How to color faces in OpenMesh?

Question

There is no example in OpenMesh documentation for coloring faces. Which function should I use to color fh0 to green? (I tried mesh.set_color but could not succeed. You can see my attempt on second part of code)

import openmesh as om
import numpy as np

mesh = om.TriMesh()

# add a a couple of vertices to the mesh
vh0 = mesh.add_vertex([0, 1, 0])
vh1 = mesh.add_vertex([1, 0, 0])
vh2 = mesh.add_vertex([2, 1, 0])
vh3 = mesh.add_vertex([0,-1, 0])
vh4 = mesh.add_vertex([2,-1, 0])

# add a couple of faces to the mesh
fh0 = mesh.add_face(vh0, vh1, vh2)
fh1 = mesh.add_face(vh1, vh3, vh4)
fh2 = mesh.add_face(vh0, vh3, vh1)

# add another face to the mesh, this time using a list
vh_list = [vh2, vh1, vh4]
fh3 = mesh.add_face(vh_list)

#  0 ==== 2
#  |\  0 /|
#  | \  / |
#  |2  1 3|
#  | /  \ |
#  |/  1 \|
#  3 ==== 4

for face in mesh.faces():
    mesh.set_color(face, [0.67578125, 0.296875, 0.3515625])

om.write_mesh('test.obj', mesh)

but this gives me

IndexError: index 3 is out of bounds for axis 0 with size 3

How can I add color to faces in OpenMesh?

Answer 1

There are several issues:

1. mesh.request_face_colors() is required for the TriMesh object to support storing face colors.
2. mesh.set_color expects an alpha channel, so your color is actually [0.67578125, 0.296875, 0.3515625, 1.]
3. write_mesh requires face_color=True argument.
4. .obj file format doesn't support vertex/face color. Color is saved in a separate .mtl file ("material"). You can use other formats instead, ie .ply.

Full code:

import openmesh as om

mesh = om.TriMesh()

# add a a couple of vertices to the mesh
vh0 = mesh.add_vertex([0, 1, 0])
vh1 = mesh.add_vertex([1, 0, 0])
vh2 = mesh.add_vertex([2, 1, 0])
vh3 = mesh.add_vertex([0,-1, 0])
vh4 = mesh.add_vertex([2,-1, 0])

# add a couple of faces to the mesh
fh0 = mesh.add_face(vh0, vh1, vh2)
fh1 = mesh.add_face(vh1, vh3, vh4)
fh2 = mesh.add_face(vh0, vh3, vh1)

# add another face to the mesh, this time using a list
vh_list = [vh2, vh1, vh4]
fh3 = mesh.add_face(vh_list)

#  0 ==== 2
#  |\  0 /|
#  | \  / |
#  |2  1 3|
#  | /  \ |
#  |/  1 \|
#  3 ==== 4

mesh.request_face_colors()

for face in mesh.faces():
    mesh.set_color(face, [0.67578125, 0.296875, 0.3515625, 1.])

for face in mesh.faces():
    print(mesh.color(face))

om.write_mesh('test.obj', mesh, face_color=True)
om.write_mesh('test.ply', mesh, face_color=True)

Console output (color attribute was successfully added to the mesh):

[0.67578125 0.296875   0.3515625  1.        ]
[0.67578125 0.296875   0.3515625  1.        ]
[0.67578125 0.296875   0.3515625  1.        ]
[0.67578125 0.296875   0.3515625  1.        ]

Content of test.obj:

# 5 vertices, 4 faces
mtllib test.mat
v 0.000000 1.000000 0.000000
v 1.000000 0.000000 0.000000
v 2.000000 1.000000 0.000000
v 0.000000 -1.000000 0.000000
v 2.000000 -1.000000 0.000000
usemtl mat0
f 1 2 3
f 2 4 5
f 1 4 2
f 3 2 5

Content of test.mat (this file specifies what "mat0" in the test.obj file means):

newmtl mat0
Ka 0.5000 0.5000 0.5000
Kd 0.67451 0.298039 0.352941
illum 1

Content of test.ply (everything in a single file):

ply
format ascii 1.0
element vertex 5
property float x
property float y
property float z
element face 4
property list uchar int vertex_indices
property uchar red
property uchar green
property uchar blue
end_header
0 1 0
1 0 0
2 1 0
0 -1 0
2 -1 0
3 0 1 2 172 76 90
3 1 3 4 172 76 90
3 0 3 1 172 76 90
3 2 1 4 172 76 90

Answer 2

Face color is one of the standard properties in OpenMesh.

To add a standard property to an entity simply use the appropriate request method.

In your case it will be mesh.request_face_colors().

referenced documentation