Write to NRRD writes the wrong data type to the file

[MattTheCuber]

Description

Writing images at NRRD files outputs the wrong type to the NRRD header.

Steps to Reproduce

import SimpleITK as sitk

image = ...  # Create sitk image
print(img.GetPixelIDTypeAsString())  # '16-bit signed integer'
sitk.WriteImage(image, "test.nrrd")

Expected behavior

NRRD0004
...
type: int16

Actual behavior

NRRD0004
...
type: short

Reproducibility

This happens 100% of the time for int16 data type, but likely happens with other data types as well.

Versions

I am using the latest Python SimpleITK wrapper.

Environment

Python 3.13 on RHEL 9.5

Additional Information

I use sed -e '/./!Q' /path/to/nrrd/file.nrrd to read the NRRD header easily.

[MattTheCuber]

img.SetMetaData("NRRD_type", "int16") did not work.

[zivy]

Hello @MattTheCuber,

Not sure how to classify this, bug or feature in ITK.

The NRRD data structure definition has a nrrdType enum which maps the type field to corresponding C type. ITK uses this enum and is what is currently written out as the type. In the definition of the type field there are multiple values that can be mapped to the same C type which is where the problem arrises.

ITK identifies the C type from the image itself and does not look at the metadata dictionary when writing.

@hjmjohnson, was there a reason for not looking at a NRRD_type set in the metadata dictionary, checking that it is one of the types corresponding to the C-type and then writing that value as the type? If the user specified type in the metadata dictionary doesn't match the C-type, ignore it, issue a warning, and use the C-type as the type written to file.