Model FAQs

What landmarks are used?

Theia3D tracks 124 unique keypoints across the entire body, providing sufficient points on each segment for all segments to be fully observable in 3D space.

We are unable to provide the specific keypoints that are tracked, however many are common anatomical landmarks that are typically used to define rigid body models of body segments.

---

Can I build custom segments?

Theia3D uses 17 rigid segments to track the movement of the body, as described in the Default Model Description page. The 6-DOF pose of all segments are exported as 4x4 pose matrices within the selected output file format (e.g. C3D, FBX, JSON), along with a select few landmarks for relevant segments (e.g. left and right heels). These segments are defined to have biomechanical and anatomical relevance, and will provide sufficient data for most users' applications.

When using post-processing software such as Visual3D, it is possible to build custom segments using approaches such as defining virtual landmarks relative to the existing segment pose matrices. However, since the raw tracked landmarks are not included as output signals, these custom segments are typically limited to having a different static orientation but tracking the identical movement as the existing output segments using the 4x4 pose matrices.

---

What is the difference between pelvis_4X4 and pelvis_shifted_4X4 signals?

For all versions of Theia3D, these pose matrix signals express the position and orientation of the pelvis segment. However, there are some very slight differences that may be present, depending on the version of Theia3D used:

For Theia3D v2023 and earlier, these two sets of signals are nearly identical, with the only difference being that the pelvis_shifted_4x4 signal has its origin located at the height of the pelvis plane, whereas the pelvis_4x4 signal has its origin located at the midpoint of the hip joint centers. Otherwise, these signals are identical, and if you were to calculate a vector between their origins you would find it to have a constant length and constant direction within the pelvis reference frame. These separate signals were used to define the proximal (pelvis_shifted_4x4) and distal (pelvis_4x4) ends of the pelvis segment in order to construct the pelvis segment in Visual3D and other similar post-processing software.

For Theia3D Apollo v2024 and later, these signals are identical and are a holdover from previous versions. The pelvis_4x4 origin is now located at the pelvis plane (i.e. where the pelvis_shifted_4x4 signal was previously), and is used to define the proximal end of the pelvis, while the distal end is defined as the midpoint between the hip joint centers which are coincident with the thigh segments origins. Assuming an up-to-date version of Visual3D is used, the pelvis segment will be constructed accordingly.