[Insight-developers] ImageBase->CopyInformation: Number of components per pixel copied

[Stephen R. Aylward]

Hi,

The code below illustrates the problem.

That code does the following:
1) Creates a vector image, sets the number of components per pixel, and 
prints the number of components per pixel.
2) Creates a "normal" (non-vector) image, calls CopyInfo passing the 
vector image, and prints the number of components per pixel.

The result is that the number of components per pixel of the 
itk::Image<char, 3> image is reported as 3.

Should we document, only call SetNumberOfComponents in CopyInfo if the 
image is a vectorimage, or other?

Thanks,
Stephen


#include "itkImage.h"
#include "itkVectorImage.h"

int main(int argc, char **argv)
   {
   // Create a vector image
   typedef itk::VectorImage< char, 3 > VectorImageType;
   VectorImageType::Pointer myVectorImage = VectorImageType::New();
   myVectorImage->SetNumberOfComponentsPerPixel( 3 );

   VectorImageType::SizeType size;
   size[0] = 25;
   size[1] = 25;
   size[2] = 25;
   myVectorImage->SetRegions( size );

   VectorImageType::SpacingType spacing;
   spacing[0] = 1;
   spacing[1] = 2;
   spacing[2] = 3;
   myVectorImage->SetSpacing(spacing);

   myVectorImage->Allocate();

   std::cout << "Vector # of comp = "
             <<  myVectorImage->GetNumberOfComponentsPerPixel( )
             << std::endl;

   // Create a char image and copy vector images's info
   typedef itk::Image< char, 3 > CharImageType;
   CharImageType::Pointer myCharImage = CharImageType::New();
   myCharImage->SetRegions( myVectorImage->GetLargestPossibleRegion() );
   myCharImage->CopyInformation( myVectorImage );
   myCharImage->Allocate();

   std::cout << "Char # of comp = "
             << myVectorImage->GetNumberOfComponentsPerPixel( )
             << std::endl;
   }


Luis Ibanez wrote:
> 
> Hi Stephen,
> 
> That's a good point.
> 
> Notice that yet nothing prevents you from changing the number of
> components of the recipient image in the GenerateData() method
> of your filter, before you fill in the content of the output
> image.
> 
> Note also, that in practice, the SetNumberOfComponentsPerPixel()
> method only performs a real action when it is invoked in an
> itkVectorImage.   In any other case, ITK images have compile-time
> defined number of components per pixel and the call is simply
> ignored.
> 
> Are you dealing with a filter that produces an itkVectorImage
> as output  ?
> 
> 
>      Luis
> 
> 
> 
> --------------------------
> Stephen R. Aylward wrote:
>> Hi,
>>
>> The CopyInformation function of ImageBase sets largestPossibleRegion, 
>> Spacing, Origin, Directions...all good.
>>
>> However, it also sets NumberOfComponentsPerPixel.   Perhaps this is 
>> inappropriate since it is not necessary for the images to have the 
>> same pixel type for CopyInformation to work.  Perhaps 
>> NumberOfComponentsPerPixel is conceptually a pixel-type-specific piece 
>> of information.   Admittedly this is arguable, but consider a filter 
>> that has a vector image as input and outputs a scalar image...we 
>> cannot use the CopyInformation command.
>>
>> Of course, the point is perhaps moot given the need for backward 
>> compatibility...I cannot think of a backward-compatible solution 
>> beyond better documentation.
>>
>> Stephen
>>
> 

-- 
=============================================================
Stephen R. Aylward, Ph.D.
Chief Medical Scientist
Kitware, Inc. - Chapel Hill Office
http://www.kitware.com
Phone: (518)371-3971 x300