Compare two images of any kind

mschwier · December 19, 2017, 2:12pm

Is there an easy way to compare two images for equality that also supports Vector Images?

Right now I am using the old itkTestMain.h which uses the itkTestingComparisonImageFilter and does not support Vector Images.

blowekamp · December 19, 2017, 2:22pm

This is an unfortunate limitation of the current method.

For SimpleITK testing we implemented a function which works with scalars, vector images, and images of complex numbers. It computes the root means squares:

github.com

SimpleITK/SimpleITK/blob/c9f21b20ebb1d2b0a6577578bd7e0cb6251e9cf8/Testing/Unit/sitkImageCompare.cxx#L95-L127


try
  {


  if ( baselineImage.GetPixelID() == sitk::sitkComplexFloat32 ||
       baselineImage.GetPixelID() == sitk::sitkComplexFloat64 )
    {


    const sitk::Image diff =  sitk::Subtract( testImage, baselineImage );
    // for complex number we multiply the image by it's complex
    // conjugate, this will produce only a real value result
    const sitk::Image conj = sitk::RealAndImaginaryToComplex( sitk::ComplexToReal( diff ),
                                                              sitk::Multiply( sitk::ComplexToImaginary( diff ), -1.0 ) );
    diffSquared = sitk::ComplexToReal( sitk::Multiply( diff, conj ) );
    }
  else if ( baselineImage.GetNumberOfComponentsPerPixel() > 1 )
    {
    const sitk::Image diff =  sitk::Subtract( sitk::Cast( testImage, sitk::sitkVectorFloat32 ), sitk::Cast( baselineImage, sitk::sitkVectorFloat32 ) );


    // for vector image just do a sum of the components
    diffSquared  = sitk::Pow( sitk::VectorIndexSelectionCast( diff, 0 ), 2.0 );

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This is build upon SimpleITK’s typeless layer, but could be ported to native ITK Filter and many people would find it useful. Dealing with the different pixel types may require some C++ meta-programming, new C++11 features may make this easier to do now.

mschwier · December 19, 2017, 2:50pm

Thanks for your answer. I’ll see if I can switch to SimpleITK or try to port it to native ITK.

fedorov · December 28, 2017, 8:46pm

For the reference, and in case someone else needs this, @mschwier implemented this comparison separately in our library by comparing all vector components (this is what makes sense for our use case), see https://github.com/millerjv/PkModeling/pull/69/commits/82faac4729dae139c84c9c661fbe349bc30b5d08