Race condition in ErodeObjectMorphologyImageFilter

[makarovNick]

GlobalDefaultNumberOfThreads affects result
Applying ErodeObjectMorphologyImageFilter on binary image produces different result with
GlobalDefaultNumberOfThreads > 1

To Reproduce
Steps to reproduce the behavior:

1. Operating system, version, and architecture

• OS: [Ubuntu 18.4]
  Architecture: x86_64
  CPU(s): 16

2. Programming language: Python 3.9.14]

3. Version of SimpleITK:
   tested on 2.1.0, 2.2.0, 2.3.1

4. How was SimpleITK installed?

• binary distribution [e.g. python -m pip install SimpleITK]

6. A minimal working example which causes the error.
   data - shape.zip

import SimpleITK as sitk
img = sitk.ReadImage("shape.nrrd")

sitk.ProcessObject.SetGlobalDefaultNumberOfThreads(1)
eroded_img = sitk.ErodeObjectMorphology(img)
result_volume = sitk.GetArrayFromImage(eroded_img).sum()
print("Expecting volume: ", result_volume)

sitk.ProcessObject.SetGlobalDefaultNumberOfThreads(16)
for _ in range(10):
    eroded_img = sitk.ErodeObjectMorphology(img)
    volume = sitk.GetArrayFromImage(eroded_img).sum()
    if result_volume != volume:
        print("Wrong result! Got", volume)

Got:

Expecting volume:  6039
Wrong result! Got 6213
Wrong result! Got 6109
Wrong result! Got 6046
Wrong result! Got 6119
Wrong result! Got 6212
Wrong result! Got 6042
Wrong result! Got 6183
Wrong result! Got 6212
Wrong result! Got 6044

Expected behavior
Expecting eroded image to be the same

Additional context
Also Is it supposed to work differently from sitk.BinaryErode ? Because sitk.BinaryErode gives different result for this image :)

[blowekamp]

I usually use the Binary Morphology filters or a Grayscale Morphology filters to perform binary morphology operations.

I was able to reproduce the race condition, and reviewed the code for the ObjectMorphology filters. It appears that there is a fundamental issue with the set of filters using the "output" image to both sample the kernel and write to it.

This is an ITK issue could you please repost the issue onto InsightSoftwareConsortium/ITK?

[blowekamp]

Thank you for reporting the issue.