[Insight-users] transform inverse?

[Hegenbart, Lars]

Dear ITK-community,

I am defining an affine transform with [2, 0, 0,  0, 2, 0,  0, 0, 2,   
0, 0, 0], basically a stretch of factor 2 in all 3 dimensions. I want to
apply it to an input image to create an output image with same
parameters (spacing, etc.). I end up getting an output which is scaled
by a factor of 0.5 instead of 2. If I choose 0.5 in the diagonal of the
matrix of the affine transform, I end up with the wanted result.  
It seems somehow to be inverted. What's wrong or is that normal?

Thanks for any hints.

Best regards
Lars

PS: here some relevant code parts:

	typedef itk::Image< unsigned char, Dimension > InputImageType;
	typedef itk::Image< unsigned char, Dimension > OutputImageType;
	InputImageType::Pointer i_image;
	OutputImageType::Pointer o_image;

	InputImageType::SpacingType spacing;
	spacing[0] = 6; //spacing along x
	spacing[1] = 6; //spacing along y
	spacing[2] = 6; //spacing along z
	i_image->SetSpacing(spacing);
	InputImageType::PointType origin;
	origin[0] = 0;
	origin[1] = 0;
	origin[2] = 0;
	i_image->SetOrigin(origin);
	InputImageType::SizeType size;
	size[0] = 100; // size along x
	size[1] = 100; // size along y
	size[2] = 100; // size along z

	typedef itk::AffineTransform< double, Dimension >

TransformType;	
	TransformType::Pointer transform;

	typedef TransformType::ParametersType     ParametersType;
	const unsigned int numberOfParameters = transform-
>GetNumberOfParameters();
     	ParametersType parameters( numberOfParameters );
     	parameters_affine[ 0]=  2;
     	parameters_affine[ 1]=  0;
     	parameters_affine[ 2]=  0;
     	parameters_affine[ 3]=  0;
     	parameters_affine[ 4]=  2;
     	parameters_affine[ 5]=  0;
     	parameters_affine[ 6]=  0;
     	parameters_affine[ 7]=  0;
     	parameters_affine[ 8]=  2;
     	parameters_affine[ 9]=  0;
     	parameters_affine[10]= 0;
     	parameters_affine[11]= 0;
     	transform->SetParameters( parameters);

	typedef itk::ResampleImageFilter< InputImageType,
OutputImageType >   
FilterType;
	FilterType::Pointer filter = FilterType::New();
	filter->SetTransform(transform);

	typedef itk::NearestNeighborInterpolateImageFunction<
InputImageType, double > InterpolatorType;
	InterpolatorType::Pointer interpolator =
InterpolatorType::New();
	filter->SetInterpolator(interpolator);
	filter->SetDefaultPixelValue(0);

	filter->SetOutputSpacing(i_image->GetSpacing()); // same spacing
like InputImage
	filter->SetOutputOrigin(i_image->GetOrigin()); // same like
InputImage
	filter->SetSize(size); // same like InputImage
	filter->SetInput(i_image);
	
	o_image = filter->GetOutput();