume, wave the wand in a variety of orientations and throughout the volume.
    • Extra wanding in the target area you wish to capture will improve the tracking in the target region.
    • Wanding the edges of the volume helps improve the lens distortion calculations. This may cause Motive to report a slightly worse overall calibration report, but will provide better quality calibration; explained below.
    • Starting/stopping the calibration process with the wand in the volume may help avoid getting rough samples outside your volume when entering and leaving.


    Calibration Results

    Calibration reports and analyzing the reported error is a complicated subject because the calibration process uses its own samples for validation. For example, sampling near the edge of the volume may improve the accuracy of the system but provide slightly worse calibration results. This is because the samples near the edge will have more errors to be corrected. Acceptable mean error varies based on the size of your volume, the number of cameras, and desired accuracy. The key metrics to keep an eye on are the Mean 3D Error for the Overall Reprojection and the Wand Error. Generally, use calibrations with the Mean 3D Error less than 0.80 mm and the Wand Error less than 0.030 mm. These numbers may be hard to reproduce in regular volumes. Again, the acceptable numbers are subjective, but lower numbers are better in general.


    Tracking[edit]


    Marker Type[edit]

    In general, passive retro-reflective markers will provide better tracking accuracy. The boundary of the spherical marker can be more clearly distinguished on passive markers, and the system can identify an accurate position of the marker centroids. The active markers, on the other hand, emit light and the illumination may not appear as spherical on the camera view. Even if a spherical diffuser is used, there can be situations where the light is not evenly distributed. This could provide inaccurate centroid data. Furthermore, the active markers that illuminates in pattern may involve some amount of interpolation on the off-frames, which is not preferred for precision tracking applications. For this reason, passive markers are preferred for precision tracking applications.

    Marker Placement[edit]

    Two markers are placed too close to each other and their reflections are getting merged.

    For close-up capture, it could be inevitable to place markers close to one another, and when markers are placed in close vicinity, their reflections may be merged as seen by the camera’s imager. Merged reflections will have an inaccurate centroid location, or they may even be completely discarded by the circularity filter or the intrusion detection feature. For best results, keep the circularity filter at a higher setting (>0.6) and decrease the intrusion band in the camera group filter settings to make sure only relevant reflections are reconstructed. The optimal balance will depend on the number and arrangement of the cameras in the setup.

    There are editing methods to discard or modify the missing data. However, for most reliable results, such marker intrusions should be prevented before the capture by separating the marker placements or by optimizing the camera placements.

    Refine Rigid Body Definition[edit]

    Once a rigid body is defined from a set of reconstructed points, utilize the Rigid Body Refinement feature to further refine the rigid body definition for precision tracking. The tool allows Motive to collect additional samples in the live mode for achieving more accurate tracking results.

    See: Rigid Body Refinement

    Using the RigidBody Refinement tool for improving asset definitions.

    Attention: Temperature[edit]


    Temperature of the processor board and the ringlight board displayed in the camera info.

    In a mocap system, camera mount structures and other hardware components may be affected by temperature fluctuations. Refer to linear thermal expansion coefficient tables to examine which materials are susceptible to temperature changes. Avoid using a temperature sensitive material for mounting the cameras. For example, aluminum has relatively high thermal expansion coefficient, and therefore, mounting cameras onto aluminum mounting structures may distort the calibration quality. For best accuracy, routinely recalibrate the capture volume, and take the temperature fluctuation into an account both when selecting the mount structures and before collecting data.

    Ambient Temperature[edit]

    An ideal method of avoiding influence from environmental temperature is to install the system in a temperature controlled volume. If such option is unavailable, routinely calibrate the volume before capture, and recalibrate the volume in between sessions when capturing for a long period. The effects are especially noticeable on hot days and will significantly affect your results. Thus, consistently monitor the average residual value and how well your rays converge to individual markers.

    Camera Heat[edit]

    The cameras will heat up with extended use, and change in internal hardware temperature may also affect the capture data. For this reason, avoid capturing or calibrating right after powering the system. Tests have found that the cameras need to be warmed up in Live mode for about an hour until it reaches a stable temperature. For Ethernet camera models, camera temperatures can be monitored from the Camera Preview (2D) pane in Motive (Camera Preview (2D) pane > Eye Icon > Camera Info).

    Attention: Vibrations[edit]


    Especially for measuring at sub-millimeters, even a minimal shift of the setup can affect the recordings. Re-calibrate the capture volume if your average residual values start to deviate. In particular, watch out for the following:

    • Avoid touching the cameras and the camera mounts.
    • Keep the capture area away from heavy foot traffic. People shouldn't be walking around the volume while capture is taking place.
    • Closing doors, even from the outside, may be noticeable during recording.


    Reconstruction Verification[edit]


    The following methods can be used to check the tracking accuracy and to better optimize the reconstructions settings in Motive.

    Verification Method 1

    First, go into the perspective view pane > select a marker, then go to the Camera Preview pane > Eye Button Viewport16.png > Set Marker Centroids: True. Make sure the cameras are in the object mode, then zoom into the selected marker in the 2D view. The marker will have two crosshairs on it; one white and one yellow. The amount of offset between the crosshairs will give you an idea of how closely the calculated 2D centroid location (white) aligns with the reconstructed position (yellow). Switching between the grayscale mode and the object mode will make the errors more distinguishable. The below image is an example of a poor calibration. A good calibration should have the yellow and white lines closely aligning over each other.

    Calculated 2D centroid location from the camera's perspective (white) and centroid location derived from reconstructed marker position (yellow).


    Verification Method 2

    The calibration quality can also be analyzed through checking the convergence of the tracked rays into a marker. This is not as precise as the first method, but the tracked rays can be used to check calibration quality of multiple cameras at once. First of all, make sure tracked rays are visible; Perspective View pane > Eye button > Tracked Rays. Then, select a marker in the perspective view pane. Zoom all the way into the marker (you may need to zoom into the sphere), and you will be able to see the tracking rays (green) converging into the center of the marker. A good calibration should have all the rays converging into approximately one point, as shown in the following image. Essentially, this is a visual way of examining the average residual offset of the converging rays.

    Monitoring convergence of the tracked rays.