# Get the normal vector of a polygon using vtkPolyDataNormals
**URL:** https://discourse.vtk.org/t/get-the-normal-vector-of-a-polygon-using-vtkpolydatanormals/3751
**Category:** Support
**Created:** [July 11, 2020, 9:04pm UTC](https://discourse.vtk.org/t/get-the-normal-vector-of-a-polygon-using-vtkpolydatanormals/3751 "2020-07-11T21:04:04Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@efdiloreto](https://discourse.vtk.org/u/efdiloreto)
#### Post date: [July 11, 2020, 9:04pm UTC](https://discourse.vtk.org/t/get-the-normal-vector-of-a-polygon-using-vtkpolydatanormals/3751/1 "2020-07-11T21:04:04Z")
I have a simple polygon where all points belong to the same plane, therefore the normal vectors at each point should be the same. The problem I have is that I can’t get those vectors.
For example, I have the following code.
```
normals = vtk.vtkPolyDataNormals()
normals.SetInputData(poly_data) # existing polydata
normals.ComputePointNormalsOn()
normals.ComputeCellNormalsOn()
normals.SplittingOff()
normals.ConsistencyOn()
normals.AutoOrientNormalsOn()
normals.Update()
```
From here I do not know how to obtain these vectors. I have tried different ways but apparently I am doing something wrong.
---
### Author:  [@lassoan](https://discourse.vtk.org/u/lassoan)
#### Post date: [July 11, 2020, 10:23pm UTC](https://discourse.vtk.org/t/get-the-normal-vector-of-a-polygon-using-vtkpolydatanormals/3751/2 "2020-07-11T22:23:31Z")
Point normal is computed by averaging neighbor polygon cell normals, so it is expected to be different from the polygon cell normal. See details in [Chapter 9 of VTK textbook](https://lorensen.github.io/VTKExamples/site/VTKBook/09Chapter9/).
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### Author:  [@efdiloreto](https://discourse.vtk.org/u/efdiloreto)
#### Post date: [July 11, 2020, 10:27pm UTC](https://discourse.vtk.org/t/get-the-normal-vector-of-a-polygon-using-vtkpolydatanormals/3751/3 "2020-07-11T22:27:05Z")
Thanks Andras. My polygon only has one cell, does this also apply in this case?
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### Author:  [@lassoan](https://discourse.vtk.org/u/lassoan)
#### Post date: [July 11, 2020, 11:32pm UTC](https://discourse.vtk.org/t/get-the-normal-vector-of-a-polygon-using-vtkpolydatanormals/3751/4 "2020-07-11T23:32:57Z")
Can you provide a full example (including setting of `poly_data` and retrieving of normal vectors)?
What result do you expect and what do you get?
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### Author:  [@efdiloreto](https://discourse.vtk.org/u/efdiloreto)
#### Post date: [July 11, 2020, 11:48pm UTC](https://discourse.vtk.org/t/get-the-normal-vector-of-a-polygon-using-vtkpolydatanormals/3751/5 "2020-07-11T23:48:31Z")
Yes of course.