[Insight-developers] Advice on implementing Physical Space Math Op filters.

[Williams, Norman K]

We have a need for the various Math Operation filters --
itk::AddImageFilter, itk::SubtractImageFilter, etc to sometimes operate in
physical space.


Use case:  Add 2 images with different spacing, origin etc.

Right now there's an implicit assumption in classes derived from
itk::BinaryFunctorImageFilter.h, that the 2 input images have the same
size, and the operations take space in index space.

What I'd like to do is to add two methods to BinaryFunctorImageFilter:

void SetUsePhysicalSpace(bool);
void SetInterpolator(ImageFunction<TInputImage2,
                     TOutputImage::PixelType> *interpolator)

The Interpolator has to be an itk::ImageFunction, if we're going to
support Vector voxel types, as it's the common base class for
itk::InterpolateImageFunction and it::VectorInterpolateImageFunction.

The complication I'm not sure how to address: Are the
InterpolateImageFunction classes thread-safe, or do I need an interpolater
per thread?

If they aren't all thread-safe, then I need to instantiate an interpolator
per thread, which would require the Interpolator Type as a template
parameter. This would mean changing the template parameter signature for
every class derived from BinaryFunctorImageFilter.

Looking through the available interpolators, it looks like the should be
all thread-safe -- they have member data, but it looks to be data that
would be set once at initialization, and not written to during calls to
EvaluateAtContinuousIndex.



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