First Steps: Creating a simple mesh

This section demonstrates how to create a new mesh, add some vertices and faces to it and then modify the newly inserted points.

First, we will import the openmesh and numpy modules:

import openmesh as om
import numpy as np

Next, we can create an empty mesh:

mesh = om.TriMesh()

OpenMesh provides two mesh types: One for polygonal meshes (PolyMesh) and one for triangle meshes (TriMesh). You should use triangle meshes whenever possible, since they are usually more efficient. In addition, some algorithms are only implemented for triangle meshes while triangle meshes inherit the full functionality of polygonal meshes.

Now that we have our empty mesh object we can add a couple of vertices:

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])

The add_vertex() member function takes numpy arrays with shape (3,) as point coordinates and returns a handle to the newly inserted vertex. As shown in the code above we can also pass lists with 3 elements as point coordinates. The lists are automatically converted to numpy arrays.

In order to add a new face to our mesh we have to call add_face(). This function takes the handles of the vertices that make up the new face and returns a handle to the newly inserted face:

fh0 = mesh.add_face(vh0, vh1, vh2)
fh1 = mesh.add_face(vh1, vh3, vh4)
fh2 = mesh.add_face(vh0, vh3, vh1)

We can also pass a list of vertex handles to add_face():

vh_list = [vh2, vh1, vh4]
fh3 = mesh.add_face(vh_list)

Our mesh should now look like this:

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

We can access the point coordinates of each vertex by calling point(). This member function takes a vertex handle and returns a numpy array with shape (3,):

point = mesh.point(vh0)

We can also get an array containing all points of a mesh by calling points(). The returned array has shape (n, 3), where n is the number of vertices:

point_array = mesh.points()

The latter is useful if we want to update all points of a mesh at once. For example, we can translate our mesh along the x-axis like this:

point_array += np.array([1, 0, 0])

The arrays returned by point() and points() both reference the underlying mesh data. This means that changes made to either one of these arrays affect the original mesh.

The complete source for this section looks like this:

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

# get the point with vertex handle vh0
point = mesh.point(vh0)

# get all points of the mesh
point_array = mesh.points()

# translate the mesh along the x-axis
point_array += np.array([1, 0, 0])