# Is there a way to programmatically mark/select cells for extraction?
**URL:** https://discourse.vtk.org/t/is-there-a-way-to-programmatically-mark-select-cells-for-extraction/14379
**Category:** Support
**Created:** [August 9, 2024, 1:43pm UTC](https://discourse.vtk.org/t/is-there-a-way-to-programmatically-mark-select-cells-for-extraction/14379 "2024-08-09T13:43:45Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@rexthor](https://discourse.vtk.org/u/rexthor)
#### Post date: [August 9, 2024, 1:43pm UTC](https://discourse.vtk.org/t/is-there-a-way-to-programmatically-mark-select-cells-for-extraction/14379/1 "2024-08-09T13:43:45Z")
Perhaps there is a more elegant way of doing this, but I wanted to select cells where either the normal had a positive dot product with a given unit vector, or that had a positive dot product with a direction to a point in space. (Like directional light and point light in a graphics sense).
Right now I am doing it with something like this (and the selection is frustratingly not-quite-hemispherical):
```python
import numpy as np
import vtkmodules.vtkRenderingOpenGL2
from vtkmodules.util.numpy_support import vtk_to_numpy
from vtkmodules.vtkFiltersCore import vtkPolyDataNormals
from vtkmodules.vtkFiltersExtraction import vtkExtractSelection
from vtkmodules.vtkFiltersGeneral import vtkShrinkFilter
from vtkmodules.vtkFiltersSources import vtkSelectionSource, vtkSphereSource
from vtkmodules.vtkInteractionStyle import vtkInteractorStyleTrackballCamera
from vtkmodules.vtkRenderingCore import (
vtkActor,
vtkDataSetMapper,
vtkRenderer,
vtkRenderWindow,
vtkRenderWindowInteractor)
s = vtkSphereSource()
s.SetPhiResolution(20)
s.SetThetaResolution(20)
nf = vtkPolyDataNormals()
nf.ComputeCellNormalsOn()
nf.SetInputConnection(s.GetOutputPort())
nf.Update()
pd = nf.GetOutput()
normals = vtk_to_numpy(pd.GetCellData().GetNormals())
direction = np.array((1, 1, 0))
direction = direction/np.sqrt(np.dot(direction, direction))
ss = vtkSelectionSource()
ss.SetContentType(3) # supposed to be SelectionContent::INDICES ... python equiv?
ss.SetFieldType(0) # supposed to be SelectionField::CELL ... python equiv?
ci = pd.NewCellIterator()
ci.InitTraversal()
while True:
ci.GoToNextCell()
if ci.IsDoneWithTraversal():
break
idx = ci.GetCellId()
cell_normal = normals[idx, :]
if np.dot(cell_normal, direction) >= 0:
ss.AddID(-1, idx)
es = vtkExtractSelection()
es.SetInputConnection(0, nf.GetOutputPort())
es.SetInputConnection(1, ss.GetOutputPort())
es.Update()
sf = vtkShrinkFilter()
sf.SetInputConnection(es.GetOutputPort())
sf.SetShrinkFactor(0.9)
mapper = vtkDataSetMapper()
mapper.SetInputConnection(sf.GetOutputPort())
actor = vtkActor()
actor.SetMapper(mapper)
ren = vtkRenderer()
ren_win = vtkRenderWindow()
ren_win.SetWindowName("Selected Sphere")
ren_win.SetSize(600, 600)
ren_win.AddRenderer(ren)
iren = vtkRenderWindowInteractor()
iren.SetRenderWindow(ren_win)
style = vtkInteractorStyleTrackballCamera()
iren.SetInteractorStyle(style)
ren.AddActor(actor)
ren.ResetCamera()
ren_win.Render()
iren.Start()
```
---
### Author:  [@Jens\_Munk\_Hansen](https://discourse.vtk.org/u/Jens_Munk_Hansen)
#### Post date: [August 10, 2024, 2:21pm UTC](https://discourse.vtk.org/t/is-there-a-way-to-programmatically-mark-select-cells-for-extraction/14379/2 "2024-08-10T14:21:32Z")
Instead of a selection source, I have often done this. Don’t know if this is better?
```auto
polydataConnect = vtkPolyDataConnectivityFilter()
polydataConnect.SetExtractionModeToCellSeededRegions()
polydataConnect.InitializeSeedList()
for i in range(nCells):
// Here your criteria on the dot product
polydataConnect.AddSeed(i)
polydataConnect.SetInputData(pd);
polydataConnect.Update()
```
---
### Author:  [@rexthor](https://discourse.vtk.org/u/rexthor)
#### Post date: [August 12, 2024, 12:47pm UTC](https://discourse.vtk.org/t/is-there-a-way-to-programmatically-mark-select-cells-for-extraction/14379/3 "2024-08-12T12:47:24Z")
That’s an interesting class, but for my particular use case there isn’t a topological way of extracting the region (I don’t think there is …).
---
### Author:  [@mau\_igna\_06](https://discourse.vtk.org/u/mau_igna_06)
#### Post date: [August 12, 2024, 4:24pm UTC](https://discourse.vtk.org/t/is-there-a-way-to-programmatically-mark-select-cells-for-extraction/14379/4 "2024-08-12T16:24:02Z")
I think you’ll need to use [VTK: vtkThreshold Class Reference](https://vtk.org/doc/nightly/html/classvtkThreshold.html)
---
### Author:  [@rexthor](https://discourse.vtk.org/u/rexthor)
#### Post date: [August 12, 2024, 6:27pm UTC](https://discourse.vtk.org/t/is-there-a-way-to-programmatically-mark-select-cells-for-extraction/14379/5 "2024-08-12T18:27:39Z")
Oh, I like that. So, for instance, I would define a node/cell data array, assign integers to it (inlet pressure = 1, wall = 2, etc.) and then threshold:
(swiped from [here](https://examples.vtk.org/site/Python/Medical/GenerateCubesFromLabels/)):
```python
selector = vtkThreshold()
selector.SetInputArrayToProcess(0, 0, 0, vtkDataObject().FIELD_ASSOCIATION_CELLS,
vtkDataSetAttributes().SCALARS)
selector.SetInputConnection(pad.GetOutputPort())
selector.SetLowerThreshold(start_label)
selector.SetUpperThreshold(end_label)
selector.Update()
```