Usage of "root_orient" and "trans"

[glisca]

Hi there,

I try to use the root_orient[1] and trans[2] fields in order to position and orient the SMPL model relative to the global reference frame.

When I instantiate the SMPL model with the following code:

# pose_id goes along frames stored in "./ACCAD/Male2Walking_c3d/B17 -  Walk to hop to walk_poses.npz"

root_orient = torch.Tensor(smpl_poses['poses'][pose_id:pose_id+1, :3]).to(computing_device) # controls the global root orientation
pose_body =   torch.Tensor(smpl_poses['poses'][pose_id:pose_id+1, 3:66]).to(computing_device) # controls the body
pose_hand =   torch.Tensor(smpl_poses['poses'][pose_id:pose_id+1, 66:]).to(computing_device) # controls the finger articulation
betas =       torch.Tensor(smpl_poses['betas'][:10][np.newaxis]).to(computing_device) # controls the body shape
dmpls =       torch.Tensor(smpl_poses['dmpls'][pose_id:pose_id+1]).to(computing_device) # controls soft tissue dynamics
root_trans =  torch.Tensor(smpl_poses['trans'][pose_id]).to(computing_device) # controls the global root orientation

smpl_in_pose_id = smpl_model(
    pose_body=pose_body, pose_hand=pose_hand,
    betas=betas, dmpls=dmpls,
    root_trans=root_trans, root_orient=root_orient)

and plot the trajectories of SMPL vertices with indexes 412 (head) and 3021 (pelvis) then I get this plot:

When I instantiate the SMPL model with the following code:

# pose_id goes along frames stored in "./ACCAD/Male2Walking_c3d/B17 -  Walk to hop to walk_poses.npz"

root_orient = torch.Tensor(smpl_poses['poses'][pose_id:pose_id+1, :3]).to(computing_device) # controls the global root orientation
pose_body =   torch.Tensor(smpl_poses['poses'][pose_id:pose_id+1, 3:66]).to(computing_device) # controls the body
pose_hand =   torch.Tensor(smpl_poses['poses'][pose_id:pose_id+1, 66:]).to(computing_device) # controls the finger articulation
betas =       torch.Tensor(smpl_poses['betas'][:10][np.newaxis]).to(computing_device) # controls the body shape
dmpls =       torch.Tensor(smpl_poses['dmpls'][pose_id:pose_id+1]).to(computing_device) # controls soft tissue dynamics
root_trans =  torch.Tensor(smpl_poses['trans'][pose_id]).to(computing_device) # controls the global root orientation

smpl_in_pose_id = smpl_model(
    pose_body=pose_body, betas=betas)

then manually assemble the transformation matrix from root_trans vector and root_orient rotation matrix, apply it to the same SMPL vertices with indexes 412 (head) and 3021 (pelvis) and plot their trajectories then I get this plot:

Could you please help me to correctly use the values stored in:

• smpl_poses['poses'][pose_id:pose_id+1, :3] and
• smpl_poses['trans'][pose_id] ?

Thank you very much!

kind regards,

[1] root_orient
[2] trans

[yizhouzhao]

I am kind of confused either. But I have found some clues:

1. smpl_poses['poses'][pose_id:pose_id+1, :3] stores the joint information

https://meshcapade.wiki/SMPL#smpl-x

2. smpl_poses['trans'][<frame_idx>] stores the global position of the body at one frame.

[Coondinator]

I have also encountered this issue when I try to tracking the root transform in Blender. The real rendering result is always has a such offset comparing to the 'trans' term. Have you solved it?