[ITK] Strange behavior of morphological operations

[Timothee Evain]

Hello everyone,

I was testing morphological operations when I stumbled on a strange behavior when you are using an asymmetrical structuring element.
For example :

   itk::Neighborhood<float,3> SE4;
   int SE4Radius=2;
   SE4.SetRadius(SE4Radius);
   for (int i = 0; i < SE4.GetSize()[0]*SE4.GetSize()[1]*SE4.GetSize()[2]; i++)
   {
      SE4[i]=0;
   }
   SE4[SE4.GetCenterNeighborhoodIndex()]=1;
   for (int i = 1; i <= SE4Radius; i++)
   {
      itk::Neighborhood<float,3>::OffsetType SE4Off1={i,0,0};
      itk::Neighborhood<float,3>::OffsetType SE4Off2={0,i,0};
      itk::Neighborhood<float,3>::OffsetType SE4Off3={0,0,i};

      SE4[SE4Off1]=1; SE4[SE4Off2]=1; SE4[SE4Off3]=1; 
   } 

If you dilate with this element using the BinaryDilateImageFilter, your object will be dilated toward positive X, Y and Z dimensions, which is quite expected, but if you are using the BasicDilateImageFilter, the object will be dilated toward negative X, Y and Z dimensions which doesn't fit the intuition since we inputed a structuring element with ON "voxels" only on positive offsets.
I would say it is coming from its parent, the MorphologicalImageFilter, since a custom implemented filter deriving from it have the same strange behavior.

Is there a reason for this ?

Tim