[Insight-users] Using ScaleTransform to stretch an image

[Dan Mueller]

Hi David,

Aha. After closer inspection I see that you are forgetting the concept
of spacing. You need to think carefully about what happens to spacing
after a scale.

Please find code below which I think does what you want. In fact,
because you want to change the output size, you don't even need to use
a ScaleTransform -- an IdentityTransform works just fine.

#include "itkImage.h"
#include "itkIdentityTransform.h"
#include "itkImageFileReader.h"
#include "itkImageFileWriter.h"
#include "itkResampleImageFilter.h"
#include "itkNearestNeighborInterpolateImageFunction.h"

typedef itk::Image<unsigned char, 2> ImageType;
typedef ImageType::SizeType SizeType;
typedef ImageType::SpacingType SpacingType;
typedef  itk::ImageFileWriter<ImageType> WriterType;
typedef itk::IdentityTransform<double, 2> TransformType;
typedef itk::ResampleImageFilter<ImageType, ImageType> ResampleImageFilterType;
typedef itk::NearestNeighborInterpolateImageFunction<ImageType>
InterpolatorType;

static void CreateImage(ImageType::Pointer image);

int main(int argc, char *argv[])
{
    // Create input image
    ImageType::Pointer input = ImageType::New();
    CreateImage(input);
    SizeType inputSize = input->GetLargestPossibleRegion().GetSize();
    SpacingType inputSpacing = input->GetSpacing();
    std::cout << "Input size: " << inputSize << std::endl;

    std::cout << "Writing input... " << std::endl;
    WriterType::Pointer writer = WriterType::New();
    writer->SetFileName("C:/Temp/input.png");
    writer->SetInput(input);
    writer->Update();
    input->DisconnectPipeline();

    // Resize
    std::cout << "Resizing... " << std::endl;
    SizeType outputSize;
    outputSize.Fill(200);
    SpacingType outputSpacing;
    for (unsigned int d=0; d<ImageType::ImageDimension; d++)
    {
        outputSpacing[d] = input->GetSpacing()[d] *
((double)inputSize[d] / (double)outputSize[d]);
    }
    ResampleImageFilterType::Pointer resample = ResampleImageFilterType::New();
    resample->SetInput(input);
    resample->SetSize(outputSize);
    resample->SetOutputSpacing(outputSpacing);
    resample->SetTransform(TransformType::New());
    resample->SetInterpolator(InterpolatorType::New());
    resample->UpdateLargestPossibleRegion();
    ImageType::Pointer output = resample->GetOutput();
    output->DisconnectPipeline();
    std::cout << "Output size: " <<
output->GetLargestPossibleRegion().GetSize() << std::endl;

    std::cout << "Writing output... " << std::endl;
    WriterType::Pointer outputWriter = WriterType::New();
    outputWriter->SetFileName("C:/Temp/output.mha");
    outputWriter->SetInput(output);
    outputWriter->Update();

    return EXIT_SUCCESS;
}

void CreateImage(ImageType::Pointer image)
{
    // Allocate empty image
    itk::Index<2> start; start.Fill(0);
    itk::Size<2> size; size.Fill(100);
    ImageType::RegionType region(start, size);
    image->SetRegions(region);
    image->Allocate();
    image->FillBuffer(0);

    // Make a white square
    for(unsigned int r = 40; r < 60; r++)
    {
        for(unsigned int c = 40; c < 60; c++)
        {
            ImageType::IndexType pixelIndex;
            pixelIndex[0] = r;
            pixelIndex[1] = c;
            image->SetPixel(pixelIndex, 255);
        }
    }
}

HTH

Cheers, Dan

On 9 November 2011 06:04, David Doria <daviddoria at gmail.com> wrote:
> On Tue, Nov 8, 2011 at 2:43 PM, Dan Mueller <dan.muel at gmail.com> wrote:
>> Hi David,
>>
>> I think you forgot to set the center of the transform. Try something
>> like the following:
>>
>>  typedef itk::ScaleTransform<double, 2> TransformType;
>>  TransformType::Pointer scaleTransform = TransformType::New();
>>  itk::FixedArray<float, 2> scale;
>>  scale[0] = 1.5;
>>  scale[1] = 1.5;
>>  scaleTransform->SetScale(scale);
>>  TransformType::PointType center;
>>  // NOTE: The below code assumes no offset/origin and unity spacing
>>  center[0] = image->GetLargestPossibleRegion().GetSize()[0] / 2;
>>  center[1] = image->GetLargestPossibleRegion().GetSize()[1] / 2;
>>  scaleTransform->SetCenter(center);
>>
>> HTH
>>
>> Cheers, Dan
>
> Hm, that makes sense, but it didn't fix it.
>
> Updated code: http://www.itk.org/Wiki/ITK/Examples/WishList/ImageProcessing/ScaleTransform
>
> David
>