[Insight-developers] Integrating the New Statistics framework into ITK

[Stephen Aylward]

There is the philosophy that if you're going to break backward
compatibility, then you should REALLY break backward compatibility.

In this case it might be best to do what it takes to "get it right"
this time, rather trying to preserve the old system.

Similarly, regarding making things "consistent with itk", in my
opinion it is ok to deviate from the ITK ways of doing things if it
makes more sense in the statistics context - it is not surprising that
a good statistics library doesn't resemble a good image processing
library.

So, the consist thread is that neither backward compatibility nor
consistency should be the driving force in this implementation.   Get
a statistics library that people use should be.   Heck, I'd be open to
swapping out the whole things for calls to the R library...

Stephen

On Thu, Apr 9, 2009 at 4:41 PM, Bill Lorensen <bill.lorensen at gmail.com> wrote:
> Ah, but we did release Calculators in Statistics. I'm not suggesting
> that Calculators be implemented as Filters, rather that Filters be
> implemented using Calculators.
>
> We never released a  MedianImageCalculator, so there is no need to add
> one now. But we did release Calculators in Statistics and we all
> agreed to that API.
>
> I realize that it is probably not practical to keep the new Statistics
> 100% backward compatible, but shouldn't we try as best we can?
>
> Bill
>
> On Thu, Apr 9, 2009 at 4:22 PM, Luis Ibanez <luis.ibanez at kitware.com> wrote:
>> Bill Lorensen wrote:
>>>
>>> Just a few initial comments:
>>>
>>> I see in general that Calculators have been replaced with Filters. It
>>> is a great idea to have filters. However, I'm suprised that the
>>> calculators are gone. In other parts of ITK, we have both Calculators
>>> and Filters. The Filters are implemented using the Calculators.
>>>
>>> I wonder was there any consideration to minimize backward compatibility
>>> issues.
>>>
>> ---------------
>>
>> Bill,
>>
>> The main purpose of the refactoring was to make the Statistics
>> Framework compatible with the pipeline architecture of the
>> rest of the toolkit, and to solve for a number of structural
>> problems in the implementations (e.g. SubSamples of SubSamples).
>>
>> In that regard the main change was to convert:
>>
>>     * Samples    ----->  DataObjects
>>     * Calculator ----->  Filters
>>
>> In the current statistics framework most operations are done by
>> Calculators. This was mostly due to the disconnection with the
>> pipeline architecture of the rest of the toolkit.
>>
>> We could indeed reimplement the old Calculators based on using
>> the new Filters, but it will not correspond to the duality that
>> you refer to in other sections of the toolkit.
>>
>> To be more concrete:
>>
>> In ITK we have the ImageStatistics filter and we have the
>> Calculators that will compute min, max, variance ... etc.
>>
>> But...
>> we don't have both a MedianImageFilter and a MedianImageCaculator,
>> we don't have AnisotropicImagefilter and AnisotropicImageCalculators
>> ... etc... That is, not every filter is duplicated into a calculator.
>>
>> This latter will be more a duplication of functionality (not to
>> mention a maintenance nightmare).
>>
>>
>> Calculators are a functional programming approach and they break away
>> from the data pipeline architecture of the rest of the toolkit.
>>
>>
>> If users are going to migrate and to learn a new framework,
>> we should avoid cluttering it with the same weaknesses that
>> we are trying to get away from.
>>
>>
>> Given that we are not going to remove the existing Statistics
>> Framework anyways, there is not much point into crippling the
>> new Framework with the burden of being backward compatible with
>> the structure that we already identified as having a number of
>> design limitations.
>>
>>
>> ---
>>
>> The rationale for the DensityFrequencyContainer being renamed as "2"
>> is detailed in the Migration Guide, at:
>> http://www.itk.org/Wiki/Proposals:Refactoring_Statistics_Framework_2007_Migration_Users_Guide#Frequency_Containers
>>
>> It also lists the deperecated API (methods from the old class
>> that are no longer available in the new class), and the New API
>> (methods that replace the deprecated methods).
>>
>>
>>
>>   Luis
>>
>>
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-- 
Stephen R. Aylward, Ph.D.
Director of Medical Imaging
Kitware, Inc. - North Carolina Office
http://www.kitware.com
(518) 371-3971 x300