# Converting vtkUnstructuredGrid to vtkPolyData
**URL:** https://discourse.vtk.org/t/converting-vtkunstructuredgrid-to-vtkpolydata/12508
**Category:** Support
**Tags:** python
**Created:** [October 19, 2023, 2:00pm UTC](https://discourse.vtk.org/t/converting-vtkunstructuredgrid-to-vtkpolydata/12508 "2023-10-19T14:00:43Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@rexthor](https://discourse.vtk.org/u/rexthor)
#### Post date: [October 19, 2023, 2:00pm UTC](https://discourse.vtk.org/t/converting-vtkunstructuredgrid-to-vtkpolydata/12508/1 "2023-10-19T14:00:43Z")
I have a vtkUnstructuredGrid that is a contiguous mass of connected vtkHexahedrons (it is an appended grouping of vtkUnstructuredGrid). I want to use some algorithms that only work with vtkPolyData (namely I want to delete some cells where a variable is small). The data is very large, and I want to make sure that I am not deep copying.
Right now, this is how the script looks:
```python
# data is a vtkUnstructuredGrid
# data.GetPointData().GetNumberOfArrays() --> 9
# data.GetNumberOfPoints() --> more than 12 million
# I'm not sure if these 'Set' functions make deep or shallow copies ...
pd = vtk.vtkPolyData()
pd.SetPoints(data.GetPoints())
pd.SetPolys(data.GetCells())
for a_idx in range(data.GetPointData().GetNumberOfArrays()):
pd.GetFieldData().AddArray(data.GetPointData().GetArray(a_idx))
```
---
### Author:  [@rexthor](https://discourse.vtk.org/u/rexthor)
#### Post date: [October 19, 2023, 4:29pm UTC](https://discourse.vtk.org/t/converting-vtkunstructuredgrid-to-vtkpolydata/12508/2 "2023-10-19T16:29:52Z")
It does occur to me that I can’t add vtkHexahedrons to a vtkPolyData … ultimately I want the faces of the hexahedrons in there … but the above code for sure will not work.
---
### Author:  [@Charles\_Gueunet](https://discourse.vtk.org/u/Charles_Gueunet)
#### Post date: [October 23, 2023, 7:42am UTC](https://discourse.vtk.org/t/converting-vtkunstructuredgrid-to-vtkpolydata/12508/3 "2023-10-23T07:42:16Z")
If you only want the external surface of your mesh, you may want to look at the `vtkGeometryFilter`.
If you want to all the internal faces, do you need them to stay connected ? do you need them to stay quads ?
---
### Author:  [@rexthor](https://discourse.vtk.org/u/rexthor)
#### Post date: [October 23, 2023, 12:31pm UTC](https://discourse.vtk.org/t/converting-vtkunstructuredgrid-to-vtkpolydata/12508/4 "2023-10-23T12:31:21Z")
Thanks - I hadn’t heard of vtkGeometryFilter, I actually do need the insides of the object - I mostly wanted to convert the thing to vtkPolyData so that I could run vtkCleanPolyData on it. I am beginning to realize that the real answer to my question is “it depends.” In my, very strange, circumstance the code that generated what I needed was:
```python
# FIXME: hardcoded: Temperature is index 7
logging.debug("generating vtkCellArray of unique faces")
CELL_NODE_COUNT = data.GetCell(0).GetNumberOfPoints()
FACE_NODE_COUNT = data.GetCell(0).GetFace(0).GetNumberOfPoints()
temp_array = pd.GetPointData().GetAbstractArray(7)
unique_quads = set()
ca = vtk.vtkCellArray()
for c_idx in range(data.GetNumberOfCells()):
# Skip those cells for which the average nodal temperature is <= T_LOWER_LIMIT
cell = data.GetCell(c_idx)
cell_node_temps = [temp_array.GetTuple1(cell.GetPointId(n)) for n in range(CELL_NODE_COUNT)]
if np.mean(cell_node_temps) <= T_LOWER_LIMIT:
continue
# Break cell into faces, and only add unique faces.
for f_idx in range(data.GetCell(0).GetNumberOfFaces()):
face = data.GetCell(c_idx).GetFace(f_idx)
face_ids = [face.GetPointId(x) for x in range(FACE_NODE_COUNT)]
sorted_ids = tuple(sorted(face_ids))
reversed_ids = tuple(reversed(sorted(face_ids)))
sorted_ids = min(sorted_ids, reversed_ids)
if sorted_ids not in unique_quads:
unique_quads.add(sorted_ids)
ca.InsertNextCell(FACE_NODE_COUNT, face_ids)
pd.SetPolys(ca)
logging.debug("cleaning final vtkPolyData")
cpd = vtk.vtkCleanPolyData()
cpd.SetInputData(pd)
cpd.Update()
```