Skeleton Tracking

Main PageMotive DocumentationAssetsSkeleton Tracking


In Motive, skeleton assets are used for tracking human motions. These assets auto-label specific sets of markers attached to human subjects, or actors, and create skeletal models. Unlike rigid body assets, skeleton assets require additional calculations to correctly identify and label 3D reconstructed markers on multiple semi-rigid body segments. In order to accomplish this, Motive uses pre-defined skeleton markerset templates, which is a collection of marker labels and their specific positions on a subject. According to the selected markerset, retroreflective markers must be placed on pre-designated locations of the body. This page details instructions on how to create and use skeleton assets in Motive.

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Note:

  • Motive license: Skeleton features are supported only in Motive:Body or Motive:Body - Unlimited.
  • Skeleton Count: Standard Motive:Body license supports up to 3 skeletons. For tracking higher number of skeletons, activate with Motive: Body - Unlimitted license.
  • Height requirement: For skeleton tracking, the subject must be between 1'7" ~ 9' 10" tall.
  • Use the default create layout to open related panels that are necessary for skeleton creation. (CTRL + 2)


Skeleton Marker Placement


When it comes to tracking human movements, a proper marker placement becomes especially important. Motive utilizes pre-programmed skeleton markersets, and each marker is used to indicate anatomical landmarks when modeling the skeleton. Thus, all of the markers must be placed at their appropriate locations. If any of markers are misplaced, the skeleton asset may not be created, and even if it is created, bad marker placements may lead to labeling problems. Thus, taking extra care in placing the markers on intended locations is very important and can save time in post-processing of the data.

Attaching markers directly onto a person’s skin can be difficult because of hairs, oils, and moistures from sweat. Plus, dynamic human motions tend to move the markers during capture, so use appropriate skin adhesives for securing marker bases onto the skin. Alternatively, mocap suits allow velcro marker bases to be used.

Select a Markerset

Open Builder pane and go to the skeleton creation feature. Select the markerset you desire to use from the drop-down menu. A total number of required markers for each skeleton is indicated in the parenthesis after each markerset name, and corresponding marker locations are displayed over an avatar that shows up in the Builder pane. Instruct the subject to strike a calibration pose (T-pose or A-pose) and carefully follow the figure and place retroreflective markers at corresponding locations of the actor or the subject.

  • The marker arrangement displayed over an avatar in the Builder pane, skeleton creation options.
  • Markers placed accordingly.
  • Placing a joint marker on the elbow joint axis

Placing the Markers

All markers need to be placed at respective anatomical locations of a selected skeleton as shown in the Builder pane. Skeleton markers can be divided into two categories: markers that are placed along joint axes (joint markers) and markers that are placed on body segments (segment markers).

Joint Markers

Joint markers need to be placed carefully along corresponding joint axes. Proper placements will minimize marker movements during a range of motions and will give better tracking results. To accomplish this, ask the subject to flex and extend the joint (e.g. knee) a few times and palpate the joint to locate the corresponding axis. Once the axis is located, attach the markers along the axis where skin movement is minimal during a range of motion.

Segment Markers

Segment markers are markers that are placed on skeleton body segments, but not around a joint. For best tracking results, each segment marker placement must be incongruent to an associated segment on the opposite side of the skeleton (e.g. left thigh and right thigh). Also, segment markers must be placed asymmetrically within each segment for the best tracking results. This helps the skeleton solve to thoroughly distinguish, left-side and right-side of the corresponding skeleton segments throughout the capture. This asymmetrical placement is also emphasized in the avatars shown in the Builder pane.
Segment markers that can be slightly moved to different places on the same segment is highlighted on the 3D avatar in the skeleton creation window on the Builder pane.
SkeletonTracking SegmentMarkers.png

Additional Placement Tips

  • Wipe out any moisture or oil on the skin before attaching the marker.
  • Avoid wearing clothing or shoes with reflective materials since they can introduce extraneous reflections.
  • Tie up hair which can occlude the markers around the neck.
  • Remove reflective jewelry.
  • Place markers in an asymmetrical arrangement by offsetting the related segment markers (markers that are not on joints) in slightly different height.
  • See also: Baseline Markerset Placements

Biomechanics Markersets

When using the biomechanics markersets, markers must be placed precisely with extra care because these placements directly relate to coordinate system definition of each respective segment; thus, affecting the resulting biomechanical analysis. The markers need to be placed on the skin for direct representation of the subject’s movement. Mocap suits are not suitable for biomechanic applications. While the basic marker placement must follow the avatar in the Builder pane, additional details on the accurate placements can be found on the following page: Biomechanics Markersets.

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Additional Tips

  • All markers need to be placed at the respective anatomical landmarks.
  • Place markers where you can palpate the bone or where there are less soft tissues in between. These spots have fewer skin movements and provide secure marker attachment.
  • Joint markers are vulnerable to skin movements because of the range of motion in the flexion and extension cycle. In order to minimize the influence, a thorough understanding of the biomechanical model used in the post-processing is necessary. In certain circumstances, the joint line may not be the most appropriate location. Instead, placing the markers slightly superior to the joint line could minimize soft tissue artifact, still taking care to maintain parallelism with the anatomical joint line.
  • Use appropriate adhesives to place each markers and make sure they are securely attached.

Creating Skeletons


Skeleton Creation Steps

Defining skeleton from a skeleton markerset.

Step 1.

From the skeleton creation options on the Builder pane, select a skeleton markerset template from the Template drop-down menu. This will bring up a skeleton avatar displaying where the markers need to be placed on the subject.

Step 2.

Refer to the avatar and place the markers on the subject accordingly. For accurate placements, ask the subject to stand in the calibration pose while placing the markers. It is important that these markers get placed at the right spots on the subject's body for the best skeleton tracking. Thus, extra attention is needed when placing the skeleton markers.

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The magenta markers indicate the segment markers that can be placed at a slightly different position within the same segment.

Step 3.

Double-check the marker counts and their placements. It may be easier to use the 3D viewport in Motive to do this. The system should be tracking the attached markers at this point.

Step 4.

In the Builder pane, make sure the numbers under the Markers Needed and Markers Detected sections are matching. If the skeleton markers are not automatically detected, manually select the skeleton markers from the 3D perspective view.

Step 5.

Select a desired set of marker labels under the Labels section. Here, you can just use the Default labels to assign labels that are defined by the markerset template. Or, you can also assign custom labels by loading previously prepared marker-name XML files in the label section.

Step 6.

Next step is to select the skeleton creation pose settings. Under the Pose section drop-down menu, select the desired calibration post you want to use for defining the skeleton. This is set to the T-pose by default.

Step 7.

Ask the subject to stand in the selected calibration pose. Here, standing in a proper calibration posture is important because the pose of the created skeleton will be calibrated from it. For more details, read the calibration poses section.

Step 8.

Click Create to create the skeleton. Once the skeleton model has been defined, confirm all skeleton segments and assigned markers are located at expected locations. If any of the skeleton segment seems to be misaligned, delete and create the skeleton again after adjusting the marker placements and the calibration pose.

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In Edit Mode

If you are creating a skeleton in the post-processing of captured data, you will have to auto-label the Take to see the skeleton modeled and tracked in Motive.

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Reset Skeleton Tracking

When skeleton tracking is not acquired successfully during the capture for some reason, you can use the CTRL + R hotkey to trigger the solver to re-boot the skeleton asset.

Skeleton Properties

By configuring Skeleton Properties, you can modify the display settings as well as skeleton creation pose settings for skeleton assets. For newly created skeletons, default skeleton creation properties are configured under the Application Settings pane. Properties of existing, or recorded, skeleton assets are configured under the Properties pane while the respective skeletons are selected in Motive.

  • Example of the Visual property set to Segment under skeleton properties.
  • Properties of the selected skeleton is listed the Properties pane.

Calibration Pose

A proper calibration posture is necessary because the pose of the created skeleton will be calibrated from it. Read through the following explanations on proper T-poses and A-poses.

T pose

The T-pose is commonly used as the reference pose in 3D animation to bind two characters or assets together. Motive uses this pose when creating skeletons. A proper T-pose requires straight posture with back straight and head looking directly forward. Both arms are stretched to each side, forming a “T” shape. Both arms and legs must be straight, and both feet need to be aligned parallel to each other.

  • Front view of the T-pose.
  • Back view of the T-pose.

Calibration Markers

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Calibration markers exists only in the biomechanics markersets

Many skeleton markersets do not have medial markers because they can easily collide with other body parts or interfere with the range of motion, all of which increase the chance of marker occlusions.

However, medial markers are beneficial for precisely locating joint axes by associating two markers on the medial and lateral side of a joint. For this reason, some biomechanics markersets use medial markers as calibration markers. Calibration markers are used only when creating skeletons but removed afterward for the actual capture. These calibration markers are highlighted in red from the 3D view when a skeleton is first created.

After creating a skeleton from the Builder pane, calibration markers need to be removed. First, detach the calibration markers from the subject. Then, in Motive, right-click on the skeleton in the perspective view to access the context menu and click Skeleton → Remove Calibration Markers. Check the assigned marker positions to make sure that the skeleton no longer expects markers in the corresponding medial positions.

  • A skeleton asset with calibration markers.
  • Calibration markers removed.

Recalibrating Skeleton

Existing skeleton assets can be recalibrated using the existing skeleton information. Basically, the recalibration recreates the selected skeleton using the same skeleton markerset. This feature recalibrates the skeleton asset and refreshes expected marker locations on the assets.

To recalibrate skeletons, select all of the associated skeleton markers from the perspective view and click Recalibrate From Markers which can be found in the skeleton context menu from either the Assets pane or the Perspective View pane. When using this feature, select a skeleton and the markers that are related to the corresponding asset.

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Skeleton recalibration do not work with skeleton templates with added markers.

  • Recalibrate from selected marker in the Assets pane.
  • Recalibrate from selected marker in the Perspective View pane.

Marker Colors and Marker Sticks


Skeleton marker colors and marker sticks can be viewed in the perspective view pane. They provide color schemes for clearer identification of skeleton segments and individual marker labels from the perspective viewport. To make them visible, enable the Marker Sticks and Marker Colors under the visual aids Viewport16.png in the perspective view pane. A default color scheme is assigned when creating a skeleton asset. To modify marker colors and labels, you can use the Constraints pane.

Constraints basically store information of marker labels, colors, and marker sticks which can be modified, exported and re-imported as needed. For more information on doing this, please refer to the Constraints XML Files page.

  • (Left) Marker colors enabled in the perspective view. (Right) Both marker sticks and marker colors enabled in the perspective view.
  • Generating constraints will update the skeleton markers with the default constraints template.

Adding/Removing Skeleton Markers


Related markers for a skeleton segment indicated when Marker Lines advanced skeleton property is enabled

Skeleton markersets can be modified slightly by adding or removing markers to or from the template. Follow the below steps for adding/removing markers. Note that modifying, especially removing, skeleton markers is not recommended since changes to default templates may negatively affect the skeleton tracking when done incorrectly. Removing too many markers may result in poor skeleton reconstructions while adding too many markers may lead to labeling swaps. If any modification is necessary, try to keep the changes minimal.

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When adding, or removing, markers in the Edit mode, the Take needs to be auto-labeled again to re-label the skeleton markers.

Steps

You can add or remove asset model markers from a rigid body or a skeleton using the Builder pane. This is basically adding or removing markers to the existing rigid body and/or skeleton definition. Follow the below steps to add or remove markers:

To Add

  1. Access the Modify tab on the Builder pane.
  2. Select a skeleton segment that you wish to add extra markers onto.
  3. Then, CTRL + left-click on an the marker that we wish to add to the template
  4. On the Asset Model Markers tool in the Builder pane, click + to add and associate the selected marker to the selected segment
  5. Reconstruct and Auto-label the Take.
  6. When you add extra markers to skeletons, the markers will be labeled as Skeleton_CustomMarker#. You can use the Constraints pane to change the label as needed.

To Remove

  1. Enable selection of Asset Model Markers from the visual aids option in perspective view.
  2. [Optional] Under the advanced properties of the target skeleton, enable Marker Lines property to view which markers are associated with different skeleton bones.
  3. Access the Modify tab on the Builder pane.
  4. Select the skeleton segment that you wish to modify and select the associated Asset Model Marker that you wish to dissociate.
  5. Delete the association by clicking on the "-" next to the Asset Model Markers tool in the Builder pane while both the target marker and the target segment is selected.
  6. Reconstruct and Auto-label the Take.
  • Adding an extra chest marker to a skeleton
  • Changing the name of newly added marker from the Constraints pane

Export Assets Definition


Assets can be exported into Motive user profile (.MOTIVE) file if it needs to be re-imported. The user profile is a text-readable file that can contain various configuration settings in Motive; including the asset definitions.

When the asset definition(s) is exported to a MOTIVE user profile, it stores marker arrangements calibrated in each asset, and they can be imported into different takes without creating a new one in Motive. Note that these files specifically store the spatial relationship of each marker, and therefore, only the identical marker arrangements will be recognized and defined with the imported asset.

To export the assets, go to Files tab → Export Assets to export all of the assets in the Live-mode or in the current TAK file. You can also use Files tab → Export Profile to export other software settings including the assets.

  • Exporting Assetsinto the User Profile.
  • Exporting user profile that includes assets. This dialogue window is from the Export Profile As... option.
SKLTracking ExportSKL 30.png

Relative Skeleton Joint Angles


There are two ways of obtaining skeleton joint angles. Rough representations of joint angles can be obtained directly from Motive, but the most accurate representations of joint angles can be obtained by pipelining the tracking data into a third-party biomechanics analysis and visualization software (e.g. Visual3D or The MotionMonitor).

For biomechanics applications, joint angles must be computed accurately using respective skeleton model solve, which can be accomplished by using biomechanical analysis software. Export C3D files or stream tracking data from Motive and import them into an analysis software for further calculation. From the analysis, various biomechanics metrics, including the joint angles can be obtained.

Joint angles generated and exported from Motive are intended for basic visualization purposes only and should not be used for any type of biomechanical or clinical analysis. A rough representation of joint angles can be obtained by either exporting or streaming the skeleton rigid body tracking data. When exporting the tracking data into CSV, set the Use World Coordinates export setting to Local to obtain bone segment position and orientation values in respect to its parental segment, roughly representing the joint angles by comparing two hierarchical coordinate systems. When streaming the data, set Local Rigid Bodies to true in the streaming settings to get relative joint angles.

Constraints XML: Customize Marker Labels, Colors, and Sticks


Each skeleton asset has its marker templates stored in an XML file. By exporting, customizing, and importing the constraint XML files, a skeleton markerset can be modified. Specifically, customizing the XML files will allow you to modify skeleton marker labels, marker colors, and marker sticks within a skeleton asset. For detailed instructions on modifying skeleton XML files, read through Constraints XML Files page.

To export skeleton constraints XML file

To export a skeleton XML file, right-click on a skeleton asset under the Assets pane and use the Export Constraints feature to export corresponding skeleton marker XML file.

To import skeleton constraints XML file

You can import marker XML file under the Labels section of the Builder pane when first creating a new skeleton. To import a constraints XML file on an existing skeleton, right-click on a skeleton asset under the Assets pane and click Import Constraints.

See Also