[Insight-users] When to set image spacing?

[David Doria]

The following code works correctly, and produces
rpi.edu/~doriad/test_ok.vtk(there was a few more lines to set 3 voxels
to non-zero values). Visualized
with 'representation=slice' in paraview, the first slice is uniform in color
(should be all 0 except one corner pixel)

typedef itk::Image< float, 3 > ImageType;
ImageType::Pointer image = ImageType::New();

    ImageType::IndexType start;
    start[0] = 0;  // first index on X
    start[1] = 0;  // first index on Y
    start[2] = 0;  // first index on Z

    ImageType::SizeType  size;
    size[0] = 20;  // size along X
    size[1] = 20;  // size along Y
    size[2] = 20;  // size along Z

    ImageType::RegionType region;
      region.SetSize( size );
    region.SetIndex( start );
    image->SetRegions( region );

    image->Allocate();

    ImageType::SpacingType spacing;
    spacing[0] = .5;
    spacing[1] = .5;
    spacing[2] = .5;
    image->SetSpacing(spacing);


However, if the spacing setting is moved to before the allocate(), the z=0
voxels are not uniformly zero (again, via visualization in paraview)
http://www.rpi.edu/~doriad/test_bad.vtk


typedef itk::Image< float, 3 > ImageType;
ImageType::Pointer image = ImageType::New();

    ImageType::IndexType start;
    start[0] = 0;  // first index on X
    start[1] = 0;  // first index on Y
    start[2] = 0;  // first index on Z

    ImageType::SizeType  size;
    size[0] = 20;  // size along X
    size[1] = 20;  // size along Y
    size[2] = 20;  // size along Z

    ImageType::RegionType region;
      region.SetSize( size );
    region.SetIndex( start );
    image->SetRegions( region );

  //this block is now moved BEFORE allocate
    ImageType::SpacingType spacing;
    spacing[0] = .5;
    spacing[1] = .5;
    spacing[2] = .5;
    image->SetSpacing(spacing);

    image->Allocate();


Is it supposed to work like that? (i.e. you have to allocate() the image
before you set it's properties like spacing/origin?). Also, is there a way
to initialize the image so all pixels have a constant non-zero value? I can
post the entire code in a file if necessary, but the problem seemed
localized to this bit.

Thanks,

David
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