[SID-6921]

Bug

Crop.apply_transform (src/torchio/transforms/spatial/crop.py) and _crop_image_lazy (src/torchio/transforms/spatial/crop_or_pad.py) both build slice stops as:

si - i1 or None

si - i1 already equals si (an open-ended stop, same as None) when i1 == 0. The or None doesn't help that case — but it also converts a stop of exactly 0 into None. A stop of 0 happens when i1 == si, i.e. the user crops an amount equal to the axis's full size (intending to remove the whole axis). slice(i0, None) means "to the end," the opposite of the intended empty slice — the axis is silently left completely uncropped instead of becoming length 0. No error, no warning.

Reproduction:

import torch, torchio as tio
image = tio.ScalarImage(torch.rand(1, 20, 20, 20))
subject = tio.Subject(t1=image)
result = tio.Crop(cropping=(0, 20, 0, 0, 0, 0))(subject)
print(result.t1.shape)  # (1, 20, 20, 20) -- should be (1, 0, 20, 20)

Fix

Remove the unneeded or None: si - i1 is correct as a literal stop value in every case (open-ended, partial, and full-axis crop alike), since slicing to si on an axis of length si is identical to slicing to None.

Tests

Added one test per affected path:

• test_crop_entire_axis_from_the_end / test_crop_entire_axis_from_the_end_all_axes (tests/test_crop.py) for the eager Crop.apply_transform path.
• test_crop_image_lazy_entire_axis_from_the_end (tests/test_crop_or_pad.py) for the lazy _crop_image_lazy path.

Mutation-verified: reverting either file's fix back to ... or None fails exactly the new test(s) for that file, with the rest of tests/test_crop.py and tests/test_crop_or_pad.py (84 tests total) still green. Full tests/test_crop.py + tests/test_crop_or_pad.py suite: 84 passed. Broader -k "crop or pad or spatial" slice: 252 passed.

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[SID-6921]

Checking in on this one — still green and mergeable, no review yet. Happy to make any further changes if needed.