Skeleton Tracking

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In Motive, skeleton assets are used for tracking human motions. Skeleton 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 recognize and label reconstructed markers on multiple semi-rigid body segments. In order to simplify skeleton definition and calculation, Motive uses pre-defined skeleton marker sets (a collection of marker labels and their specific positions on a subject), and retroreflective markers must be placed on pre-designated locations of the body. This page details instructions on how to create skeleton assets in Motive and associated features.

Note: Skeleton features are supported only in Motive:Body. 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 will not be created. Even if the skeleton is created, bad marker placements can lead to labeling problems, and the created skeleton may appear to be crooked. Accordingly, taking extra care to arrange markers correctly can save time in post-processing of the data.

Open Skeleton pane and select the markerset you desire to use from the dropdown menu. A total number of required markers for each skeleton is indicated in the parenthesis after the name of each markerset, and corresponding marker locations are displayed over an avatar shows up in the Skeleton 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. Attaching markers onto a person’s skin can be difficult because hairs, oils, and moistures from sweat can affect the adhesiveness. 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.

  • The marker arrangement displayed over an avatar in the Skeleton pane.
  • Markers placed accordingly.
  • Placing a joint marker on the elbow joint axis

Placing the Markers

All markers need to be placed on the respective anatomical location of the skeleton. In other words, joint markers need to be carefully placed along the corresponding joint axis. For example, ask the subject to flex the knee about 90 degrees, and palpate the knee joint to locate the axis. Place the markers on the axis so that they do not move with the knee movement.

For best results, associated segment markers (left and right) must be placed asymmetrically within a skeleton. By doing so, left and right side will be more thoroughly distinguished throughout the capture. Segment markers are markers that are placed within a body segment, but not around a joint. For example, placing the right thigh marker slightly higher than the left thigh marker helps Motive to differentiate them. This asymmetry is also reflected in the avatars shown in the Skeleton pane.

See also: Baseline Markerset Placements

Placement Tips

  • For joint markers, ask the subject to repeat few rounds of flexion and extension and examine and place the markers on the spot where skin movements are minimal. This spot usually lies along the joint axis (i.e. on the epicondyle of the bone).
  • 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.

Biomechanics Markersets

Markers for the Biomechanics markersets must be placed precisely with extra attention because these placements directly relate to coordinate system definition of each respective segment, and thus influencing the resulting biomechanical analysis. Mocap suits are not suitable for biomechanic application because an additional uncertainty is introduced from the fabric layer between the marker and the subject. The markers need to be placed on the skin for direct representation of the subject’s movement. While all the marker placement must follow the details from the avatar in the Skeleton pane, there are more considerations that need to be made when attaching markers.

See also: Biomech Markerset Placements and Rizzoli Markerset Placements

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

Creating a skeleton from Project Pane. Properties for the selected asset is described at the bottom.
  1. Open the Skeleton pane.
  2. Select the desired skeleton marker set from the dropdown menu
  3. Place the markers according to the placement shown on the avatar. Ask the subject to stand in the calibration post for more accurate placements.
  4. Double check the marker placement. Since Motive is tracking the attached markers at this point, you can also check the marker setup in Motive.
  5. If not detected automatically, select the skeleton markers from the 3D perspective view pane.
  6. Make sure Markers Needed and Markers Detected are matched.
  7. Ask the subject to stand in the calibration pose defined under the Skeleton Properties. A proper calibration posture is necessary because the pose of the created skeleton will be calibrated from it.
  8. Click Create in the Skeleton Pane
  9. After creating the skeleton model, confirm all skeleton segments and assigned markers are located at the expected locations. If not, delete and create the skeleton again after adjusting the marker placements and the calibration pose.

Creating a skeleton asset in Motive.

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 Skeleton Pane or the Application Settings pane. Properties of existing, or recorded, skeleton assets are configured under the properties section of the Project pane.

Example display settings in skeleton assets

Calibration Poses

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. The T-pose requires a proper standing posture with back straight and head looking directly forward. Then, both arms are stretched to sides, forming a “T” shape. Both arms and legs must be straight, and both feet need to be aligned together. For finger tracking marker sets, all fingers must be fully extended and aligned together creating a flat surface. For best results, thumbs need to be tucked in slightly to improve the alignment of the fingers. The following diagrams, taken from the Skeleton Pane, show examples of a proper T-pose.

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

A pose

The A-pose is another type of calibration pose that is used to create skeletons. Set the Skeleton Create Pose setting to the A-pose you wish to calibrate with. This pose is especially beneficial for subjects who have restrictions in lifting the arm. Unlike the T-pose, arms are abducted at approximately 40 degrees from the midline of the body, creating an A-shape. There are three different types of A-pose: Palms down, palms forward, and elbows bent.

  • Palms Down: Arms straight. Abducted, sideways, arms approximately 40 degrees, palms facing downwards.
  • Palms forward: Arms straight. Abducted, sideways, arms approximately 40 degrees, palms facing forward. Be careful not to over rotate the arm.
  • Elbows Bent: Similar to all other A-poses. arms approximately 40 degrees, bend elbows so that forearms point towards the front. Palms facing downwards, both forearms aligned.

Calibration Markers

In most skeleton marker sets, medial markers are avoided because they may introduce tracking problems. These markers 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 modeling joint axes by associating two markers on the medial and lateral side of the joint. For this reason, some biomechanics marker sets use medial markers as calibration markers. Calibration markers are used as indicators when creating skeletons, but are removed for the actual capture. The medial markers highlight in red from the 3D view when a skeleton is first created.

After creating a skeleton from the Skeleton Pane, detach the calibration markers from the subject, then remove the calibration markers in Motive. Access the context menu (right-click) of the skeleton in the perspective view, and go to Skeleton → Remove Calibration Markers. You can 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

For Motive 1.10 and above, existing skeleton assets can be recalibrated using the existing skeleton information. Essentially, the recalibration recreates the selected skeleton using the same skeleton markerset. This feature allows to adjust any displacements of the skeleton markers after the skeleton has been created. To do this, select 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 Project pane or the Perspective View pane. When using this feature, select only the markers that are associated with the corresponding markerset template. Extra markers that were added to the skeleton will not be included in the recalibration.

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

Adding/Removing Skeleton Markers

Related markers for a skeleton segment indicated when Assign Markers To Skeleton is enabled

Skeleton templates can be slightly modified by adding new markers to the set. Follow the below steps for adding/removing markers. After the skeleton marker set has been modified, auto-label the Take again to re-label the skeleton markers. Note that modifying (especially removing) skeleton markers is not recommended since changes to default templates can corrupt the tracking data when it is done inappropriately. 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. Otherwise, contact us for possible solutions for non-standard skeleton marker sets.

To Add

  1. First, enable the Assign Markers To Skeleton Label AssignMarkerSKL.png feature from the perspective view pane. When selecting a skeleton segment with this feature enabled, associated markers will be highlighted.
  2. Select a skeleton segment that you wish to add extra markers onto.
  3. Then, associate the marker(s) and the segment by Ctrl + left-clicking on the markers. If done properly, a blue line will connect the selected marker and the segment.
  4. Reconstruct and Auto-label the Take.
  5. When you manually add extra markers to skeletons, the markers will be labeled Segment_# in relation to the associated segments, and they will be included at the end of the skeleton XML file.

To Remove

  1. First, enable the Assign Markers To Skeleton Label AssignMarkerSKL.png and the Assigned Marker Selection Assets SelectAssigned.png from the perspective view pane.
  2. Select the skeleton segment that you wish to modify and select the associated Assigned Marker or solved marker.
  3. Delete the association by pressing the Delete key while the marker and the segment is selected.
  4. A pop-up dialogue asking whether to un-assign currently selected marker from the skeleton. Select Yes.
  5. Reconstruct and Auto-label the Take.

Marker Colors and Marker Sticks

For Motive 1.10 and above, 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 view. To view them, enable the Marker Sticks and Marker Colors under the Application Settings or under the visual aids Viewport16.png in the perspective view pane. Default color scheme will be assigned when creating a marker set. To modify them, export and edit the skeleton template XML file, where the custom marker labels can also be assigned.

Updating skeleton markers to visualize marker sticks and color for skeletons created Motive versions below 1.10.

The marker colors and sticks are featured only in Motive 1.10 and above, and skeletons created using Motive versions before 1.10 will not include the colors and sticks. For the Takes recorded before 1.10, the skeleton assets will need to be updated from the Project pane by right-clicking onto an asset and selecting Update Markers. The Update Markers feature will apply the default XML template to skeleton skeleton assets.

(Left) Marker colors enabled in the perspective view. (Right) Both marker sticks and marker colors enabled in the perspective view.

Export Assets Definition

Assets in a Take can be exported into Motive compatible rigid body (TRA) and skeleton (SKL) definition files. These files record calibrated spatial relationship of markers in each asset, and they are used to import the asset into different takes without creating a new one in Motive. Note that these files contain the spatial relationship and only the identical marker arrangements will be recognized and defined with the imported asset.

Rigid body definitions are exported as TRA files from the Export Rigid Body option under the File tab. When there are multiple rigid bodies in a Take, individual definitions can be exported by selecting a rigid body and clicking the Export Rigid Body from the context menu in the Project Pane. Likewise, skeleton definitions are exported as SKL files from the Export Skeleton option under the File tab, and individual definitions are exported through the Export Skeleton option located in the context menu from the Project Pane.

TIP: Rigid body and Skeleton definitions can also be quickly copied into different takes by dragging and dropping selected assets into a Take from the Project Pane.

Relative Joint Angles

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

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

A rough representation of joint angles can be obtained either when exporting a tracking data into CSV files or when streaming the skeleton rigid body data. When exporting into CSV, set Coordinate Space 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 hierarchial coordinate systems. This can also be done in data streaming by setting the Local Rigid Bodies to true in the data streaming pane.

Skeleton Marker 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 skeleton 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 Skeleton Tracking: Skeleton XML Files page.

  • To export skeleton marker XML file: To export a skeleton XML file, right-click on a skeleton asset under the project pane and use the Export Markers feature to export corresponding skeleton marker XML file.
  • To import skeleton marker XML file: To import a skeleton XML file on an existing skeleton, right-click on a skeleton asset under the project pane and use Rename Markers feature. To import the XML file for a new skeleton, specify the file directory under Marker Names section in the skeleton pane when creating a skeleton from reconstructed markers.