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 onto human subjects or actors and create skeletal models. Unlike a rigid body asset, a skeleton asset requires additional calculations to correctly recognize and label 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 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 for 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. The number of markers required for each skeleton is indicated in the parenthesis after the name of each markerset, and corresponding marker locations are displayed over the avatar in the Skeleton pane. Instruct the subject to strike a calibration pose (T-pose or A-pose) and carefully follow the figure and place the markers at the corresponding location 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

  • Place markers where skin movements are minimal. For joint markers, ask the subject to repeat few rounds of flexion and extension and examine for spot where skin is comparably stationary. 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 jewelries.
  • Place markers in asymmetrical arrangement by offsetting the related segment markers (markers that are not on joints) in slightly different height.

Biomechanics Markersets

Marker placements for Biomechanics must be done precisely since each location is directly related to analyzing the biomechanical movements. Mocap suits are not suitable for biomechanical analysis because an extra uncertainty variable is introduced from the fabric layer between the marker and the subject, and the markers needs to be placed onto the skin for more 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 some considerations that needs 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 less skin movements and provide secure marker attachment.
  • Joint markers are more vulnerable to skin movements because of the range of motion in the flexion and extension cycle. In order to minimize the influence, joint markers need to be placed along the pivot joint axis.
  • 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

Additional Skeleton Settings can be found in default creation properties from the Application Settings or Skeleton Pane. Under skeleton properties you can configure settings that are associated with skeleton assets. For newly created skeletons, default skeleton properties can be accessed from the Skeleton Pane or the Application Settings pane. Asset specific skeleton properties 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 mid-line 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

Note that modifying, specifically removing, skeleton markers is not advised since changes to default templates can corrupt the tracking data when it is done inappropriately. Skeleton templates can be slightly modified by adding new markers to the set. After the skeleton marker set has been modified, reconstruct and auto-label the Take again to examine the changed skeleton. Please be careful when modifying skeleton markers. Removing too many markers may result in poor skeleton reconstructions, while adding too many markers may lead to labeling swaps. If the modification is necessary, try to keep the changes minimal. Otherwise, consult with OptiTrack Support for solutions for non-standard marker sets.

To Add

  1. First, enable the Assign Markers To Skeleton Label AssignMarkerSKL.png feature from the perspective view pane. This will highlight the associated markers when a skeleton segment is selected.
  2. Select a skeleton segment that you wish to add extra markers to.
  3. Then, associate the marker(s) and the segment by Ctrl-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 unassign 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.

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 have them available. For the Takes recorded before 1.10, the assets will need to be recreated using Motive 1.10 or above.

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

Export Assets Definition

Assets can be exported into Motive-compatible rigid body (TRA) and skeleton (SKL) definition files. These files record marker arrangements calibrated 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 specifically store the spatial relationship of each marker, and therefore, 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 Assets 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 Assets 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 Data Management pane.

Skeleton XML: Customize Marker Labels, Colors, and Sticks

In order to customize the skeleton marker labels, colors, and sticks, a skeleton XML file needs be exported from an existing skeleton, customized, and loaded onto the skeleton of the same markerset. For example, if you exported a Baseline (41) skeleton and modified the labeling XML file, same Baseline (41) needs to be created in order to import the customized XML file. The following section describes the steps for customizing skeleton XML templates.

1. Export a Skeleton XML

a) First, choose a markerset from the Skeleton Pane, and create a skeleton.

b) Open the Export Skeleton dialogue window, which can be found under the file tab or from the assets context menu in the Project Pane.

c) In the Export Skeleton dialog window, select Marker Name Template (.xml) file, and export.

  • Export Skeleton option shown in the Project pane.
  • Selecting XML format in the Skeleton export dialog window.

2. Customize the XML

Customize Marker Labels

a) Open the exported XML file using a text editor. It will contain corresponding marker label information under the MarkerNameMap section.

b) Customize the marker labels from the XML file. Note that in the tag <Original_Name>Custom_Name</Original_Name> only the Custom_Name part should be modified, and the original labels within the bracket <Original_Name> should remain the same. The order of the markers should also remain the same.

c) If you changed marker labels, the corresponding marker names must also be renamed within the Marker and MarkerSticks definitions as well. Otherwise, the marker colors and marker sticks will not be defined properly.

  • Individual marker labels defined within the exported XML file.
  • Modified marker labels

Customize Marker Sticks and Colors

a) To customize the Marker Colors and Sticks, open the exported XML file using a text editor and scroll down to the Markers and MarkerSticks section. If the Markers and MarkerSticks section does not exist within the exported XML file, you could be using an old skeleton created before Motive 1.10. Creating and exporting a new skeleton will provide these sections in the XML.

  • MarkerColors definition section in the skeleton template XML file.
  • MarkerSticks definition section in the skeleton template XML file.

b) Here, you can customize the marker colors and the marker sticks. For each marker name, you must use exactly same marker labels that was defined from the MarkerNameMap section of the same XML file. If any marker label was changed in the MarkerNameMap section, the changed name must be reflected on the respective colors and sticks definitions as well. In other words, if a Custom_Name was assigned in the MarkerNameMap section (<Original_Name>Custom_Name</Original_Name>), the same Custom_Name must be used to rename all the respective marker names within Marker and MarkerSticks instances of the XML.

  • Marker Colors: For each marker in a skeleton, there will be a respective name and color definitions under the Markerssection of the XML. To change corresponding marker colors for the template, edit the RGB parameter and save the XML file.
  • Marker Sticks: A marker stick is simply a line interconnecting two labeled markers within the skeleton. Each marker stick definition consists of two marker labels for creating a marker stick and a RGB value for its color. To modify the marker sticks, edit the marker names and the color values. You can also define additional marker sticks by copying the format from the other marker stick definitions.

3. Import Skeleton XML

Creating new skeletons

Now that you have customized the XML file, it can be loaded each time when creating new skeletons. In the Skeleton Pane, select the corresponding markerset. Then, under the Marker Names drop down menu, choose (…) to browse to import the XML file. When you Create the skeleton, the custom marker labels, marker sticks, and marker labels will be applied. You will need to auto-label the take again if you are working on a recorded TAK file.

If you manually added extra markers to a skeleton, you must rename the skeleton after creating it. See more at the Added Markers section.

  • Loading skeleton XML when creating a new skeleton
  • Customized skeleton markers labels.

Renaming Markers on existing Skeleton

You can also apply customized XML into an existing skeleton using the renaming feature. Right-click on a skeleton asset in the Project pane and select the Rename Markers from the context menu, and this will bring up a dialog window for importing a skeleton XML template. Import the customized XML template and modified labels will be applied. This feature must be used if extra markers were added to the default XML template.

Renaming existing skeleton by importing a XML template file.

Added Markers

XML definitions can also be applied to added markers on a skeleton asset. When extra markers were added to a skeleton, its XML file, when exported, will have the corresponding marker labels logged at the end of the MarkerNameMap section, and the labels for these markers can be customized from the exported XML file. The marker color and sticks definitions for extra markers will not be automatically logged. To assign the marker colors and sticks for the extra markers, you will need to type additional instances which exactly copies the format that are used in other instances.

A newly created skeleton will not contain the added markers within the asset. To apply the customized XML for the extra markers, you must first create the skeleton and add the extra markers before importing the XML. When adding multiple markers, it is important that they are added in exactly the same order that it was added on the skeleton which was exported; Otherwise, the extra labels will be assigned incorrectly. After adding the corresponding markers, use the Rename Markers feature to apply the customized XML file. Lastly, auto-labeled the Take to assign the corresponding marker definitions onto the skeleton.

Applying Customized XML with Added Markers

1) Motive: Skeleton pane Create the skeleton using the markers without including the extra markers.

2) Motive: Perspective View pane Add the extra markers onto the selected skeleton asset. See how to add markers.

3) Motive: Project pane: Select the skeleton and click Rename Markers to import the customized skeleton XML template.

4) Motive: Project pane: When working with a recorded Take, auto-label the Take to label the markers using the imported XML file.