# problem with interpolating vtkImageData on vtkPolyData with vtkPointInterpolator
**URL:** https://discourse.vtk.org/t/problem-with-interpolating-vtkimagedata-on-vtkpolydata-with-vtkpointinterpolator/6594
**Category:** Support
**Created:** [September 9, 2021, 1:34pm UTC](https://discourse.vtk.org/t/problem-with-interpolating-vtkimagedata-on-vtkpolydata-with-vtkpointinterpolator/6594 "2021-09-09T13:34:20Z")
**Posts on this page:** 11
**Page:** 1
### Author:  [@martinick](https://discourse.vtk.org/u/martinick)
#### Post date: [September 9, 2021, 1:34pm UTC](https://discourse.vtk.org/t/problem-with-interpolating-vtkimagedata-on-vtkpolydata-with-vtkpointinterpolator/6594/1 "2021-09-09T13:34:20Z")
Hi all,
I’m trying to interpolate a .vti volume on a .vtp mesh using vtkPointInterpolator but I get “Segmentation fault (core dumped)” when I’m updating the vtkPointInterpolator filter. Here is my code:
```auto
import vtk
# Read vtkPolyData
reader = vtk.vtkXMLPolyDataReader()
reader.SetFileName('surfacemesh.vtp')
reader.Update()
surface = reader.GetOutput()
# Read vtkImage
reader = vtk.vtkImageReader()
reader.SetFileName('volume.vti')
reader.Update()
image3D = reader.GetOutput()
# Gaussian kernel
gaussian_kernel = vtk.vtkGaussianKernel()
gaussian_kernel.SetSharpness(2)
gaussian_kernel.SetRadius(12)
interpolator = vtk.vtkPointInterpolator()
interpolator.SetInputData(surface)
interpolator.SetSourceData(image3D)
interpolator.SetKernel(gaussian_kernel)
interpolator.Update()
```
Any ideas to solve this issue?
Thanks!
---
### Author:  [@dgobbi](https://discourse.vtk.org/u/dgobbi)
#### Post date: [September 9, 2021, 2:00pm UTC](https://discourse.vtk.org/t/problem-with-interpolating-vtkimagedata-on-vtkpolydata-with-vtkpointinterpolator/6594/2 "2021-09-09T14:00:59Z")
Try the vtkXMLImageDataReader (the vtkImageReader is intended for raw files, not .vti files).
---
### Author:  [@martinick](https://discourse.vtk.org/u/martinick)
#### Post date: [September 9, 2021, 2:41pm UTC](https://discourse.vtk.org/t/problem-with-interpolating-vtkimagedata-on-vtkpolydata-with-vtkpointinterpolator/6594/3 "2021-09-09T14:41:16Z")
Hi David,
thanks for your advice but I got the same error. Here you can find the data: `surfacemesh.vtp` and `volume.vti`
[surfacemesh.vtp](https://discourse.vtk.org/uploads/short-url/64CRq1wZnCjMdqbHRSMmHrHS80Y.vtp) (20.3 KB)
[volume.vti](https://discourse.vtk.org/uploads/short-url/pvmqHBsmfweIIkzt1d8WBiWhCS9.vti) (239.7 KB)
---
### Author:  [@dgobbi](https://discourse.vtk.org/u/dgobbi)
#### Post date: [September 9, 2021, 3:30pm UTC](https://discourse.vtk.org/t/problem-with-interpolating-vtkimagedata-on-vtkpolydata-with-vtkpointinterpolator/6594/4 "2021-09-09T15:30:21Z")
Hmm, the segfault occurs in vtkStaticPointLocator. Maybe a bug?
Try using a different locator to see if it works:
```python
interpolator.SetLocator(vtk.vtkMergePoints())
```
---
### Author:  [@martinick](https://discourse.vtk.org/u/martinick)
#### Post date: [September 10, 2021, 9:08am UTC](https://discourse.vtk.org/t/problem-with-interpolating-vtkimagedata-on-vtkpolydata-with-vtkpointinterpolator/6594/5 "2021-09-10T09:08:10Z")
Thanks for your help @dgobbi , now it works ! Please find below the corrected code and the result.
**CODE**
```python
import vtk
vtp_filename = 'surfacemesh.vtp'
vti_filename = 'volume.vti'
## Read vtkPolyData
reader = vtk.vtkXMLPolyDataReader()
reader.SetFileName(vtp_filename)
reader.Update()
surface = reader.GetOutput()
# Read vtkImage
reader = vtk.vtkXMLImageDataReader()
reader.SetFileName(vti_filename)
reader.Update()
image = reader.GetOutput()
# Gaussian kernel
gaussian_kernel = vtk.vtkGaussianKernel()
gaussian_kernel.SetSharpness(2)
gaussian_kernel.SetRadius(12)
# Set vtkPointInterpolator
interpolator = vtk.vtkPointInterpolator()
interpolator.SetInputData(surface)
interpolator.SetSourceData(image)
interpolator.SetLocator(vtk.vtkMergePoints())
interpolator.SetKernel(gaussian_kernel)
interpolator.Update()
# Write result
writer = vtk.vtkXMLPolyDataWriter()
writer.SetInputData(interpolator.GetOutput())
writer.SetFileName('interpolated.vtp')
writer.Update()
```
**RESULT**
---
### Author:  [@dgobbi](https://discourse.vtk.org/u/dgobbi)
#### Post date: [September 10, 2021, 1:19pm UTC](https://discourse.vtk.org/t/problem-with-interpolating-vtkimagedata-on-vtkpolydata-with-vtkpointinterpolator/6594/6 "2021-09-10T13:19:00Z")
Good to hear that it works. It seems that the vtkStaticPointLocator cannot be used on vtkImageData, and that’s why a different locator must be used.
Note that if you are using VTK 9 and need extra speed, the [vtkImageProbeFilter](https://vtk.org/doc/nightly/html/classvtkImageProbeFilter.html) is specifically designed to efficiently interpolate points from an image (and it’s SMP, too, like vtkPointInterpolator).
```python
interpolator = vtk.vtkImageProbeFilter()
interpolator.SetInputData(surface)
interpolator.SetSourceData(image3D)
interpolator.Update()
```
It uses linear interpolation by default, but it can also do higher order interpolation.
---
### Author:  [@martinick](https://discourse.vtk.org/u/martinick)
#### Post date: [September 14, 2021, 7:49am UTC](https://discourse.vtk.org/t/problem-with-interpolating-vtkimagedata-on-vtkpolydata-with-vtkpointinterpolator/6594/7 "2021-09-14T07:49:48Z")
Many thanks, I’tried the `vtkImageProbeFilter ` and it’s really much faster than the `vtkStaticPointLocator` on vtkImageData 👍
---
### Author:  [@kerim](https://discourse.vtk.org/u/kerim)
#### Post date: [October 27, 2021, 11:04pm UTC](https://discourse.vtk.org/t/problem-with-interpolating-vtkimagedata-on-vtkpolydata-with-vtkpointinterpolator/6594/8 "2021-10-27T23:04:22Z")
@dgobbi thank you, this helped me to solve similar problem.
I’m interpolating custom object inherited from `vtkDataSet` and I’m wondering is it possible to somehow tell the interpolator that it should look for the points in some order?
My dataset has some [partly predictable structure](https://discourse.vtk.org/t/writing-custom-class-based-on-vtkdataset/6950/3) and I would like to experiment with perfomance (the dataset may take pretty much of memory).
Is it `vtkPointLocator` I should try to look at and probably inherit from it? Or `vtkDataSet::GetPoint()/GetCell()` are the only methods I should improve?
---
### Author:  [@dgobbi](https://discourse.vtk.org/u/dgobbi)
#### Post date: [October 28, 2021, 1:35pm UTC](https://discourse.vtk.org/t/problem-with-interpolating-vtkimagedata-on-vtkpolydata-with-vtkpointinterpolator/6594/9 "2021-10-28T13:35:42Z")
Hi Kerim, if your data set has cells in addition to points, then try using vtkProbeFilter to do the interpolation. If it’s necessary to use vtkPointInterpolator instead of vtkProbeFilter, then a custom vtkPointLocator subclass is probably the best way to go (the locators have a lot of interface methods, so it might require a lot of work).
---
### Author:  [@kerim](https://discourse.vtk.org/u/kerim)
#### Post date: [November 7, 2021, 2:27am UTC](https://discourse.vtk.org/t/problem-with-interpolating-vtkimagedata-on-vtkpolydata-with-vtkpointinterpolator/6594/10 "2021-11-07T02:27:49Z")
Thank you for the hints,
I’m facing some problems when using `vtkProbeFilter` even though it works fine in most cases but sometimes there is an incorrect result.
I’m trying to debug and I set the breakpoint in `MyDataset::FindCell()` method but the program never goes there. I don’t understand why but [according to the documentation](https://vtk.org/doc/nightly/html/classvtkProbeFilter.html#a3f8ecf7772d00409715704b2a36b8216) it should.
What maybe the reason of that?
```cpp
vtkNew interpolator;
interpolator->SetFindCellStrategy(nullptr);
interpolator->SetCellLocatorPrototype(nullptr);
interpolator->SetSourceData(raisedGrid); // this is my dataset
interpolator->SetInputData(imageData); // I want to interpolate my dataset `raisedGrid` on `vtkImageData`
interpolator->Update();
```
---
### Author:  [@kerim](https://discourse.vtk.org/u/kerim)
#### Post date: [November 8, 2021, 1:23am UTC](https://discourse.vtk.org/t/problem-with-interpolating-vtkimagedata-on-vtkpolydata-with-vtkpointinterpolator/6594/11 "2021-11-08T01:23:05Z")
I have looked to the source code of `vtkProbeFilter` and understood that when it somehow works with `vtkImageData` it doesn’t use `vtkDataSet::FindCell()` but instead it implements some more effective logic.
