[Insight-developers] OtsuThresholdCalculator versus OtsuMultipleThresholdsCalculator

Tue Jul 2 11:04:54 EDT 2013

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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