# vtkGradientFilter segmentation fault
**URL:** https://discourse.vtk.org/t/vtkgradientfilter-segmentation-fault/10516
**Category:** Support
**Created:** [January 22, 2023, 7:49am UTC](https://discourse.vtk.org/t/vtkgradientfilter-segmentation-fault/10516 "2023-01-22T07:49:50Z")
**Posts on this page:** 2
**Page:** 1
### Author:  [@FanWang10](https://discourse.vtk.org/u/FanWang10)
#### Post date: [January 22, 2023, 7:49am UTC](https://discourse.vtk.org/t/vtkgradientfilter-segmentation-fault/10516/1 "2023-01-22T07:49:50Z")
Hello,
I am using to calculate the gradient of each vertex in an unstructured grid. I followed this example and made some changes: [https://gitlab.kitware.com/vtk/vtk/-/blob/v9.2.0/Accelerators/Vtkm/Filters/Testing/Cxx/TestVTKMGradient.cxx](https://gitlab.kitware.com/vtk/vtk/-/blob/v9.2.0/Accelerators/Vtkm/Filters/Testing/Cxx/TestVTKMGradient.cxx)
But my codes always gives me segmentation fault when i want to access the gradient data(with the type of vtkDoubleArray\*)
```auto
int main()
{
vtkNew reader;
reader->SetFileName("testInput.vtu");
reader->Update();
vtkNew gradientFilter;
gradientFilter->SetInputConnection(reader->GetOutputPort());
gradientFilter->SetInputScalars(vtkDataObject::FIELD_ASSOCIATION_POINTS, "scalars");
gradientFilter->SetResultArrayName("gradient");
gradientFilter->Update();
vtkDoubleArray* gradientArray =
vtkArrayDownCast(vtkDataSet::SafeDownCast(gradientFilter->GetOutput())
->GetPointData()
->GetArray("gradient"));
// seg fault here!
gradientArray->GetNumberOfComponents();
}
```
[testInput.vtu](https://discourse.vtk.org/uploads/short-url/xg8GIMbWpCpjdXKxpknhherjU5V.vtu) (1.0 KB)
Thank you in advance!
---
### Author:  [@dgobbi](https://discourse.vtk.org/u/dgobbi)
#### Post date: [January 22, 2023, 9:44pm UTC](https://discourse.vtk.org/t/vtkgradientfilter-segmentation-fault/10516/2 "2023-01-22T21:44:12Z")
According to the documentation of vtkImageGradient, for the output,
> data type will be either float or double
More specifically, the output data type will be float unless the input data type is double, long long, or unsigned long long.
A VTK down cast will return nullptr if the type is incorrect. The “safe” just means that it’s a dynamic cast, not a static cast. You still need to check for nullptr to make it truly safe.