[FAQ] Common mistakes when parsing SMPL UV data

[Lotayou]

@Lotayou (a.k.a myself) I just found that my SMPL UV data at hand is messed up, UV vertices' topology does not match that of SMPL 3D mesh. Here's the result:

This error hampers my further development since the color inconsistency induced by UV data error serious affects the interpolation accuracy of (x,y,z) coords. I'm afraid I cannot do anything further until this problem is solved.

Any help would be deeply appreciated!

[radvani]

How did you extract the UV map? From the FBX file?

In the code it looks like you're parsing a file named 'SMPL_template_UV_map.obj'. Do you have that file in the repository?

[Lotayou]

@radvani I downloaded the FBX file from official website and convert it into .obj using an online converter , the result is the aforementioned SMPL_template_UV_map.obj.
I'll upload the obj file for your evaluation.

[radvani]

Interesting, today I'm trying to extract the UV map directly from the FBX file, using the FBX SDK. I will let you know if it looks different than yours (perhaps the FBX -> OBJ conversion corrupted something?)

[Lotayou]

Cool! You can visualize the results with the following code snippets:

# create a half-red-half-blue 3D mesh
ones_vec = np.ones(verts.shape[0], dtype=verts.dtype)
    zeros_vec = np.zeros(verts.shape[0], dtype=verts.dtype)
    verts = np.stack((
        np.where(verts[:,1] > 0.5, ones_vec, zeros_vec),
        zeros_vec,
        np.where(verts[:,1] > 0.5, zeros_vec, ones_vec),
        ),axis=1
    )

# write colorized point cloud file 
def write_ply(ply_name, coords, rgbs):
    if rgbs.max() < 1.000001:
        rgbs = (rgbs * 255.).astype(np.uint8)
    
    with open(ply_name, 'w') as f:
        # headers
        f.writelines([
            'ply\n'
            'format ascii 1.0\n',
            'element vertex {}\n'.format(coords.shape[0]),
            'property float x\n',
            'property float y\n',
            'property float z\n',
            'property uchar red\n',
            'property uchar green\n',
            'property uchar blue\n',
            'end_header\n',
            ]
        )
        
        for i in range(coords.shape[0]):
            str = '{:10.6f} {:10.6f} {:10.6f} {:d} {:d} {:d}\n'\
                .format(coords[i,0],coords[i,1],coords[i,2],
                    rgbs[i,0], rgbs[i,1], rgbs[i,2])
            f.write(str)
    

[radvani]

Which FBX file were you using from the official website? Was it basicModel_f_lbs_10_207_0_v1.0.2, or basicModel_m_lbs_10_207_0_v1.0.2? My preliminary printout of texcoords from FBX show that they're different from what's in your OBJ file.

Also, regarding visualizing the results, I think I'm missing something. It looks like you're generating the human model (shown above) by creating the PLY file using the snippet in the previous comment, but how are you generating the images of the red/blue UV map?

[radvani]

Nevermind my first question above, it looks like you're using the female model. I generated a new OBJ and UV map directly from the FBX file and will try to test it now (still working through how to do that exactly).

[Lotayou]

Weird. As I recall SMPL model is gender-independent, and I never bother to find the difference... I guess male model has an extra "part" eh?
It's not necessary to generate the whole map (that requires barycentric interpolation which is kinda tricky), I think the scatter plot (like the one on the right) will be adequate. For that you can just quantize the UV texture coordinates to integers (denpending on your output solution, I use 300), and colorize each UV texture vertex with [0,1] normalized 3D coordinates of the corresponding mesh vertex.

[radvani]

Actually it may take me a bit to test this myself, but I've attached the OBJ I generated from the FBX SDK in case you want to try out. You can use your usual methods to generate the UV map from this (in get_SMPL_UV_map.py).

smpl_torch.obj.zip

[radvani]

I'm getting the following when using your OBJ file, and it seems fairly consistent?

I wonder if there's actually an issue in get_SMPL_UV_map, in that in your version you're creating a 1-1 mapping between texcoord indices and vertices. I don't think that's correct, in that a single texcoord index can correspond to multiple vertices (there are no restrictions against this in OBJ). I changed this so that instead there's a mapping between faces: e.g. each tuple of texcoords maps to the corresponding tuple of vertices. This resulted in my graphic above.

[Lotayou]

I think you got a point, it's indeed much safer to use face correspondences for UV plot... Think you can submit a PR?

[radvani]

Sure, I think my changes broke the render method in that class but I'll fix that and submit a PR tonight.

[radvani]

Submitted the PR! There's a new method in there as well for rendering the scatter plot; when you run get_SMPL_UV_map.py it spits out an image like the following. Let me know if looks right to you.

[Lotayou]

Works perfectly! Thanks a lot for your help! Closing this issue now.