ITK Python and output of WatershedImageFilter

[brad-t-moore]

Description

WatershedImageFilter produces an itk.ULL (IdentifierType) Image. CastImageFilter doesn't wrap itk.ULL.

https://github.com/InsightSoftwareConsortium/ITK/blob/f98f6a8920c2b8af175865458fade50439a81c3a/Modules/Segmentation/Watersheds/include/itkWatershedImageFilter.h

Expected behavior

The current Python example: https://itk.org/ITKExamples/src/Segmentation/Watersheds/SegmentWithWatershedImageFilter/Documentation.html

works by using ScalarToRGBColormapImageFilter which is wrapped for ULL.

Would like to be able to have either ULL supported/wrapped in CastImageFilter or modifying WaterShedImageFilter to take OutputImageType (with default IdentifierType) as a parameter. Thoughts?

Versions

itk-5.2.0.post2

If the commit number is required, run $ git rev-parse --short HEAD. -->

Environment

Python 3.7.6
Windows 10

[dzenanz]

Watershed filter tends to produce a lot of segments. It is probably not enough to just add an output image template type which defaults to the current output type. If anyone takes up modifying it, they will have to ensure that segment count overflows don't crash the filter and are handled somehow.

Adding a type specification for ULL->UL (and maybe some other target types) should be relatively easy and straightforward.

[brad-t-moore]

Sure, so just add a ULL to UL and ULL to double in the CastImageFilter wrapping?

[brad-t-moore]

With dimensions 2,3,4 as that is what itk.Image is wrapped for (don't see any ULL in vector images).

[dzenanz]

Yes, I think that will do it.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. Thank you for your contributions.

[hjmjohnson]

Closing as fixed.

itk::CastImageFilter is now wrapped to allow casting from unsigned long long to all signed/unsigned integer types, which is exactly what WatershedImageFilter's ULL output needs to be reinterpreted into a Python-displayable type. From Modules/Filtering/ImageFilterBase/wrapping/itkCastImageFilter.wrap:

unique(types "${WRAP_ITK_SCALAR};UC;UI;SI;UL")
itk_wrap_image_filter_combinations("${types}" "${types}")
...
# Enable casting 64 bit unsigned int to smaller integer types if we haven't done so already
if(NOT ITK_WRAP_unsigned_long_long
   AND NOT ${ITKM_IT} STREQUAL "ULL")
  itk_wrap_image_filter_combinations("ULL" "${WRAP_ITK_INT}")
endif()

Python users can now itk.cast_image_filter(watershed_output, ttype=(itk.Image[itk.ULL,3], itk.Image[itk.UI,3])) and feed the result into a viewer.

Triaged as part of the 2026-04-10 open-issue relevance review.