Meaning of global root orientation and trans

[glisca]

Hi there,

I would like to compute the trajectory of a vertex belonging to the SMPL model relative to the global coordinate system. More specifically I would like to double check my understanding of the first 3 values of poses entry (global root orientation) and of the trans entry. These are my questions:

1. How is the global coordinate system oriented? Does it keep the same orientation across all data sets?
2. What does global root orientation describe? The orientation of SMPL's root joint relative to the global coordinate system?
3. What does trans describe? The translation of SMPL's root joint relative tot the global coordinate system?
4. The coordinates of a vertex belonging to the SMPL model are relative to SMPL's root joint?

Many thanks in advance!

[nghorbani]

good questions.
the root of the SMPL model is the joint zero of its kinematic tree. Any other joint rotation and translation is computed relative to its parent joint in the tree, therefore root joint's rotation and translation (being a parent of all) will effect any others.
now answering your questions:

1. the same orientation is kept across all AMASS subsets.
2. Yes. If you rotate the root joint by providing root_orient vector to an instance of BodyModel you will rotate all the body vertices.
3. global translation of the body. If you translation the root joint by providing trans vector to an instance of BodyModel you can place SMPL model anywhere in a scene.
4. the position of a vertex on the surface of SMPL depends on the body shape, pose of the body, global orientation and translation of the body. I would suggest having a look at the original SMPL paper.

[glisca]

Thanks a lot! I very much appreciate your detailed answers!

[charsyme]

To which coordinate system does global root rotation correspond?

[glisca]

To my understanding the global root rotation is expressed in the world coordinate system.

[amituofo1996]

it seems global root rotation of AMASS is different from that in SMPL.I want to know how it transfer.

[Combo1]

"Normally, SMPL bodies are rotated 180 degrees on the x-axis (meshes are upside down)" mkocabas/VIBE#26