# How to extract the ids of the boundary points of a surface?
**URL:** https://discourse.vtk.org/t/how-to-extract-the-ids-of-the-boundary-points-of-a-surface/2530
**Category:** Support
**Created:** [February 1, 2020, 2:30am UTC](https://discourse.vtk.org/t/how-to-extract-the-ids-of-the-boundary-points-of-a-surface/2530 "2020-02-01T02:30:34Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@normanius](https://discourse.vtk.org/u/normanius)
#### Post date: [February 1, 2020, 2:30am UTC](https://discourse.vtk.org/t/how-to-extract-the-ids-of-the-boundary-points-of-a-surface/2530/1 "2020-02-01T02:30:34Z")
I know that it is possible to extract boundary edges using [`vtkFeatureEdges`](https://vtk.org/doc/nightly/html/classvtkFeatureEdges.html). See [this example](https://lorensen.github.io/VTKExamples/site/Cxx/Meshes/BoundaryEdges/). The edges are returned as a new `vtkPolyData` consisting of one or more `vtkPolyLines`.
However, I require the point _ids_ of the input surface, and not an independent poly with new points and edges. How to achieve that?
I tried to use `vtkIdFilter` (in the hope that `vtkFeatureEdges` preserves point data), to no avail. Alternatively, I could use a point locator to find the boundary points again in the original surface. However, this looks a bit awkward to me. Is there a simpler way to achieve this?
---
### Author:  [@Paulo\_Carvalho](https://discourse.vtk.org/u/Paulo_Carvalho)
#### Post date: [February 1, 2020, 5:38pm UTC](https://discourse.vtk.org/t/how-to-extract-the-ids-of-the-boundary-points-of-a-surface/2530/2 "2020-02-01T17:38:35Z")
My first attempt would be to assign the id’s to a scalar field (to the vertexes) in the original mesh as `vtkFeatureEdges` is supposed to preserve scalars in the output. This would be a little less awkward, IMO, than performing a spatial search.
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### Author:  [@normanius](https://discourse.vtk.org/u/normanius)
#### Post date: [February 1, 2020, 7:01pm UTC](https://discourse.vtk.org/t/how-to-extract-the-ids-of-the-boundary-points-of-a-surface/2530/3 "2020-02-01T19:01:46Z")
I totally agree. Using [`vtkIdFilter`](https://vtk.org/doc/nightly/html/classvtkIdFilter.html) is the right way (it creates such a scalar array with the ids as point data). I had messed up something in my tests. The following works:
```
def extractBoundaryIds(source):
idFilter = vtk.vtkIdFilter()
idFilter.SetInputConnection(source.GetOutputPort())
idFilter.SetIdsArrayName("ids")
idFilter.SetPointIds(True)
idFilter.SetCellIds(False)
# Available for vtk>=8.3:
#idFilter.SetPointIdsArrayName(arrayName)
#idFilter.SetCellIdsArrayName(arrayName)
idFilter.Update()
edges = vtk.vtkFeatureEdges()
edges.SetInputConnection(idFilter.GetOutputPort())
edges.BoundaryEdgesOn()
edges.ManifoldEdgesOff()
edges.NonManifoldEdgesOff()
edges.FeatureEdgesOff()
edges.Update()
array = edges.GetOutput().GetPointData().GetArray("ids")
n = edges.GetOutput().GetNumberOfPoints()
boundaryIds = []
for i in range(n):
boundaryIds.append(array.GetValue(i))
return boundaryIds
```