[Insight-users] concept checking in AccumulateImageFilter

[Luis Ibanez]

Hi Gaetan,

We agree with you that according to the Doxygen documentation
of the filter, the dimensions of the input image and the output
image should be the same.


However, what is a stake here is the conceptual correctness
of the filter. (we can fix the Doxygen documentation accordingly).



Since the filter is integrating an image across one dimension,
it is equivalent to computing the integral of a function of
N-variables along a range of one of those N variables.


Something like:


          Sum{ F(x,y,z) }    from Z=-inf to Z=inf


which we would expect to become a function of the remaining
variables,


something like


          G(x,y).


If you take a CT scan and integrate the values along the Z
direction, conceptually this is close to generating a DRR
(an X-Ray) of that 3D CT image, and therefore it seems
reasonable to expect that output image to be a 2D image.


Expecting the integral of a 3D function to be a 3D degenerate
function (a 3D image of 1 slice) is a dangerous approach, because
then filters that are applied to the accumulated image will
assume that is a 3D image. For example a gradient will not work
properly on the accumulated image.



I don't see any difficulty on writing the code for a (N) to (N-1)
dimensional image. We only need a linear iterator, and a while loop.
The GenerateData method shouldn't require more than 50 lines of code.



Have you used this filter ?


Could you post some of the cases for which you use this filter ?



That could help us to understand if the output should be a
N-dimensional or an (N-1)-dimensional image.



    Please let us know,



        Thanks



           Luis



--------------------
Gaetan Lehmann wrote:
> Hi,
> 
> The check is correct. The output is an image of the same dimension to keep the 
> info of the output image on the same dimension (the docstring is surely a lot 
> more clear than me).
> I don't think it will work if you try to build it with a dim=3 in input and 
> dim=2 in output.
> It would be nice to be able to set the dim=3 or dim=2 in output, but the code 
> will surely be difficult to write.
> 
> Gaetan
> 
> 
> On Wednesday 01 March 2006 21:04, Amy Squillacote wrote:
> 
>>Hi all,
>>
>>There is currently a check in the header file of the
>>AccumulateImageFilter (in Code/BasicFilters) for whether the input
>>and output images have the same dimension.  Another ITK developer
>>pointed out to me a couple days ago that this concept check is
>>probably wrong.  He suggested that the dimension of the output should
>>actually be one less than that of the input.  Luis and I discussed
>>this earlier today, and we believe that the output's dimension should
>>be one less than that of the input.  However, if we make this change,
>>anyone's code depending on the input and output to have the same
>>dimensionality would break.
>>
>>What has been the experience of other people using this filter?  What
>>is your opinion about the dimensionality of the input and output of
>>this filter?
>>
>>- Amy
>>
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