Painting faces instead of vertices for meshes

[lucabergamini]

Is your feature request related to a problem? Please describe.
Currently, it's only possible to colour meshes via the vertex_colors attribute. However, this may introduces artifacts due to interpolation between the 3 points of a face if the colour source is related to faces instead of vertices (as a Blender's material).

Describe the solution you'd like
It would be great to have the possibility to set the colour for faces along with vertices (two different modes maybe?).

Describe alternatives you've considered
As the issue is related with the size of the faces, subdividing the mesh's triangles reduces its magnitude but also increases computation.

[yxlao]

Mesh is colored by vertices in open3d. To support "pure-color face", we can create 3 vertices for each triangle, and assign the same color to all 3 vertices for each triangle. That is, we'll end up with N triangles and exactly 3N vertices.

[griegler]

@yxlao The problem with this approach is that it then isn't a triangle mesh anymore, but a triangle soup. Hence, algorithms like filtering will not work as expected. We should consider facet colors in our new design of the geometry classes.

[lucabergamini]

What @yxlao proposes does work in fact, but as @griegler points out the mesh's triangles are no longer connected.

Still, it's the only fix at the moment and should be enough for what I'm trying to do.
Here is a quick implementation to convert a mesh in independent triangles apt to be colourised as faces:

def split_triangles(mesh: o3d.TriangleMesh) -> o3d.TriangleMesh:
    """
    Split the mesh in independent triangles    
    """
    triangles = np.asarray(mesh.triangles).copy()
    vertices = np.asarray(mesh.vertices).copy()

    triangles_3 = np.zeros_like(triangles)
    vertices_3 = np.zeros((len(triangles) * 3, 3), dtype=vertices.dtype)

    for index_triangle, t in enumerate(triangles):
        index_vertex = index_triangle * 3
        vertices_3[index_vertex] = vertices[t[0]]
        vertices_3[index_vertex + 1] = vertices[t[1]]
        vertices_3[index_vertex + 2] = vertices[t[2]]

        triangles_3[index_triangle] = np.arange(index_vertex, index_vertex + 3)

    mesh_return = deepcopy(mesh)
    mesh_return.triangles = o3d.Vector3iVector(triangles_3)
    mesh_return.vertices = o3d.Vector3dVector(vertices_3)
    return mesh_return

[yxlao]

@griegler Right, this will become a triangle soup. So it's a temporary workaround.

[errissa]

This feature is planned for 0.12 release.

[rubenverhack]

This feature is planned for 0.12 release.

Is there any news on this? For me this makes open3d unusable for my use case.

[germanros1987]

@errissa is this available now?

[errissa]

This is available now. The visualizer supports per-face colors. However, only t::geometry::TriangleMesh supports setting colors per-face using Set/GetTriangleColors. Also, if a TriangleMesh has both vertex and triangle colors the visualizer will use the vertex colors.

[andy-beer]

@errissa, Is there a python binding for SetTriangleColors?

[errissa]

@andy-beer SetTriangleColors is a convenience function in C++ that isn't really necessary in Python. In Python the following is the exact equivalent of using SetTriangleColors:

mesh.triangle["colors"] = o3d.core.Tensor(...)

[Jerrypiglet]

@errissa Thanks for the info! However I found open3d.geometry.TriangleMesh has no property named 'triangle' as indicated here http://www.open3d.org/docs/release/python_api/open3d.geometry.TriangleMesh.html# Any ideas?

[lucagrementieri]

The property triangle 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

The comment refers to a mesh object from this class, the color assignment works and colors can be visualized correctly using o3d.visualization.draw.