Pytorch3d: Request for documentation on how to use multiple textures & multiple meshes per scene

[aluo-x]

🚀 Feature

Would appreciate more documentation or functionality to allow for rendering multiple meshes in a single scene, each with its own texture or vertex colors. In neural mesh renderer this can be achieved by setting a tensor with size (1, F, T, T, T, 3), where F are number of faces, and T are "texture size", while in the Nvidia Kaolin repository, there is the vertex texture mode for DIB-R.

Requesting an example of how this could be accomplished using this renderer.

Motivation

Complex scene require multiple objects, often with its own texture. This functionality is present in other differentiable renderers.

Pitch

Possible pseudo-code:

```
MeshList = [Mesh1(faces1, verts1, text1), Mesh2(faces2, verts2, text2) ... ]

a=Mesh()

for i in MeshList:
a = a.mesh_join(i)

img = Renderer(a)

[nikhilaravi]

@aluo-x If I understand your question correctly, you want to concatenate multiple meshes into one mesh and then render one image.

This is definitely possible with PyTorch3d. You would just need to format the input data correctly yourself as internally we assume that for a batch of meshes, each mesh is rendered onto a separate image (the same assumption made in NMR and Kaolin).

For example, take two ico spheres where one has a blue texture and the other has a red texture. We can initialize the textures as an RGB color per vertex. We can then create one mesh which contains both the meshes and render them to a single image.

from pytorch3d.utils import ico_sphere
from pytorch3d.structures import Textures 

# Initialize two ico spheres of different sizes
mesh1 = ico_sphere(3)  # (42 verts, 80 faces)
mesh2 = ico_sphere(4)  # (162 verts, 320 faces)
verts1, faces1 = mesh1.get_mesh_verts_faces(0)
verts2, faces2 = mesh2.get_mesh_verts_faces(0)

# Initalize the textures as an RGB color per vertex
tex1 = torch.ones_like(verts1) 
tex2 = torch.ones_like(verts2)
tex1[:, 1:] *= 0.0  # red
tex2[:, :2] *= 0.0  # blue

# Create one mesh which contains two spheres of different sizes.
# To do this we can concatenate verts1 and verts2
# but we need to offset the face indices of faces2 so they index
# into the correct positions in the combined verts tensor. 

# Make the red sphere smaller and offset both spheres so they are not overlapping
verts1 *= 0.25  
verts1[:, 0] += 0.8
verts2[:, 0] -= 0.5
verts = torch.cat([verts1, verts2])  #(204, 3)

#  Offset by the number of vertices in mesh1
faces2 = faces2 + verts1.shape[0]  
faces = torch.cat([faces1, faces2])  # (400, 3)

tex = torch.cat([tex1, tex2])[None]  # (1, 204, 3)
textures = Textures(verts_rgb=tex)

mesh = Meshes(verts=[verts], faces=[faces], textures=textures)

# Initialize a renderer separately and then render the mesh
image = renderer(mesh)   # (1, H, W, 4)

The output as seen below is a single RGBA image which contains both meshes.

Let me know if that answered your question. Bear in mind that the texturing API is still experimental and we are working on improvements and more functionality.

---

NOTE

To learn how to initialize a renderer please refer to one of the tutorials e.g. camera position optimization

---

[aluo-x]

Wow, thanks for the extremely fast response and the great tool!
The example is very clear and is exactly what I was looking for. Somehow I missed the verts_rgb option while browsing the texture structure code. The documentation between the world/camera/image space is also a missing part of many other renderers.

One minor suggestion, this method (like the vertex color mode in DIB-R) are limited by the mesh resolution to some degree. It would still be a minor improvement to allow for barycentric like interpolation of textures per face. So instead of [F, 3] for texture, you can specify [F, T, T, T, 3] for texture.

This solves my problem so I will close the issue for now.

[nikhilaravi]

@aluo-x Great! :) Specifying a texture atlas with a T*T*3 texture map per face is a feature we are planning to add. Stay tuned!

[rahuldey91]

I am facing problems with rendering a mesh with Texture initialized using verts_rgb. I used the same code as above and my renderer is like:

MeshRenderer(
  (rasterizer): MeshRasterizer()
  (shader): TexturedSoftPhongShader()
)

When I try to render using these

tex = torch.cat([tex1, tex2])[None]  # (1, 204, 3)
textures = Textures(verts_rgb=tex)

mesh = Meshes(verts=[verts], faces=[faces], textures=textures)

# Initialize a renderer separately and then render the mesh
image = renderer(mesh)   # (1, H, W, 4)

I get the error the following error:

-> image = renderer(mesh)   # (1, H, W, 4)
(Pdb) c
Traceback (most recent call last):
  File "main.py", line 93, in <module>
    main()
  File "main.py", line 68, in main
    loss_train = trainer.train(epoch, loaders)
  File "~/3DMM/pytorchnet_3d/train.py", line 369, in train
    image = renderer(mesh)   # (1, H, W, 4)
  File "~/miniconda3/envs/pytorch3d/lib/python3.6/site-packages/torch/nn/modules/module.py", line 532, in __call__
    result = self.forward(*input, **kwargs)
  File "~/miniconda3/envs/pytorch3d/lib/python3.6/site-packages/pytorch3d/renderer/mesh/renderer.py", line 69, in forward
    images = self.shader(fragments, meshes_world, **kwargs)
  File "~/miniconda3/envs/pytorch3d/lib/python3.6/site-packages/torch/nn/modules/module.py", line 532, in __call__
    result = self.forward(*input, **kwargs)
  File "~/miniconda3/envs/pytorch3d/lib/python3.6/site-packages/pytorch3d/renderer/mesh/shader.py", line 269, in forward
    texels = interpolate_texture_map(fragments, meshes)
  File "~/miniconda3/envs/pytorch3d/lib/python3.6/site-packages/pytorch3d/renderer/mesh/texturing.py", line 43, in interpolate_texture_map
    faces_uvs = meshes.textures.faces_uvs_packed()
  File "~/miniconda3/envs/pytorch3d/lib/python3.6/site-packages/pytorch3d/structures/textures.py", line 147, in faces_uvs_packed
    return list_to_packed(self.faces_uvs_list())[0]
  File "~/miniconda3/envs/pytorch3d/lib/python3.6/site-packages/pytorch3d/structures/utils.py", line 116, in list_to_packed
    N = len(x)
TypeError: object of type 'NoneType' has no len()

However, the rendering works only when the Texture is initialized with verts_uvs, faces_uvs and texture_maps.

[nikhilaravi]

TexturedSoftPhongShader supports only textures specified as texture maps and vertex uv coordinates. Please use SoftPhongShader instead.

[nikhilaravi]

@aluo-x support for textures as a per face atlas of shape (F, R, R, 3) has now been added (based on the SoftRas implementation). Here is a complete example of how to load the textures as an atlas, create a mesh and and render it: https://github.com/facebookresearch/pytorch3d/blob/master/tests/test_render_meshes.py#L468.