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fix(data): avoid divide-by-zero in pydicom affine for single-slice volumes#8956

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fix(data): avoid divide-by-zero in pydicom affine for single-slice volumes#8956
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SID-6921:fix/pydicom-affine-single-slice-zero-div

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Summary:

  • fix PydicomReader._get_affine to skip lastImagePositionPatient z-step derivation when there is only one slice
  • add regression tests for single-slice and multi-slice metadata paths

Why:
For single-slice 3D DICOM segmentation metadata, n == 1 caused division by zero in affine z-step computation.

Validation:

  • python -m pytest tests/data/test_init_reader.py -k pydicom_reader_get_affine -q
  • python -m pre_commit run --files monai/data/image_reader.py tests/data/test_init_reader.py

Closes #8925

…lumes

Signed-off-by: SID <nandasiddhardha@gmail.com>
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No actionable comments were generated in the recent review. 🎉

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📒 Files selected for processing (1)
  • tests/data/test_init_reader.py
🚧 Files skipped from review as they are similar to previous changes (1)
  • tests/data/test_init_reader.py

📝 Walkthrough

Walkthrough

In PydicomReader._get_affine, the slice-spacing calculation from lastImagePositionPatient now also requires MetaKeys.SPATIAL_SHAPE[-1] > 1, avoiding a divide-by-zero case for single-slice inputs. tests/data/test_init_reader.py adds two _get_affine tests for single-slice and multi-slice metadata and checks the Z-related affine values.

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🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name Status Explanation Resolution
Docstring Coverage ⚠️ Warning Docstring coverage is 25.00% which is insufficient. The required threshold is 80.00%. Write docstrings for the functions missing them to satisfy the coverage threshold.
✅ Passed checks (4 passed)
Check name Status Explanation
Title check ✅ Passed The title clearly summarizes the main fix for single-slice pydicom affine handling.
Description check ✅ Passed The description covers the fix, motivation, tests, and issue link, though it does not follow the template headings exactly.
Linked Issues check ✅ Passed The code guards the lastImagePositionPatient z-step logic with the single-slice case and adds regression tests as requested.
Out of Scope Changes check ✅ Passed The changes are limited to the requested affine fix and targeted regression tests.
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tests/data/test_init_reader.py

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Actionable comments posted: 2

Caution

Some comments are outside the diff and can’t be posted inline due to platform limitations.

⚠️ Outside diff range comments (1)
monai/data/image_reader.py (1)

729-771: 📐 Maintainability & Code Quality | 🟡 Minor | ⚡ Quick win

Document the new _get_affine fallback.

This definition changed, but its docstring still omits a Returns section and does not say that lastImagePositionPatient is ignored unless MetaKeys.SPATIAL_SHAPE[-1] > 1. Please document that single-slice inputs keep the default third column. As per path instructions, "Docstrings should be present for all definition which describe each variable, return value, and raised exception in the appropriate section of the Google-style of docstrings."

🤖 Prompt for AI Agents
Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@monai/data/image_reader.py` around lines 729 - 771, The _get_affine docstring
is missing the new fallback behavior and return documentation. Update the
Google-style docstring for _get_affine to include a Returns section for the
affine matrix, and note that lastImagePositionPatient is only used when
MetaKeys.SPATIAL_SHAPE[-1] > 1; for single-slice inputs the default third column
remains unchanged.

Source: Path instructions

🤖 Prompt for all review comments with AI agents
Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@tests/data/test_init_reader.py`:
- Around line 104-135: Add Google-style docstrings to the two new test methods
in PydicomReader’s test class:
test_pydicom_reader_get_affine_single_slice_with_last_position and
test_pydicom_reader_get_affine_multi_slice_uses_last_position. Briefly describe
the purpose of each test, the key metadata variables they set up, and the
expected affine assertions in the Args/Returns/Raises sections as appropriate,
matching the project’s docstring requirements for test definitions.
- Around line 120-134: The PydicomReader._get_affine multi-slice test is too
weak because the current lastImagePositionPatient and MetaKeys.SPATIAL_SHAPE
values produce the same affine result as the default branch, so it does not
prove the special path executed. Update
test_pydicom_reader_get_affine_multi_slice_uses_last_position to use a
lastImagePositionPatient value and spatial shape that yield a non-default k3
term, then assert that affine[2, 2] (or the full third column) reflects the
branch-specific result so the test fails if _get_affine skips the
lastImagePositionPatient logic.

---

Outside diff comments:
In `@monai/data/image_reader.py`:
- Around line 729-771: The _get_affine docstring is missing the new fallback
behavior and return documentation. Update the Google-style docstring for
_get_affine to include a Returns section for the affine matrix, and note that
lastImagePositionPatient is only used when MetaKeys.SPATIAL_SHAPE[-1] > 1; for
single-slice inputs the default third column remains unchanged.
🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

  • Push a commit to this branch (recommended)
  • Create a new PR with the fixes

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Review profile: CHILL

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Run ID: 2ddf1ea8-6e46-466f-b496-8d91deca2125

📥 Commits

Reviewing files that changed from the base of the PR and between 4ba89bd and 4a51549.

📒 Files selected for processing (2)
  • monai/data/image_reader.py
  • tests/data/test_init_reader.py

Comment on lines +104 to +135
def test_pydicom_reader_get_affine_single_slice_with_last_position(self):
reader = PydicomReader()
metadata = {
"00200037": {"Value": [1.0, 0.0, 0.0, 0.0, 1.0, 0.0]},
"00200032": {"Value": [10.0, 20.0, 30.0]},
"00280030": {"Value": [0.5, 0.25]},
"lastImagePositionPatient": np.array([10.0, 20.0, 30.0]),
MetaKeys.SPATIAL_SHAPE: np.array([64, 64, 1]),
}

affine = reader._get_affine(metadata, lps_to_ras=False)

np.testing.assert_allclose(affine[0, 2], 0.0)
np.testing.assert_allclose(affine[1, 2], 0.0)
np.testing.assert_allclose(affine[2, 2], 1.0)

def test_pydicom_reader_get_affine_multi_slice_uses_last_position(self):
reader = PydicomReader()
metadata = {
"00200037": {"Value": [1.0, 0.0, 0.0, 0.0, 1.0, 0.0]},
"00200032": {"Value": [0.0, 0.0, 0.0]},
"00280030": {"Value": [1.0, 1.0]},
"lastImagePositionPatient": np.array([0.0, 0.0, 4.0]),
MetaKeys.SPATIAL_SHAPE: np.array([8, 8, 5]),
}

affine = reader._get_affine(metadata, lps_to_ras=False)

np.testing.assert_allclose(affine[0, 2], 0.0)
np.testing.assert_allclose(affine[1, 2], 0.0)
np.testing.assert_allclose(affine[2, 2], 1.0)

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📐 Maintainability & Code Quality | 🟡 Minor | ⚡ Quick win

Add docstrings to the new tests.

Both new test defs are missing Google-style docstrings. As per path instructions, "Docstrings should be present for all definition which describe each variable, return value, and raised exception in the appropriate section of the Google-style of docstrings."

🤖 Prompt for AI Agents
Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@tests/data/test_init_reader.py` around lines 104 - 135, Add Google-style
docstrings to the two new test methods in PydicomReader’s test class:
test_pydicom_reader_get_affine_single_slice_with_last_position and
test_pydicom_reader_get_affine_multi_slice_uses_last_position. Briefly describe
the purpose of each test, the key metadata variables they set up, and the
expected affine assertions in the Args/Returns/Raises sections as appropriate,
matching the project’s docstring requirements for test definitions.

Source: Path instructions

Comment on lines +120 to +134
def test_pydicom_reader_get_affine_multi_slice_uses_last_position(self):
reader = PydicomReader()
metadata = {
"00200037": {"Value": [1.0, 0.0, 0.0, 0.0, 1.0, 0.0]},
"00200032": {"Value": [0.0, 0.0, 0.0]},
"00280030": {"Value": [1.0, 1.0]},
"lastImagePositionPatient": np.array([0.0, 0.0, 4.0]),
MetaKeys.SPATIAL_SHAPE: np.array([8, 8, 5]),
}

affine = reader._get_affine(metadata, lps_to_ras=False)

np.testing.assert_allclose(affine[0, 2], 0.0)
np.testing.assert_allclose(affine[1, 2], 0.0)
np.testing.assert_allclose(affine[2, 2], 1.0)

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🎯 Functional Correctness | 🟡 Minor | ⚡ Quick win

Make the multi-slice fixture prove the branch ran.

With lastImagePositionPatient = [0, 0, 4] and MetaKeys.SPATIAL_SHAPE[-1] = 5, _get_affine() computes (k1, k2, k3) == (0, 0, 1), which is exactly the same as the pre-branch defaults. This test still passes if the lastImagePositionPatient path is skipped entirely.

Proposed fix
     def test_pydicom_reader_get_affine_multi_slice_uses_last_position(self):
         reader = PydicomReader()
         metadata = {
             "00200037": {"Value": [1.0, 0.0, 0.0, 0.0, 1.0, 0.0]},
             "00200032": {"Value": [0.0, 0.0, 0.0]},
             "00280030": {"Value": [1.0, 1.0]},
-            "lastImagePositionPatient": np.array([0.0, 0.0, 4.0]),
+            "lastImagePositionPatient": np.array([2.0, 4.0, 8.0]),
             MetaKeys.SPATIAL_SHAPE: np.array([8, 8, 5]),
         }

         affine = reader._get_affine(metadata, lps_to_ras=False)

-        np.testing.assert_allclose(affine[0, 2], 0.0)
-        np.testing.assert_allclose(affine[1, 2], 0.0)
-        np.testing.assert_allclose(affine[2, 2], 1.0)
+        np.testing.assert_allclose(affine[0, 2], 0.5)
+        np.testing.assert_allclose(affine[1, 2], 1.0)
+        np.testing.assert_allclose(affine[2, 2], 2.0)
📝 Committable suggestion

‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.

Suggested change
def test_pydicom_reader_get_affine_multi_slice_uses_last_position(self):
reader = PydicomReader()
metadata = {
"00200037": {"Value": [1.0, 0.0, 0.0, 0.0, 1.0, 0.0]},
"00200032": {"Value": [0.0, 0.0, 0.0]},
"00280030": {"Value": [1.0, 1.0]},
"lastImagePositionPatient": np.array([0.0, 0.0, 4.0]),
MetaKeys.SPATIAL_SHAPE: np.array([8, 8, 5]),
}
affine = reader._get_affine(metadata, lps_to_ras=False)
np.testing.assert_allclose(affine[0, 2], 0.0)
np.testing.assert_allclose(affine[1, 2], 0.0)
np.testing.assert_allclose(affine[2, 2], 1.0)
def test_pydicom_reader_get_affine_multi_slice_uses_last_position(self):
reader = PydicomReader()
metadata = {
"00200037": {"Value": [1.0, 0.0, 0.0, 0.0, 1.0, 0.0]},
"00200032": {"Value": [0.0, 0.0, 0.0]},
"00280030": {"Value": [1.0, 1.0]},
"lastImagePositionPatient": np.array([2.0, 4.0, 8.0]),
MetaKeys.SPATIAL_SHAPE: np.array([8, 8, 5]),
}
affine = reader._get_affine(metadata, lps_to_ras=False)
np.testing.assert_allclose(affine[0, 2], 0.5)
np.testing.assert_allclose(affine[1, 2], 1.0)
np.testing.assert_allclose(affine[2, 2], 2.0)
🤖 Prompt for AI Agents
Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@tests/data/test_init_reader.py` around lines 120 - 134, The
PydicomReader._get_affine multi-slice test is too weak because the current
lastImagePositionPatient and MetaKeys.SPATIAL_SHAPE values produce the same
affine result as the default branch, so it does not prove the special path
executed. Update test_pydicom_reader_get_affine_multi_slice_uses_last_position
to use a lastImagePositionPatient value and spatial shape that yield a
non-default k3 term, then assert that affine[2, 2] (or the full third column)
reflects the branch-specific result so the test fails if _get_affine skips the
lastImagePositionPatient logic.

Signed-off-by: SID <nandasiddhardha@gmail.com>
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fix: division by zero in DICOMReader._get_affine for single-slice 3D volumes

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