[Insight-developers] OtsuThresholdCalculator versus OtsuMultipleThresholdsCalculator

Tue Jul 2 15:44:53 EDT 2013

I concur with Brad and Hans.

Adding Gaëtan in CC.

Thanks,
Matt

On Tue, Jul 2, 2013 at 5:08 PM, Johnson, Hans J <hans-johnson at uiowa.edu> wrote:
> I agree with brad.  The two should produce the same results, even tough it
> may introduce a different numerical result in the
> OtsuMultipleThresholdCalculator.
>
> -----Original Message-----
> From: Bradley Lowekamp <blowekamp at mail.nih.gov>
> Date: Tuesday, July 2, 2013 12:01 PM
> To: "Padfield, Dirk R (GE Global Research)" <padfield at research.ge.com>
> Cc: ITK <insight-developers at itk.org>, "Richard.Beare at ieee.org"
> <Richard.Beare at ieee.org>
> Subject: Re: [Insight-developers] OtsuThresholdCalculator
> versus  OtsuMultipleThresholdsCalculator
>
> Dirk,
>
> I would vote to have the two produce the same results, and create a little
> migration guide which notes the change.
>
> Regarding, the refactoring, is there any difference in the algorithm
> complexity of the two? Any measurements of the performance difference
> between the two?
>
> Brad
>
> On Jul 2, 2013, at 11:04 AM, "Padfield, Dirk R (GE Global Research)"
> <padfield at research.ge.com> wrote:
>
>> Hi Richard,
>>
>> Thank you for your response and insight.  If anything would need to be
>>changed, I would also lean towards changing the OtsuMultiple because I
>>think we shouldn't change Otsu since I am sure many more people are using
>>the Otsu because it is a standard thresholding algorithm.  I will do some
>>comparisons of the two against other implementations to see what I find.
>>
>> But the question still remains: is it okay to change the OtsuMultiple to
>>give the same output as Otsu for one threshold?  I am thinking in terms
>>of those people who use OtsuMultiple whose results will then be slightly
>>different.  Here are the advantages and disadvantages for keeping things
>>the same:
>>
>> Advantages: everyone's code still works as it did before
>> Disadvantages: the two implementations are inconsistent with each other
>>even though they are the same algorithm.  The overlapping code cannot be
>>merged (inheritance).  And future enhancements will need to be made in
>>both places.
>>
>> My vote is: change OtsuMultiple to be consistent with Otsu.
>>
>> What do others think?
>>
>> Dirk
>>
>>
>> ________________________________
>> From: Richard Beare [richard.beare at gmail.com]
>> Sent: Monday, July 01, 2013 5:40 PM
>> To: Bradley Lowekamp
>> Cc: Padfield, Dirk R (GE Global Research); <insight-developers at itk.org>
>>Developers
>> Subject: Re: [Insight-developers] OtsuThresholdCalculator versus
>>OtsuMultipleThresholdsCalculator
>>
>> Hi,
>> I think the order was - I introduced new filters copied from ImageJ,
>>then Gaetan started refactoring to use the histogram framework. We both
>>did some work to make that correspond to old versions. I don't remember
>>working on the MultipleThreshold  version, but the code does look
>>similar, so perhaps it was done somewhere along the way - will need to
>>check the logs.
>>
>> I'm pretty sure that Otsu was producing the same results that it used to
>>- I didn't compare to other implementations. Thus, if the original Otsu
>>was correct then the current one should be too, which would suggest that
>>the MultipleThresholds version should probably change.
>>
>> Not sure when I'll get a chance to look at this in detail.
>>
>> I don't have a current email for Gaetan to CC for confirmation.
>>
>>
>> On Mon, Jul 1, 2013 at 11:02 PM, Bradley Lowekamp
>><blowekamp at mail.nih.gov<mailto:blowekamp at mail.nih.gov>> wrote:
>> Dirk,
>>
>> I believe Richard Beare did the refactoring of the thresholding
>>framework an Insight Journal Article. He will likely know why it is this
>>way better than anyone else.
>>
>> You also didn't say which implementation is correct.
>>
>> Brad
>>
>>
>> On Jun 30, 2013, at 10:03 PM, "Padfield, Dirk R (GE Global Research)"
>><padfield at research.ge.com<mailto:padfield at research.ge.com>> wrote:
>>
>>> Hi ITK Developers,
>>>
>>> I was just looking through the OtsuThresholdCalculator and
>>>OtsuMultipleThresholdsCalculator to see whether I could refactor them so
>>>that the Otsu inherits from the OtsuMultiple since the latter is a more
>>>general case of the former.  Currently, the code for these two filters
>>>is totally different resulting in significant code duplication and a
>>>need to keep both filters in sync.
>>>
>>> As a first step, I wrote a CMake test to check that the output of the
>>>OtsuMultiple with 1 threshold is the same as the output of the Otsu.
>>>Unfortunately, they are not!  The two filters output thresholds that are
>>>different by 1 histogram bin!  This can be a quite extreme difference
>>>when the numberOfHistogramBins is low, and it leads to different
>>>thresholds even when the numberOfHistogramBins is reasonably high (say
>>>256).  I tracked it down to this code in the Calculators:
>>>
>>> The relevant code from Otsu:
>>>  const double tolerance = 0.00001;
>>>  if ( (varBetween - tolerance) > maxVarBetween )
>>>    {
>>>    maxVarBetween = varBetween;
>>>    maxBinNumber = j;
>>>    }
>>>  }
>>> this->GetOutput()->Set( static_cast<OutputType>(
>>>histogram->GetMeasurement( maxBinNumber + 1, 0 ) ) );
>>>
>>> The relevant code from MultipleOtsu:
>>>  if ( varBetween > maxVarBetween )
>>>    {
>>>    maxVarBetween = varBetween;
>>>    maxVarThresholdIndexes = thresholdIndexes;
>>>    }
>>>  }
>>> for ( j = 0; j < m_NumberOfThresholds; j++ )
>>>  {
>>>  m_Output[j] = histogram->GetBinMax(0, maxVarThresholdIndexes[j]);
>>>  }
>>>
>>> The difference is that the Otsu adds one to the computed threshold
>>>whereas the MultipleOtsu does not.  This is problematic because users
>>>would expect them to give the same result.
>>>
>>> My question is: how should we proceed?  If we change one or the other,
>>>people's code that use the changed one will give slightly different
>>>answers.  If we don't change them, the two filters will give different
>>>outputs for the same input, and it will not be possible to refactor them
>>>to share code.
>>>
>>> What are your thoughts?
>>>
>>> Thanks,
>>> Dirk
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>>
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