Theia Model Description
There are currently three model configurations available in Theia3D, described below, which can be selected in the Analysis Preferences tab:
Default (Upper and Lower Body) Model
Details: Default (Upper and Lower Body) Model
Consists of two kinematic chains: lower body (pelvis and legs) and upper body (torso, arms, and head).
No abdomen or neck segments.
Shoulder joints and head segment are allowed 6 degrees of freedom.
Default Model
Full Body Model
Details: Full Body Model
Primarily intended for use when FBX file exports are required.
Consists of one, whole-body kinematic chain.
Abdomen and neck segments included.
Shoulder joints are allowed 6 degrees of freedom.
Head is allowed 3 degrees of freedom.
Head, Arms, Torso, and Lower-Body Model
Details: Head, Arms, Torso, and Lower-Body Model
Consists of five kinematic chains: lower body (pelvis and legs), torso, left arm, right arm, and head.
No abdomen or neck segments.
Shoulder joints and head segment are allowed 6 degrees of freedom.
These models have identical joint constraints, but differ in how the body is split into kinematic chains. As a result of the varying kinematic chains, the main difference between models will be the extent to which segments or limbs are lost during any tracking dropouts. If a segment is not trackable due to limited visibility or another reason, the entire kinematic chain of which that segment is a part will be lost.
For example, if the left hand cannot be tracked, each model option would result in the following:
Full Body Model: the entire body would be lost
Default (Upper and Lower Body) Model: the upper body would be lost
Head, Arms, Torso, and Lower-Body Model: only the left arm (left hand, left forearm, left upper arm) would be lost
Pose
Model pose can be exported to .c3d, .fbx, and .json files.
The .c3d files contain the 4x4 pose matrices for each model segment and the local coordinates of the anatomical landmarks of the distal segments of the model (feet, hands, head). These files can be processed using Visual3D.
The .fbx files contain the hierarchical skeleton, pose, and bone meshes of the animation model. The first frame of the file contains the model in a “T-Pose”. The skeleton must be solved using the Full Body Model in order to save pose files in FBX format.
The .json files contain information about how the trial was processed (Theia3D version, model, preferences, etc.) and the 4x4 pose matrices for each body segment for every frame of the trial.
Inertial Properties
With each model, inertial properties are estimated using established regression equations similar to Dempster’s, which provide values for segment masses, centers of mass, and moments of inertia based on subject-specific anthropometric estimates and validated population data. Theia3D combines deep learning with inverse kinematics to estimate 3D body movement, using calculated segment positions, orientations to support further biomechanical analysis.
Coordinate System
The 4x4 pose matrices for each body segment convey the position and orientation of the segment’s local coordinate system, which are visualized with the x-axis as red, y-axis as green, and z-axis as blue. Local coordinate system origins are visualized as white spheres
Last updated
Was this helpful?