[Insight-developers] MatrixOffsetTransformBase

[Miller, James V (Research)]

Stephen, 

I am putting in code to optimize the ResampleImageFilter for the case of linear transformations. I am but surprised that there is not a LinearTransform class.  It looks like MOST of the linear transforms subclass MatrixOffsetTransformBase.  The IdentityTransform is one linear transform that does not.  I probably need to check the rest of the transforms to see which are not derived from MatrixOffsetTransformBase.

Can I assume that a MatrixOffsetTransformBase is indeed linear? From the template parameters and internal typedefs, I think it pretty much has to be linear (in the parameters).  But one could model a quadratic transform with the MatrixOffsetTransformBase as taking 7D "points" (x^2 xy y^2 x y 1) and producing 2D points.  This is still linear in the parameters but would defeat the optimizations that I put in the ResampleImageFilter.  However, I think a quadratic transform would have to be modelled differently anyway (going 2D-2D verses 7D-2D) for the ResampleImageFilter to work anyway. 

An alternative is to add methods like IsLinear() method to Transform that subclasses can override.	

Should there be a LinearTransform class?

Jim Miller 
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Visualization & Computer Vision
GE Research
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millerjv at research.ge.com <mailto:millerjv at research.ge.com>
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