[Insight-users] SetDirection in itkImage

[Alex Taylor]

I have a question about the SetDirection member function of the itkImageBase class. I am attempting to pass image data that is stored in a column major convention to ITK, which is row major. I start with a given definition of X and Y in the column major environment with X pointing in the direction of increasing column indices and Y pointing in the direction of increasing row indices. I want to keep the same coordinate system convention relative to my image data when I construct my images in ITK.

To accomplish this, I believe I want to swap the X and Y direction definitions on my itkImage object as follows:

    typename imageType::DirectionType direction = image->GetDirection();

    direction[0][0] = 0;
    direction[0][1] = 1;
    direction[1][0] = 1;
    direction[1][1] = 0;

    image->SetDirection(direction);

My question is regarding the SetSpacing and SetOrigin methods of the ITKImage object. The documentation for each member function reads:

SETORIGIN

Set the origin of the image. The origin is the geometric coordinates of the image origin (pixel [0,0]). It is stored internally as double but may be set from float.

SETSPACING

Set the spacing (size of a pixel) of the image. The spacing is the geometric distance between image samples along each dimension. It is stored internally as double, but may be set from float. These methods also pre-compute the Index<http://www.itk.org/Doxygen/html/classitk_1_1Index.html> to Physical point transforms of the image.

After I set direction, I call SetOrigin and SetSpacing. Do SetOrigin and SetSpacing expect the origin and spacing in [X Y] order, or are they in the order [SpacingAlongRows, SpacingAlongColumns], [RowOrigin ColumnOrigin]?

    typename imageType::DirectionType direction = image->GetDirection();
    direction[0][0] = 0;

    direction[0][1] = 1;
    direction[1][0] = 1;
    direction[1][1] = 0;
    image->SetDirection(direction);

    // Set the origin and inter pixel spacing.
    image->SetOrigin(origin);
    image->SetSpacing(spacing);

I'm seeing some strange behavior in which the numerics of my registration result are changing as a result of redefining which direction is considered X and which direction is considered Y, with all of the other inputs to registration changed to be consistent with the new X Y definition. I'm wondering whether I'm calling SetOrigin/SetSpacing correctly.

Thanks,

Alex Taylor

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