[Insight-developers] bug in BSplineDeformableTransform?

[Tomáš Kazmar]

Hi all,

  recently i tried to use second order b-splines to warp images and
the results contained sharp steps (~were not smooth). I am not sure
whether I do something wrong. Has anybody used it successfully?

It seems to me that there is an error in
BSplineInterpolatingWeightFunction in combination with
BSplineKernelFunction. The first uses kernel in the range -1..2 and
the second defines the support as -1.5..1.5.

Should I report it as a bug? There is a simple patch but once more
I am not sure whether I use it the right way.

Here is the relevant part of code:

  const unsigned int SplineOrder = 2;
  typedef itk::BSplineDeformableTransform<float, Dimension, SplineOrder> TransformType;
  TransformType::Pointer bsplineTransform = TransformType::New();
  typedef TransformType::RegionType RegionType;
  RegionType bsplineRegion;
  RegionType::SizeType size;
  const unsigned int nodesOutsideImage = SplineOrder;
  const unsigned int nodesInsideImage = 4;
  const unsigned int numberOfGridNodes = nodesInsideImage + nodesOutsideImage;
  size.Fill( numberOfGridNodes );
  bsplineRegion.SetSize( size );
  typedef TransformType::SpacingType SpacingType;
  SpacingType spacing;
  spacing[0] = 1.0 * floor( (width-1) / (double)(nodesInsideImage-1) );
  spacing[1] = 1.0 * floor( (height-1) / (double)(nodesInsideImage-1) );
  typedef TransformType::OriginType OriginType;
  OriginType origin;
  origin[0] = 0.0 - spacing[0];
  origin[1] = 0.0 - spacing[1];
  bsplineTransform->SetGridSpacing( spacing );
  bsplineTransform->SetGridOrigin( origin );
  bsplineTransform->SetGridRegion( bsplineRegion );


Regards,
Tomas