Obtain the smplx parameters in scene coordinates, confusing points about align_smpl.py and eval_dataset.py

[Chuan Guo]

Hi, thanks for your great assets. I am trying to align the smplx parameters with the scene world. I have a few questions after looking into eval_dataset.py and align_smpl.py. I am not an expert in 3D transformation, hope you don't mind my "silly" questions:

I feel some parts in eval_dataset.py and align_smpl.py are not consistent:

1. How could we get the smpl.obj from the smplx.pkl? Should we pass the smplx.pkl to vposer to otain pose parameters, and then use smplx to obtain the shape vertices. I get some related codes in utils/vis_utils.py. But not very sure if there are further transformations.
2. In align_smpl.py, the smpl.obj are loaded, rescaled (become larger), and then transformed to scene coordinates using pose2scene RT matrix. While in eval_dataset, we do nothing to smpl.obj, but to rescale and transform the smpl parameters (specifically the global orientation and translation). Also in eval_dataset, we load the scene_downsampled.ply, instead of the textured mesh. Some other confusing parts includes:

• Is scene_downsampled.ply just downsampled from textured_output.obj? Or there are still rescaling and RT transformation here.
• What is the transform_norm.txt? I notice this is to transform the scenes into some canonical space. But I didn't find explanations about this in the paper or git. Why should we apply transform_norm to smpl parameters in eval_dataset.py ?
• Why in align_smpl.py we apply scale to smpl.obj, while in eval_dataset.py we apply 1/scale to the gloabl orientation and translation.
• In align_smpl.py, we apply the rescale to the whole smpl model. While in eval_dataset.py, we only apply the rescale to the global translation, in which case the scale of the smpl remains unchanged.

I know these are A LOT of questions. I would greatly appreciate it if you could help clarify this. I believe these can also be helpful for other beginner to use this dataset.

[Zzzzz]

Hi, thanks for asking! For the 1st question, yes, the obj files are obtained from pkl files. No transformations are needed.

For question 2, scene_downsampled.ply is downsampled from textured_output.obj. You can simply open them together in meshlab to verify that. For the transformations, essentially, there are 3 coordinate systems: the smplx coordinate system (from mocap device), the gaze coordinate system (from hololens2), the scene coordinate system (from 3D scanner). In align_smpl.py, what we are doing is transforming smplx to the scene coordinate space:
$$X_{scene} = W_{p2s}sX_p$$ where s is the scale factor, and $W_{pose2scene}=[R|t]$ transforms the scaled smplx vertex $X_p$ to the 3D scene space. In eval_dataset.py, basically we are doing similar things, but there are several points to pay attention to:
(1) the 3D scene points are transformed into the canonical space using transform_norm.txt such that the pointnet++ backbone can extract more informative features. Thus, aligning smplx to the transformed scene is as follows:
$$X_{scene}' = W_nX_{scene}=W_nW_{p2s}sX_p=[R_n|t_n][R|t]sX_p$$ Note that this equals to:
$$1/sX_{scene}' = [R_n|t_n/s][R|t/s]X_p$$ (2) In eval_dataset.py we don't use smplx vertex $X_p$, and instead we use the global translation, orientation and latent vector to represent smplx poses. So we transform the global orientation $R_g$ and translation $t_g$ using the above equation, since $X_p=[R_g|t_g]X_{local}$.

So you can see in eval_dataset.py we don't scale the smplx, but we rescale the 3D scene (we don't scale global translation and orientation). That's why 1/scale. While in align_smpl.py, we simply rescale smplx vertex since it's easier and we only want to visualize. Both transformations aim to align smplx and the scene and essentially they are doing the same thing.

[Chuan Guo]

Hi, I really appreciate your detailed reply. It helps me a lot.