[Insight-users] Compare images by mean square difference

[t_s_80 at gmx.de]

Hello,

I simply want to compare two images (without registration) by mean square value. All I have found to do so, is the MeanSquaresImageToImageMetric that is also used for
registration. Is it correct to use it also in my case?

I wrote a method (you can see below) that get two identical images, apart from the image content (same size, same spacing, same origin).
It is working, but the results can't be right. The values are very large, even if the content of the images is equal. What is wrong with my method? Did I forget something?
Or is it wrong to use the Metric this way?

Thanks a lot for your help,
Marta

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void THIS::calculateMeanSquaresErrorBetweenImages( ImageType::Pointer image1, ImageType::Pointer image2 ) {
	
	/*
	//This part is actually in the header:
	int Dimension = 2;
	typedef unsigned short InputPixelType;
	typedef unsigned short OutputPixelType;
	typedef itk::Image< InputPixelType, Dimension > ImageType;
	typedef itk::Image< OutputPixelType, Dimension > OutputImageType;
	typedef itk::RescaleIntensityImageFilter< ImageType, OutputImageType > RescaleFilterType;*/

	typedef itk::MeanSquaresImageToImageMetric< OutputImageType, OutputImageType > MetricType;
	typedef itk::LinearInterpolateImageFunction< OutputImageType > LinearInterpolatorType;
	typedef itk::CenteredRigid2DTransform< double > TransformType;

	RescaleFilterType::Pointer rescaler1 = RescaleFilterType::New( );
	RescaleFilterType::Pointer rescaler2 = RescaleFilterType::New( );
	
	rescaler1->SetOutputMinimum(   0 );
	rescaler1->SetOutputMaximum( 65535 );
	
	rescaler2->SetOutputMinimum(   0 );
	rescaler2->SetOutputMaximum( 65535 );
	
	rescaler1->SetInput( image1 );
	rescaler2->SetInput( image2 );
		
	LinearInterpolatorType::Pointer interpolator = LinearInterpolatorType::New( );
	
	OutputImageType::Pointer image1 = rescaler1->GetOutput( );
	OutputImageType::Pointer image2 = rescaler2->GetOutput( );
	
	rescaler1->Update( );
	rescaler2->Update( );
	
	MetricType::Pointer metric = MetricType::New( );
	
	metric->SetFixedImage( image1 );
	metric->SetMovingImage( image2  );
	metric->SetFixedImageRegion( image1->GetLargestPossibleRegion( ) );
	
	TransformType::Pointer transform = Transform::TransformType::New( );	
	transform->SetIdentity( );
	metric->SetTransform( transform );
	metric->SetTransformParameters ( transform->GetParameters( ) );
	metric->SetInterpolator( interpolator );
	metric->Initialize( );

	std::cout << "Metrik Value: " << metric->GetValue( transform->GetParameters( ) ) << std::endl;
	
}
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