[Insight-users] vtk vs mhd images

[Luis Ibanez]

Hi David,

This is indeed strange.

It would look like the VTKImageIO class is interpreting "signed shorts"
as "unsigned shorts".

However, with the program below, I just verified that we can write
negative numbers from a "signed short" image, into a VTK file, and
obtain the correct output image.

This made suspect the next step of your pipeline.

Questions:

  A) What are you using for visualizing this .vtk and .mhd images ?

  B) What are you using for computing the histogram of these two images ?

  and of course:
  C) Is there a chance that you can post:

       1) A minimal example of source code
          where you observe this behavior

     and

       2) Could you post your .vtk and .mhd images in a public web site ?


Please let us know,


    Thanks


       Luis

------ VERIFICATION CODE FOR .vtk files BEGIN ------

#include "itkImage.h"
#include "itkImageFileWriter.h"

int main()
{
   typedef signed short PixelType;
   const unsigned int Dimension = 3;
   typedef itk::Image< PixelType, Dimension >  ImageType;
   typedef itk::ImageFileWriter< ImageType >   WriterType;

   ImageType::Pointer image = ImageType::New();

   ImageType::RegionType  region;
   ImageType::IndexType  index;
   ImageType::SizeType  size;

   index.Fill(0);
   size.Fill(100);

   region.SetIndex( index );
   region.SetSize( size );

   image->SetRegions( region );
   image->Allocate();
   image->FillBuffer( -10 );

   WriterType::Pointer writer = WriterType::New();

   writer->SetInput( image );
   writer->SetFileName( "image.vtk" );
   writer->Update();

   return 0;
}

------ VERIFICATION CODE FOR .vtk files END ------



-------------------
David Pastor wrote:
> Hi everyone!
>  
> sorry for dumping emails in the list lately, but I'm encountering 
> unexpected problems...
>  
> I'm using SubtractImageFilter to get the an output image. In order to 
> preserve the intensity profile I'm using a signed short image for 2 
> unsigned char input images.
>  
> As I don't want negative values, I get the minimum of the image (a 
> negative value) and create and offset image so adding it to the output 
> of SubtractImageFilter will return an image with positive values that i 
> template as unsigned short to avoid problems.
>  
> Well, so far so good I think. Then, i write the final image to a .mhd 
> type image and the result looks as I wanted. But if I asked it to write 
> a vtk image, the resulting image is far from what I expected. Instead, 
> my image is satured on the high levels and is generally brighter.
>  
> The histogram shows that the values have an addition offset so the 
> values closer to the maximum are satured. The mhd image has a max value 
> of 456 (which is consistent with something that come from unsigned char 
> images), but the vtk image is beyong 60000.
>  
> Any idea about this?? Please note, that the only difference is the 
> format of the output image passed in the command line. Input and code 
> are exactly the same.
>  
> Thanks
> David
> 
> 
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