# Strange dependency between GPU & CPU filters

**URL:** <https://discourse.itk.org/t/strange-dependency-between-gpu-cpu-filters/5188>\
**Category:** Algorithms\
**Tags:** gpu\
**Created:** [July 15, 2022, 5:39pm UTC](https://discourse.itk.org/t/strange-dependency-between-gpu-cpu-filters/5188 "2022-07-15T17:39:38Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Fijoy\_Vadakkumpadan](https://discourse.itk.org/user_avatar/discourse.itk.org/fijoy_vadakkumpadan/32/1287_2.png) [@Fijoy\_Vadakkumpadan](https://discourse.itk.org/u/Fijoy_Vadakkumpadan)\
**Post date:** [July 15, 2022, 5:39pm UTC](https://discourse.itk.org/t/strange-dependency-between-gpu-cpu-filters/5188/1 "2022-07-15T17:39:38Z")

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Do GPU filters in ITK have some hidden dependency on corresponding CPU filters?

I’m seeing this unexpected behavior in my application where a GPU filter usage aborts my application with an exception (the kind that cannot be caught in a try/catch block) unless I add an unused, corresponding CPU filter in the same scope. For example, the following code snippet will abort. I have confirmed with debugging that the abort happens in the call to Update().

```auto
// Various typedefs
typedef itk::Image<short, 3> imageType;
typedef itk::Image<double, dim> imageDOUBLEType;
typedef itk::GPUImage<type, dim> gpuImageType; 
typedef itk::CastImageFilter<imageType, gpuImageType> cpu2gpuCastFilterType;
typedef itk::GPUImage<double, 3> gpuImageDOUBLEType;
typedef itk::GPUGradientAnisotropicDiffusionImageFilter<gpuImageType, gpuImageDOUBLEType> gpuGradientDiffusionFilterType;
typedef itk::CastImageFilter<gpuImageDOUBLEType, imageDOUBLEType> gpu2cpuCastFilterDOUBLEType;

// Allocate pointers
gpuGradientDiffusionFilterType::Pointer gpuGradient = gpuGradientDiffusionFilterType::New();
cpu2gpuCastFilterType::Pointer cpu2gpu = cpu2gpuCastFilterType::New();
gpu2cpuCastFilterDOUBLEType::Pointer gpu2cpu = gpu2cpuCastFilterDOUBLEType::New();

// Pipeline the filters and run the pipeline
cpu2gpu->SetInput(<Pointer to an image of type imageType>);
gpuGradient->SetInput(cpu2gpu->GetOutput());
gpuGradient->SetNumberOfIterations(<iterations>);
gpuGradient->SetTimeStep(<timeStep>);
gpuGradient->SetConductanceParameter(<conductance>);
gpu2cpu->SetInput(gpuGradient->GetOutput());
gpu2cpu->Update();

```

However, if I add the a CPU filter as shown below to the above snippet, the above snippet will run fine and produce expected results.

```auto
typedef itk::GradientAnisotropicDiffusionImageFilter<imageType, imageDOUBLEType> gradientDiffusionFilterType;
gradientDiffusionFilterType::Pointer gradient = gradientDiffusionFilterType::New();

```

This behavior is not specific to the gradient smoothing filter. I’m seeing the same behavior in the itk::DiscreteGaussianImageFilter vs. itk::GPUDiscreteGaussianImageFilter pair as well.

Any idea as to what is happening here? Am I missing a step in setting up the GPU filters, e.g., registering them before use?

Thanks.

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<div class="post-metadata">

**Author:** ![dzenanz](https://discourse.itk.org/user_avatar/discourse.itk.org/dzenanz/32/1093_2.png) [@dzenanz](https://discourse.itk.org/u/dzenanz)\
**Post date:** [July 15, 2022, 7:33pm UTC](https://discourse.itk.org/t/strange-dependency-between-gpu-cpu-filters/5188/2 "2022-07-15T19:33:20Z")

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@LucasGandel or @simon.rit might be able to advise.

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<div class="post-metadata">

**Author:** ![simon.rit](https://discourse.itk.org/letter_avatar_proxy/v4/letter/s/f08c70/32.png) [@simon.rit](https://discourse.itk.org/u/simon.rit)\
**Post date:** [July 18, 2022, 5:33am UTC](https://discourse.itk.org/t/strange-dependency-between-gpu-cpu-filters/5188/3 "2022-07-18T05:33:24Z")

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I have never used this filter but it is not an expected behavior as far as I know. To debug this, I would track down what static member is initialized by the CPU filter’s constructor and not by the GPU filter’s constructor…