[Insight-developers] VectorType of itk::LevelSetFunction is FixedArray. Why not Vector?

[Karthik Krishnan]

Hi Zach,

 Its done. You can read/write FixedArrays of length 2. 

If you need to add other lengths, you should be able to easily add that
duplicating code. Look at the cvs diffs
http://www.itk.org/cgi-bin/viewcvs.cgi/Code/IO/itkImageIOBase.cxx?root=Insight&r1=text&tr1=1.56&r2=text&tr2=1.55&diff_format=h
http://www.itk.org/cgi-bin/viewcvs.cgi/Code/IO/itkImageIOBase.h?root=Insight&r1=text&tr1=1.38&r2=text&tr2=1.37&diff_format=h

(thanks to VS6 for not supporting partial specialization! ).

Thanks
karthik

On Thu, 2005-07-07 at 18:21 -0700, Zachary Pincus wrote:
> Hi Luis,
> 
> Thanks for your reply!
> 
> Josh Cates had some good commentary for why not to use itk::Vector  
> for advection fields:
> 
> > Hi Zachary,
> >
> > As I recall, FixedArray was chosed for maximum compatibility with  
> > the variety of vector types in itk.  For whatever reason, all  
> > numeric vectors do not inherit from itk::Vector.  (To wit,  
> > itk::CovariantVector is a sibling of itk::Vector and not its  
> > child.  This setup also allows floating-point RGB type images.)   
> > Images of itk::Vector should work, but I think you'll also be ok  
> > changing the typedef in your own code.
> >
> > Josh.
> 
> I agree that it's easier to make a loop that copies a subclass of  
> FixedArray into a FixedArray than copying one sibling type into  
> another. Nevertheless, your point about Vector being the appropriate  
> type for advection fields is very correct.
> 
> I'll try a build on my machine and see how that goes.
> 
> About the FixedArray read/write coverage: I can't seem to write  
> Image<FixedArray<float, 2>, 2> images out to MHA/MHD files. Where  
> would I look in the code to see what the coverage capabilities for a  
> given file reader/writer are? (I'm not too familiar with how the  
> reader/writer code is structured.)
> 
> Zach
> 
> 
> 
> On Jul 7, 2005, at 8:28 AM, Luis Ibanez wrote:
> 
> >
> >
> > Hi Zach,
> >
> > Thanks for pointing this out.
> >
> > There is no aparent reason why this VectorType couldn't
> > be an itk::Vector instead of an itk::FixedArray.
> >
> >
> > The fact that the VectorType represents an advection field
> > is actually a good argument in favor of using an itk::Vector
> > type instead of the itk::FixedArray type.
> >
> > Note that the IO argument may not be the best reason for
> > changing the type. The coverage for reading and writing
> > itk::FixedArrays should be as complete as the one for
> > itk::Vectors
> >
> > The itk::Vector class derives from the itk::FixedArray,
> > so the only reason for having them separate, is the fact
> > that Vectors represent a geometrical concept in N-D space,
> > and can therefore have a particular set of operators defined.
> > A comparative discussion on the concept of Vectors, Points,
> > Arrays... has been added to the second edition of the
> > Software Guide:
> >
> >       http://www.itk.org/ItkSoftwareGuide.pdf
> >
> > you will find it in section 8.8.1 "Geometrical Representation"
> > pdf-page 415.
> >
> >
> >
> > Probably the right way to proceed at this point, will be for
> > you to change the type in your local copy, run and submit an
> > experimental build, and if all the tests are green in the
> > Dashboard then we will proceed to change the type.
> >
> >
> > Please let us know if this option sounds reasonable to you.
> >
> >
> >
> >     Thanks
> >
> >
> >        Luis
> >
> >
> >
> >
> > -----------------------
> > Zachary Pincus wrote:
> >
> >> Hi guys,
> >> The base class for level set functions in ITK  
> >> (itk::LevelSetFunction) typedef's VectorType as a FixedArray. This  
> >> means that custom edge advection images need to have FixedArrays  
> >> as pixels, which is inconvenient because most of the ImageWriter  
> >> classes that deal with vector images only work with Vector-typed  
> >> pixels, not FixedArray-typed pixels. Moreover, there are many  
> >> appropriate numeric traits defined for Vector, but none for  
> >> FixedArray -- this makes using filters on FixedArray images more  
> >> challenging.
> >> Is there any reason why FixedArray images are used in this case  
> >> and not Vector images? Do FixedArrays have fewer data members or  
> >> fewer virtual functions so that using them is significantly  
> >> faster? Or is there some other advantage? If not, is there any  
> >> reason not to change the LevelSetFunction typedef to Vector  
> >> instead of FixedArray? (And you guys prudently wouldn't want to  
> >> make this an ITK-wide change, is there any reason I wouldn't want  
> >> change it for my own ITK checkout?)
> >> Sure, I can pixel-by-pixel copy a Vector image to a FixedArray  
> >> image, but if it's not necessary that's better for me.
> >> Zach
> >> _______________________________________________
> >> Insight-developers mailing list
> >> Insight-developers at itk.org
> >> http://www.itk.org/mailman/listinfo/insight-developers
> >>
> >
> >
> >
> > _______________________________________________
> > Insight-developers mailing list
> > Insight-developers at itk.org
> > http://www.itk.org/mailman/listinfo/insight-developers
> >
> 
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