[Insight-developers] Default boundary condition for WindowedSinc as constant?

[Bradley Lowekamp]

Hello,

As I am finally integrating the different interpolators into SimpleITK. I am giving them a close look over.

The set of WindowSincInterpolateImageFunctions takes a Boundary condition template parameter. This defaults to ConstantBoundaryCondition. That is by default the pixels are zero outside the image, and they are not zero flux. This results in quite a bit of ringing and fading around my test images. It seems just wrong.

I can easily specify this parameter as the ZeroFluxNeumannBoundaryCondition (I don't think we have a mirror/reflective boundary, which is another possibility), and things look quite good and as I expect the output to be. I was curious as to what others were doing so I perused BRAINS and ANTS, grepping for the sinc interpolator. And to my surprise they are using the default!

Is there a reason that this default is preferred? Or is it that I am not processing a single blob in the center of a black image (aka a brain)?

Also in terms of consistency across the interpolators, this is the only one which takes a boundary condition template parameters. The other interpolators appear to behave sensibly, and exhibit a zero-flux type boundary condition. I think the default for this may need to be changed.


I have this little example I have been working on in SimpleITK with the famed cthead1.png data input. Here is a code snippet:


image = image[(size[0]//2-25):(size[0]//2+25),(size[1]//2-25):(size[1]//2+25)]


iterps = [sitk.sitkNearestNeighbor,
          sitk.sitkLinear,
          sitk.sitkBSpline,
          sitk.sitkGaussian,
          sitk.sitkHammingWindowedSinc,
          sitk.sitkCosineWindowedSinc,
          sitk.sitkWelchWindowedSinc,
          sitk.sitkLanczosWindowedSinc,
          sitk.sitkBlackmanWindowedSinc]

eFactor=5

image_list = []

for i in iterps:
    image_list.append( sitk.Expand( image, [eFactor]*3, i ))

tiles = sitk.Tile( image_list, [3,0] )

And the following is the output with the different boundary conditions:

http://erie.nlm.nih.gov/~blowek1/images/expand_interp_cbc.png
http://erie.nlm.nih.gov/~blowek1/images/expand_interp_zfbc.png

Thanks for you feedback,
Brad