[Insight-developers] OtsuThresholdCalculator versus OtsuMultipleThresholdsCalculator

Wed Jul 3 14:27:10 EDT 2013

Hi Folks,

I agree as well that we should merge the two implementations and provide an update on the migration guide.  

I looked in a little more detail at the two implementations.  If I run both algorithms for a large range of numberOfHistogramBins, they produce very different results when the number is small, but very similar results when the number of bins is large.  This is disturbing in itself, so I am looking into the implementations to see why this happens.

In terms of the timing, they are both very fast for normal bin sizes.  But when I push the numberOfHistogramBins up to 1,000,000 and 10,000,000, I get some differences:

numberOfHistogramBins = 1,000,000:
Otsu = 5 sec
OtsuMultiple = 0.5 sec

numberOfHistogramBins = 10,000,000:
Otsu = 45.80 sec
OtsuMultiple = 4.58 sec

Thus, when the numberOfHistogramBins is very large, the difference is significant: the OtsuMultiple is about 10 times faster.  This is encouraging since the OtsuMultiple is the more general implementation.  

All of these tests are on the cthead1.png image that was used for the ctest.

My plan is to carefully compare the implementations to see how to make them report the same results.  And then I will merge them so that the Otsu is just a shell that inherits from OtsuMultiple.

Thanks,
Dirk

________________________________________
From: Matt McCormick [matt.mccormick at kitware.com]
Sent: Tuesday, July 02, 2013 3:44 PM
To: Johnson, Hans J
Cc: Bradley Lowekamp; Padfield, Dirk R (GE Global Research); <insight-developers at itk.org> Developers; Richard.Beare at ieee.org; Gaëtan Lehmann
Subject: Re: [Insight-developers] OtsuThresholdCalculator versus OtsuMultipleThresholdsCalculator

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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