[Insight-users] GradientRecursiveGaussianImageFilter

[Julien Michel]

Dear ITK users,

Lately we tried a pipeline of our with some images whose vertical 
spacing is negative. This did not give the expected result and after 
some investigation, we identified the error source as beeing the 
itk::GradientRecursiveGaussianImageFilter, not giving the same result 
than with the same image with positive spacing.

By reading the code from itk::GradientRecursiveGaussianImageFilter, I 
found that the internal derivative filter seemed to handle negative 
spacing (there is a specific case in the SetUp() method from 
itk::RecursiveGaussianImageFilter.

Meanwhile, in the GenerateData() method from 
itk::GradientRecursiveGaussianImageFilter, the output from the 
derivative filters is divided by the spacing along the given dimension, 
without any caution regarding negative spacing:

In file itkGradientRecursiveGaussianImageFilter.txx

00246     const RealType spacing = inputImage->GetSpacing()[ dim ];
00247
00248     it.GoToBegin();
00249     ot.GoToBegin();
00250     while( !it.IsAtEnd() )
00251       {
00252       ot.Set( it.Get() / spacing );
00253       ++it;
00254       ++ot;
00255       }

I can not figure out by myself if it is normal to divide by a possibly 
negative spacing at this step of the filter. Any idea on this issue ?

Thanks a lot,

Best regards,

Julien Michel