[Don-Yin]

Description

I corrected the minimum-size check in compute_ms_ssim to follow its stride-2 pooling. With L scales, the final SSIM calculation sees each spatial dimension divided by 2 ** (L - 1), not max(1, L - 1) ** 2.

The old check rejects valid four-scale inputs and lets undersized two-scale inputs reach a convolution error. With the default 11-pixel kernel:

• An 88 × 88 image with four scales is valid: the SSIM inputs are 88 → 44 → 22 → 11. It now returns 1 for identical images instead of raising ValueError.
• A 16 × 16 image with two scales is too small because its second scale is only 8 × 8. It now raises ValueError before convolution, with the correct minimum of 22 pixels.

I also updated the error message to identify the affected spatial dimension and report its exact minimum. The SSIM calculation and weights are unchanged; the default five-scale size threshold is unchanged.

Evidence

I checked 96 combinations of square image size and scale count using identical synthetic images and the default kernel. For the feasibility panel, I disabled only the old size precheck and ran the actual pooling and convolution operations. The old gate falsely accepts five combinations and falsely rejects two; the fixed gate agrees with feasibility in all 96 combinations. Outlined cells mark disagreements in the old gate.

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• New tests added to cover the changes.
• In-line docstrings updated.

Validation

• python -m unittest tests.metrics.test_compute_multiscalessim_metric tests.metrics.test_ssim_metric tests.losses.test_ssim_loss -v: 36 tests passed.
• The added tests cover minimum sizes and undersized spatial dimensions in 2D and 3D with one through six scales, anisotropic kernels, and the two examples above.
• black --check, isort --check-only, and ruff check passed for both changed files.

I used an AI coding assistant to help investigate and draft this; I reviewed and tested every change.

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between bbebca6 and d4dc2b2.

📒 Files selected for processing: 2

• monai/metrics/regression.py
• tests/metrics/test_compute_multiscalessim_metric.py

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📝 Walkthrough

compute_ms_ssim now checks spatial dimensions against the kernel size and number of scales, and reports the minimum required dimension when an input is too small. Tests cover minimum valid and undersized 2D and 3D inputs across one to six scales, plus default-kernel and two-scale cases.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to d4dc2

The size-validation change is ready to merge after normal checks; no material issue remains identified.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 14.29% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 7 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Title check		The title clearly and concisely describes the main change: correcting MS-SSIM size validation across different scale counts.
Description check		The description explains the change, provides examples, lists affected change types, documents validation, and identifies new tests. The issue reference line from the template is missing, and several …
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

Full details: Description check

Explanation

The description explains the change, provides examples, lists affected change types, documents validation, and identifies new tests. The issue reference line from the template is missing, and several optional checklist items remain unchecked, but the description is otherwise complete.

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