Changing mesh coordinates

Users may want to change the coordinates of an existing mesh object for certain reasons. The coordinates can be accessed as a Function through mesh.coordinates where mesh is a mesh object. For example,

mesh.coordinates.dat.data[:, 1] *= 2.0

streches the mesh in the y-direction. Another possibility is to use assign():

Vc = mesh.coordinates.function_space()
x, y = SpatialCoordinate(mesh)
f = Function(Vc).interpolate(as_vector([x, y*2.0]))
mesh.coordinates.assign(f)

This can also be used if f is a solution to a PDE.

Changing the coordinate function space

For more complicated situations, one might wish to replace the mesh coordinates with a field which lives on a different FunctionSpace (e.g. higher-order meshes).

Note

Re-assigning the coordinates property of a mesh used to be an undocumented feature. However, this no longer works:

mesh.coordinates = f  # Raises an exception

Instead of re-assigning the coordinates of a mesh, one can create new mesh object from a field f:

new_mesh = Mesh(f)

new_mesh has the same mesh topology as the original mesh, but its coordinate values and coordinate function space are from f. The coordinate function space must be a rank-1 FunctionSpace, constructed either with VectorFunctionSpace(), or by providing a VectorElement to FunctionSpace(). For efficiency, the new mesh object shares data with f. That is, changing the values of f will change the coordinate values of the mesh, and vice versa. If this behaviour is undesired, one should explicitly copy:

g = Function(f)  # creates a copy of f
new_mesh = Mesh(g)

Or simply:

new_mesh = Mesh(Function(f))

Replacing the mesh geometry of an existing function

Creating a new mesh geometry object, as described above, leaves any existing Functions untouched – they continue to live on their original mesh geometries. One may wish to move these functions over to the new mesh. To move f over to mesh, use:

g = Function(functionspaceimpl.WithGeometry(f.function_space(), mesh),
             val=f.topological)

This creates a Function g which shares data with f, but its mesh geometry is mesh.

Warning

The example above uses Firedrake internal APIs, which might change in the future.