[Insight-developers] GetInverse is unusable -- patch attached

[Tom Vercauteren]

Hi Pavel,

You definitely also have my vote for having a usable GetInverse method in ITK.

What is the current status of things? I found a bug entry here:
  http://www.kwwidgets.org/Bug/bug_view_advanced_page.php?bug_id=7876
and a wiki page here:
  http://www.itk.org/Wiki/Proposals:InverseTransform

However I didn't find the IJ paper you mentioned. Is it still under preparation?

As far as technical matters are concerned, since I didn't find the IJ
submission, I'll post some coments here.

I think that having a virtual function that returns a
itk::Transform<ScalarType,OutDim,InDim> is the way to go. The only
thing that I don't quite like in the patch from the bug tracker is the
fact that the typedef you use may be somewhat confusing for a user.

Indeed at first glance it looks like one could change the following
typedef to match the concrete transform type:
   /** Base inverse transform type. */
   typedef typename Superclass::InverseTransformType InverseTransformType;
   typedef SmartPointer< InverseTransformType > InverseTransformPointer;

Of course it would be wrong to do so as the base class function
GetInverse wouldn't be overridden anymore. The only required change
might be to either get rid of this typedef as in the above wiki page
   virtual typename Transform<ScalarType, OutDim,InDim>::Pointer
GetInverse() const;
 or to change the naming of this typedef and enhance the
documentation, e.g. something like:
   /** Base inverse transform type. This type cannot be changed to the
concrete transform type or inheritance would be lost. */
   typedef typename Superclass::InverseTransformBaseType
InverseTransformBaseType;
   typedef InverseTransformBaseType::Pointer InverseTransformBasePointer;

As a side note, for such an application, I believe that the best would
be to use covariant return types
(http://www.parashift.com/c++-faq-lite/virtual-functions.html#faq-20.8,
http://en.wikipedia.org/wiki/Covariant_return_type). Alas this is not
possible for two reasons:

1) DerivedTransformType::Pointer does not derive from
TransformType::Pointer so I don't think covariant return types can be
used. A raw pointer instead of a smart pointer would have to be used
but this wouldn't fit in ITK standards since GetInverse allocates an
object. A raw pointer could be returned if the inverse transformation
was a member of the forward transformation but this doesn't sound
quite nice.

2) No matter the first reason, ITK supports MSVC 6 but according to
C++ FAQ lite, MSVC 6 doesn't support covariant return types.

Regards,
Tom


On Fri, Oct 10, 2008 at 16:55, Pavel Koshevoy <koshevoy at sci.utah.edu> wrote:
> Hi,
>
> I've been patching ITK since version 1.8.1 to provide a usable GetInverse
> API for transforms which don't have an analytic inverse, or have an
> approximate inverse transform of a different type.
>
> Requiring a patched ITK is a hurdle in the path of developers trying to
> build my image registration apps.  I really, really hope you merge this
> patch into the trunk.
> It's a very small change.  Basically, I define an InverseTransformPointer
> type and add virtual InverseTransformPointer GetInverse() const method.  The
> patch implements this API for itk::Transform, itk::IdentityTransform,
> itk::MatrixOffsetTransformBase, itk::ScaleTransform,
> itk::TranslationTransform.
> These are useful changes that make the ITK Transform classes far more
> useful. In my image registration apps I implement this API in my Legender
> Polynomial Transform, a Radial Basis Function Transform, Triangle Mesh
> Transform, Radial Distortion Transform, Cascaded Transform, and a generic
> Newtons Method Numeric Inverse Transform.
>
> I really hope someone takes a look at this short patch and adds it to the
> main ITK trunk.
>
> Thank you,
>   Pavel.