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

 
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  [[OptiTrack Documentation Wiki|Main Page]] → [[Quick Start Guides|Quick Start Guides]] → [[Quick Start Guide: Precision Capture| Precision Capture]]
  [[OptiTrack Documentation Wiki|Back to the Main Page]] → [[Quick Start Guides|Back to the Quick Start Guides List]]
 
 
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{| align="right"
 
| __TOC__
 
|}
 
  
  
This quick start guide details specific instructions for capturing in a small to medium sized volume for extremely accurate tracking using an OptiTrack motion capture system. Directions for optimizing the capture environment and the system setup along with some important cautions are listed in this page, and lastly, commonly used precision verification methods are also included. For more general information not included in this page, please refer to the [[Quick Start Guide: Getting Started]] or corresponding workflow pages. Before going into details on how to optimize the system for precision tracking applications, let's briefly go over the residual value, which is the key reconstruction value important for analyzing and monitoring the precision of tracked markers.
+
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=
<center>[[Image:MVQSG_Residual1.png|300 px]] [[Image:MVQSG_Residual2.png|330 px]]</center>
+
----
 +
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, in mm, 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 a 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. If you select one or more markers in the Live mode or from a recorded 2D data, then the mean residual value will be displayed in the status bar located at the bottom of Motive. With a smaller residual value, tracked rays converge more precisely and more accurate 3D reconstruction is achieved. A well-tracked marker will have a sub-millimeter average residual value.
+
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>
 +
<li class="thumblist">[[Image:MVQSG_Residual1_21.png|thumb|450 px|Multiple tracked rays contributing to a 3D reconstruction.]]</li>
 +
<li class="thumblist">[[Image:MVQSG_Residual2.png|thumb|320 px|Residual offsets at the converging point of the multiple rays.]]</li>
 +
</center>
  
 +
=Capture Volume=
 +
----
  
==Capture Volume==
+
First of all, optimize the capture volume for the most precise and accurate tracking results. Avoid a 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.
  
===Setup Environment===
+
==Infrared Black Background Objects==
Avoid populated area when setting up the system and recording a capture. Clear any obstacles or trip hazards around the capture volume. Physical impart on the setup will distort the calibration quality which could be critical especially when tracking at sub-millimeter accuracy. For best results, routinely recalibrate the capture volume.
+
<div class="padded">
 +
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.
  
 +
[[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 extraneous bright pixels from the background.]]
 +
</div>
  
===Infrared Black Background Objects===
+
=Camera Placement=
 +
----
 +
<div style="float:right"><ul>
 +
<li class="thumblist">[[Image:MVQSG_Placement3.png|thumb|280px|Capturing multiple unique vantages measure more accurate positions on all of the coordinate axis]]</li>
 +
<li class="thumblist">[[Image:MVQSG_Placement4.png|thumb|250px|Cameras placed in a dome arrangement around a cubic meter volume.]]</li>
 +
</ul></div>
  
[[Image:MVQSG_IRBlack.png|300 px]] [[Image:MVQSG_IRBlack2.png|295 px]]
+
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.
  
The 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.
+
====Mounting Locations====
 +
: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.
  
==Camera Placement==
+
====Mount Securely====
[[Image:MVQSG_Placement4.png|thumb|250px|Cameras placed in a dome arrangement around a cubic meter volume.]]
+
: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.
[[Image:MVQSG_Placement3.png|thumb|300px|Capturing multiple unique vantages measure more accurate positions on all of the coordinate axis]]
 
Proper camera placement techniques can greatly improve tracking results. The following guide highlights important setup instructions for the small volume tracking. For more details on the system hardware setup, read through the [[Hardware Setup]] pages.
 
  
 +
<center>
 +
<li class="thumblist">[[Image:CameraMount_Manfrotto.png|thumb||Manfrotto clamp used to mount cameras.]]</li>
 +
<li class="thumblist">[[Image:Mount_Clamp2.png|thumb|200 px|Camera mounted onto a truss structure using the clamp.]]</li>
 +
</center>
  
===Mounting Locations===
+
=Focus and Aiming=
 
+
----
For general tracking applications, it is usually beneficial to place the cameras in a circumnavigating layout creating a ring arrangement above the volume to maximize the size of the capture volume. However, for precise tracking applications, better results will be obtained by placing the cameras closer to the target object (adjusting focus will be required) in a spherical, or a dome, camera arrangement.
+
<div class="padded">
 
+
[[Image:FnA_InFocus.png|thumb|500 px|Grayscale images of a marker captured with varying camera focus]]
Captured 2D images are used to locate the 2D positions of the markers from the each camera's perspective, but the data on the remaining dimension, in and out of the screen, can only be obtained through the reconstruction from multiple cameras in the setup. Accordingly, placing cameras right above the volume, looking down onto the tracked objected, will provide unique vantages and provide more precise and accurate positional data on all of the three coordinate axis.
 
 
 
===Mount Securely===
 
For most accurate results, cameras should be securely fastened onto a truss system or an extremely rigid object. For 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|right|500 px]]
 
For more information on adjusting the camera aiming and focus, please read through the [[Aiming and Focusing]] workflow page.
 
  
 
====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.
+
{{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. To zoom in and out from the camera view, place the mouse cursor over the [[Viewport#Camera_Preview|2D camera preview]] window in Motive and use the mouse-scroll. 
  
==Temperature==
+
For more information, please read through the [[Aiming and Focusing]] workflow page.
[[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 setup 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, iron has relatively high thermal expansion coefficient, and therefore, you have to be careful when mounting cameras onto iron mounting structures. For best accuracy, take the temperature fluctuation into an account both when selecting the mount structures and before collecting data and routinely recalibrate the capture volume.
+
</div>
  
====Ambient Temperature====
+
=Motive Settings=
An ideal setup is to have temperature controlled capture volume, but if that is not possible there are few considerations to keep in mind.  
+
----
 +
The following sections cover key configuration settings which need to be optimized for the precision tracking.
  
*Place the truss or tripods on a rigid surface such as concrete (avoid carpet). Mounting to the walls with something like speed rail can be dangerous as well because many buildings may fluctuate with changing outside temperature. This is especially noticeable on hot days and will significantly affect your results.
+
==Camera Settings==
*Aluminum has a particularly high coefficients of linear thermal expansion. So one should be wary when using aluminum based truss.  
+
<div class="padded">
 +
Camera settings are configured using the [[Devices pane]] and the [[Properties pane]] both of which can be opened under the [[Toolbar#View|view tab]] in Motive.
  
If these situations are unavoidable, then the solution is to consistently monitor the average residual value for how well your rays converge to individual markers.  
+
{| class="wikitable" style = "width:80%; margin:auto"
 +
!style ="width:10%; padding:5px"|Setting
 +
!width = 10%|Value
 +
!Description
 +
|-
 +
|Gain
 +
|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
 +
|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.
 +
|-
 +
|Threshold (THR)
 +
|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
 +
|align ="center"|15
 +
|-
 +
|Exposure (EXP)
 +
|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.
 +
|}
 +
</div>
  
====Camera Heat====
+
==Live-Reconstruction Settings==
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).
+
<div class="padded">
 +
Live-reconstruction settings can be configured under the [[Application Settings#Live-Reconstruction|application settings]] panel. These settings determine which data gets reconstructed into the 3D data, and when needed, you can adjust the filter thresholds to prevent any inaccurate data from reconstructing. Read through the [[Application Settings: Live Reconstruction|Application Settings]] page for more details on each setting. For the precision tracking applications, the key settings and the suggested values are listed below:
  
==Vibrations==
+
{| class="wikitable" style = "width:80%; margin:auto"
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. In particular, watch out for the following:
+
!Setting
 +
!width = 10%|Value
 +
!Description
 +
|-
 +
|Residual (mm)
 +
|align ="center"|&lt; 2.00
 +
|Set the allowable [[Application Settings: Live Reconstruction#Maximum_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
 +
|align ="center"|&ge; 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
 +
|align ="center"|&ge; 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
 +
|align ="center"|&ge; 0.6
 +
|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.
 +
|}
 +
</div>
 +
 
 +
=Calibration=
 +
----
 +
The following calibration instructions are specific to precision tracking. For more general information, refer to the [[Calibration]] page.
  
*Never touch the truss or mounting device for the cameras.
+
==Wands==
*Make sure your capture area is away from heavy foot traffic and that people aren’t walking around while capture is taking place.
+
<div class="padded">
*Closing doors, even those on the other side of the, may be noticeable during recording.
+
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''.
  
Re-calibrate the capture volume if your average residual values start to deviate or the capture data starts looking strange.
+
<div class="padded"><center><ul>
 +
<li class="thumblist">[[Image:MVQSG_Wandlabel.png|530 px|thumb|Factory calibrated wand length indicated on the bottom label of the CMW-125]]</li>
 +
<li class="thumblist">[[Image:MVQSG_WandCalib.png|250 px|thumb|Define the precise wand length in the [[Calibration Pane]]]]</li>
 +
</ul></center></div>
  
 +
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.
  
==Marker Intrusions==
+
Note: '''Never''' touch the marker on the CWM-250 or CWM-125 since any changes can affect the calibration and overall data.
[[Image:MVQSG_Optimization4.png|thumb|200 px|Two markers are placed too close to each other and their reflections are getting merged.]]
 
When markers are placed very close to each other, their reflections may merge in the camera’s imager and ruin the capture data. Merged marker reflections will have an inaccurate centroid location, or they may even be completely discarded by the circularity filter. There are a variety of different editing methods for discarding or trimming the data. However, for best results, such situation should be prevented in the first place. Especially for precise tracking applications, marker placements and camera placements should be optimized in order to avoid the intrusion.
 
  
 +
{{Info|'''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.
  
==Motive Settings==
+
*'''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.
The following sections cover some key configuration settings which may need to be optimized for the micron volume tracking.
 
  
===Cameras Pane Settings===
+
*Extra wanding in the target area you wish to capture will improve the tracking in the target region.
At this point, you should’ve already adjusted the focus for each camera. For obt To open the cameras pane, click the camera icon at the top of Motive. The following list describes the
 
  
*Set the Gain setting to 1: Low (Short Range) for all cameras. Higher gain settings will amplify noise in the image.
+
*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.
*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.
 
*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.
 
  
For the micron volume 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).
+
*Starting/stopping the calibration process with the wand in the volume may help avoid getting rough samples outside your volume when entering and leaving.}}
 +
</div>
  
===Reconstruction Settings===
+
==Calibration Results==
To open the cameras pane click View > Reconstruction Settings the top of Motive. Read through the [[Reconstruction]] page for specific details. For the micron volume tracking, important reconstruction settings and the appropriate values are listed below:
+
<div class="padded">
 +
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.</div>
  
{| class="wikitable" style = "margin:auto"
+
=Tracking=
!Setting
+
----
!width = 10%|Value
+
==Marker Type==
!Description
+
<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. For this reason, passive markers are preferred for precision tracking applications.
|Residual (mm)
+
</div>
|<2.00
 
|Allowable residual value should be set smaller for the micron volume tracking, because any offset above 2.00 mm residual value will be considered as inaccurate.
 
|-
 
|Minimum Rays
 
|>= 3
 
|When more rays converge within allowable residual offset, reconstructed marker will have more accurate position.
 
|-
 
|Minimum threshold pixels
 
|>= 4
 
|
 
|-
 
|Circularity
 
|>= 0.6
 
|
 
|}
 
  
 +
==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.]]
  
==Calibration==
+
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 [[Application Settings#Circularity|circularity filter]] or the [[Application Settings#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 [[Application Settings#Filters_.282D_Object_Filter.29|2D 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.
The following calibration instructions are specific to precise captures. For more general information, refer to the [[Calibration]] page.
 
  
===Wands===
+
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.
For calibration, 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 precise and constant calibration measurements. 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.
+
</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.
  
<center>[[Image:MVQSG_Wandlabel.png|500 px]] [[Image:MVQSG_WandCalib.png|250 px]]</center>
+
See: [[Rigid Body Tracking#Rigid Body Refinement|Rigid Body Refinement]]
 +
{{#ev:youtube|u3Z3N2FNpf8|500|center|Using the RigidBody Refinement tool for improving asset definitions.|frame}}
 +
</div>
  
 +
=Attention: Temperature=
 +
----
 +
[[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 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.
  
The CW-500 wand is designed for large to medium capture volumes, but it is not suited for calibrating the micron volume. Not only it has longer reference dimension, but also the wand is made with aluminum, which makes it more vulnerable to thermal expansions. In order to calibrate the volume with best accuracy, the distance between the wand markers must maintain accurate and constant. During the wanding process, Motive assumes the defined wand length, and any distortions would cause the calibrated capture volume to be scaled slightly differently, which can be significant when capturing precise measurements.
+
==Ambient Temperature==
 +
<div class="padded">
 +
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.
 +
</div>
  
Note: '''Never''' touch the marker on the CWM-250 or CWM-125 since any changes can affect the calibration and overall data.
+
==Camera Heat==
 +
<div class="padded">
 +
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>
  
{{Tip|'''Calibration Checkpoints'''
+
=Attention: Vibrations=
 +
----
 +
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:
  
*'''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 touching the cameras and the camera mounts.
*'''Try not to occlude any of the calibration markers while wanding.''' Occluding markers will reduce the quality of the calibration.
+
:*Keep the capture area away from heavy foot traffic. People shouldn't be walking around the volume while the capture is taking place.
*'''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.
+
:*Closing doors, even from the outside, may be noticeable during recording.
*Wanding the edges of the volume helps improve the lens distortion calculations, but it may also lead to a slightly worse calibration report. This is because the calibration engine creates the report using the collected wanding samples, and the samples collected near the edge of the volume will have more errors which could affect the overall result.
 
*The amount of acceptable error is a complicated subject because the calibration can only use its own samples for validation. 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 our testing we only used calibrations with the Mean 3D Error less than 0.080 mm and the Wand Error less than 0.030 mm. Again, the acceptable numbers are subjective, but generally lower numbers are better.
 
*Re-emphasizing from previous sections, use a Micron Series wand if possible, and NEVER touch or bump the markers on the micron wand.}}
 
  
==Reconstruction Verification==
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=Reconstruction Verification=
The following sections covers methods that can be used to check the tracking accuracy and to better optimize the reconstructions settings in Motive.
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----
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The following methods can be used to check the tracking accuracy and to better optimize the [[Reconstruction Settings|reconstructions settings]] in Motive.
  
 +
<div class="padded">
 
===Verification Method 1===
 
===Verification Method 1===
First, go into the perspective pane > select a marker, then go to the Camera Preview Pane > Eye Button > Marker Centroids = True. Make sure your cameras are in 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.  
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{{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 with 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.
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{{Indent|
 
+
The calibration quality can also be analyzed by 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 the 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.}}
  
<center>[[Image:MVQSG_Optimization2.png|700 px]]</center>
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[[Image:MVQSG_Optimization2.png|thumb|700 px|center|Monitoring convergence of the tracked rays.]]
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</div>

Latest revision as of 19:02, 25 June 2019

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


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, in mm, 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


    First of all, optimize the capture volume for the most precise and accurate tracking results. Avoid a 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 extraneous bright pixels from the background.

    Camera Placement


    • 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

    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

    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


    Grayscale images of a marker captured with varying camera focus

    F-stop

    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

    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. To zoom in and out from the camera view, place the mouse cursor over the 2D camera preview window in Motive and use the mouse-scroll.

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

    Motive Settings


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

    Camera Settings

    Camera settings are configured using the Devices pane and the Properties pane both of which can be opened under the view tab 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.

    Live-Reconstruction Settings

    Live-reconstruction settings can be configured under the application settings panel. These settings determine which data gets reconstructed into the 3D data, and when needed, you can adjust the filter thresholds to prevent any inaccurate data from reconstructing. Read through the Application Settings page for more details on each setting. For the precision tracking applications, the key settings and the suggested 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


    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


    Marker Type

    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. For this reason, passive markers are preferred for precision tracking applications.

    Marker Placement

    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 2D 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

    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


    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

    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

    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


    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 the capture is taking place.
    • 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.

    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 with 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 by 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 the 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.