[Insight-users] B-Spline deformable transform grid origin

[Richard Johnson]

Hi everyone,

Could someone please clarify the code below from DeformableRegistration4.cxx.

I understand why spacing[r] is set as it is, but what about the
origin?  Why isn't the grid origin set at the origin of the fixed
image?  Why would the grid spacing be subtracted from the fixed
image's origin?

Thanks a lot
---------------------
 gridSizeOnImage.Fill( 5 );
 gridBorderSize.Fill( 3 );    // Border for spline order = 3 ( 1
lower, 2 upper )
 totalGridSize = gridSizeOnImage + gridBorderSize;

 bsplineRegion.SetSize( totalGridSize );

 typedef TransformType::SpacingType SpacingType;
 SpacingType spacing = fixedImage->GetSpacing();

 typedef TransformType::OriginType OriginType;
 OriginType origin = fixedImage->GetOrigin();;

 FixedImageType::SizeType fixedImageSize = fixedRegion.GetSize();

 for(unsigned int r=0; r<ImageDimension; r++)
   {
   spacing[r] *= floor( static_cast<double>(fixedImageSize[r] - 1)  /
                 static_cast<double>(gridSizeOnImage[r] - 1) );
   origin[r]  -=  spacing[r];
   }

 transform->SetGridSpacing( spacing );
 transform->SetGridOrigin( origin );
 transform->SetGridRegion( bsplineRegion );