color per triangle?

[cwreynolds]

Checklist

• I have searched for similar issues.
• For Python issues, I have tested with the latest development wheel.
• I have checked the release documentation and the latest documentation (for master branch).

My Question

(I may not understand the second question above, but I think I am running the most recent released version: Open3D 0.17.0, Python 3.10.10, macOS 12.6.3)

For a diagram-like rendering, I would like to use color per triangle instead of interpolated color per vertex (via open3d.geometry.TriangleMesh.vertex_colors). But this comment suggests per-triangle support was added to Open3D by December 2021. If so, I am unable to get it to run. This Python test code:

import open3d as o3d
import numpy as np

def face_color_test():
    mesh = o3d.geometry.TriangleMesh.create_octahedron()
    rgb = [[0,0,0],[0,0,1],[0,1,0],[0,1,1],[1,0,0],[1,0,1],[1,1,0],[1,1,1]]
    # face_colors = o3d.utility.Vector3dVector(np.array(rgb, np.float32))
    face_colors = o3d.utility.Vector3iVector(np.array(rgb, np.int32))
    mesh.triangles["colors"] = face_colors
    vis = o3d.visualization.Visualizer()
    vis.create_window()
    vis.add_geometry(mesh)
    vis.run()
    vis.destroy_window()

gets an error on mesh.triangles["colors"] = face_colors:

Traceback (most recent call last):
  File "/Users/cwr/Documents/code/flock/flock.py", line 158, in <module>
    face_color_test()
  File "/Users/cwr/Documents/code/flock/flock.py", line 133, in face_color_test
    mesh.triangles["colors"] = face_colors
TypeError: __setitem__(): incompatible function arguments. The following argument types are supported:
    1. (self: open3d.cpu.pybind.utility.Vector3iVector, arg0: int, arg1: numpy.ndarray[numpy.int32[3, 1]]) -> None
    2. (self: open3d.cpu.pybind.utility.Vector3iVector, arg0: slice, arg1: open3d.cpu.pybind.utility.Vector3iVector) -> None

I am currently using one of the workarounds discussed on that earlier Issue. But if there is a supported way to apply face colors directly I would prefer to use that. Any suggestions? Is this feature mentioned in the documentation? I have failed to find it, and the keywords (triangles, colors) are too common for searching.

(I would expect the color data to be rgb triples of floats, but changed it from Vector3dVector to Vector3iVector based on the error message. I also tried o3d.core.Tensor(...) as suggested in the previous Issue, but got the same error.)

[cwreynolds]

Since all I know about this feature is from a comment by @errissa I tag them here in case they want to comment.

[lucagrementieri]

The property triangle cited in the comment exists for the object open3d.t.geometry.TriangleMesh based on tensors, see http://www.open3d.org/docs/release/python_api/open3d.t.geometry.TriangleMesh.html#open3d.t.geometry.TriangleMesh

In you example, you are using the legacy TriangleMesh that has no support for colors on faces.

[cwreynolds]

Thanks @lucagrementieri — just a few days ago I discovered the “t”/“tensor” subworld of Open3D, via experimenting with o3d.visualization.draw(). I made the connection with this question, thinking I should update/close it. Thanks for the reminder.

I would suggest that there is a bigger problem in the docs and sample code. After I started to understand the “t”/“legacy” distinction I looked for a description of that distinction and failed. (Similarly the doc for o3d.visualization.draw() is almost nonexistent, as mentioned here: #6094 ) As you indicate there is doc on (say)
open3d.geometry.TriangleMesh and open3d.t.geometry.TriangleMesh but no “narrative” on why both exist, nor when you should use either.

My intuition is that the “legacy” code is deprecated and that all new development has been on o3d.visualization.draw() and the tensor classes. However, all of the “getting started” docs and sample code talk about the legacy API. This steers new users astray and leads to the sort of confusion seen in this issue (#6060).

[cwreynolds]

(This issue of legacy-vs-tensor confusion came up a day later here: #6130)