[Insight-developers] Template Meta Programming and itkImageBase

[Luis Ibanez]

Bill,


       Agree,
       this was mostly a documentation issue.


The previous implementation of the code had the expression:


    #if 1
         // template meta-programming implementation here
    #else
         // standard for loops implementation here
    #endif


which obviously was intended to be used temporarily for local debugging,
but that unfortunately remained with the code...   for way too long.


The consequence is that the "else" portion has not been tested recently,
and therefore, it is broken, as probably illustrated by:

http://www.cdash.org/CDash/testSummary.php?project=2&name=itkOrientedImage2DTest01&date=2008-10-19


In addition, the "1" in the #if doesn't provide any indication
of what the "if" is about. Which is an unfortunate lack of
documentation for future maintenance of the code.


Recent tests on gcc with -O3 illustrated that the compiler is
actually able to unroll the for-loops and that the code with
template meta-programming wasn't necessarily faster.


By reconnecting this options to a CMake flag we can now:


    1) Make sure that the "else" can be easily tested,
       (maybe nightly, at least in some machines)

    2) Re-evaluate the performance comparison of both
       implementations.



Your point of backward compatibility is well taken,
therefore I'm turning the CMake flag:

    ITK_USE_TEMPLATE_META_PROGRAMMING_LOOP_UNROLLING

to be ON by default.

http://www.itk.org/cgi-bin/viewcvs.cgi/CMakeLists.txt?root=Insight&view=log
http://www.itk.org/cgi-bin/viewcvs.cgi/CMakeLists.txt?root=Insight&r1=1.318&r2=1.319


This should bring the code back to behave just as when
the  "#if 1"   was in place.


We can now use the following Nightly tests:

    itkOrientedImageProfileTest1.cxx
    itkOrientedImageProfileTest2.cxx
    itkOrientedImageProfileTest3.cxx

to verify the computation time performance of the methods:

    TransformIndexToPhysicalPoint()
    TransformPhysicalPointToIndex(_

in the

    a) itk::OrientedImage  (test 1)
    b) itk::Image          (test 2)
    c) itk::VectorImage    (test 3)


After running these profiling tests in multiple platforms
we will be in a better informed position to decide whether
we want to keep the option, or simply stick to one of the
two implementations and remove the alternative code.


My suggestion will be to base our test only on builds
that use "Release" compilation mode, although one could
argue that it may be desirable to have an option that could
make Debug builds run faster...


Thanks for any suggestions,


      Luis


---------------------
Bill Lorensen wrote:
> Luis,
> 
> I noticed that the image helper methods are now enclosed within the
> ITK_USE_TEMPLATE_META_PROGRAMMING_LOOP_UNROLLING  #ifdef. I suggest
> that we do away with this cmake option. The image helper methods have
> been the default behavior for oriented imaged from the beginning. I
> see no reason to make that code optional now that we're also using it
> in itkImage.
> 
> Bill
>