[ITK] [ITK-users] OtsuMultipleThresholdsImageFilter creates asymmetric classes for symmetric a distribution

[Dženan Zukić]

Hi Frederik,

what happens if you set number of histogram bins to 129 or 127?

Regards,
Dženan

On Tue, Oct 17, 2017 at 10:35 AM, hellman <fredrik.hellman at gmail.com> wrote:

> Hi all,
>
> I am planning to use OtsuMultipleThresholdsImageFilter to segment an image
> into a number of classes. I started by testing it for a small image with
> only a single threshold to see that I understand how it works. The results
> surprise me though.
>
> I create a 1D-image with four pixels: 1000, 2000, 3000 and 4000. Computing
> a
> threshold to split this distribution into two classes of maximum
> between-class variance should give a threshold between 2000 and 3000.
> However, OtsuMultipleThresholdsImageFilter gives me a value 1996.17, which
> puts 1000 in one class and 2000, 3000, 4000 in the other class. This does
> not give the smallest between-class variance.
>
> The code that I tried can be found below. Do I misunderstand how this
> filter
> should work or is there a bug in its implementation?
>
> Running the following code with ITK 4.12.0 and ITK 4.12.2 prints 1996.17:
>
> #include <itkOtsuMultipleThresholdsImageFilter.h>
>
> typedef itk::Image<float, 1> ImageType;
> typedef itk::Image<int, 1> LabelImageType;
>
> int main(int argc, char *argv[])
> {
>   typename ImageType::Pointer input = ImageType::New();
>
>   ImageType::RegionType region;
>   region.GetModifiableSize()[0] = 4;
>   input->SetRegions(region);
>   input->Allocate();
>
>   ImageType::IndexType index;
>   index[0] = 0;
>   input->SetPixel(index, 1000);
>   index[0] = 1;
>   input->SetPixel(index, 2000);
>   index[0] = 2;
>   input->SetPixel(index, 3000);
>   index[0] = 3;
>   input->SetPixel(index, 4000);
>
>   auto otsuFilter = itk::OtsuMultipleThresholdsImageFilter<ImageType,
> LabelImageType>::New();
>   otsuFilter->SetInput(input);
>   otsuFilter->SetNumberOfHistogramBins(128);
>   otsuFilter->SetNumberOfThresholds(1);
>   otsuFilter->Update();
>
>   std::cout << otsuFilter->GetThresholds()[0] << std::endl;
>
>   return 0;
> }
>
> Best wishes,
> Fredrik Hellman
>
>
>
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