Fix multi-axis shear transform to compose individual shear matrices

[aymuos15]

Fix Shear transform

[coderabbitai]

📝 Walkthrough

Walkthrough

Refactors internal shear matrix construction in monai/transforms/utils.py to compose elementary single-axis shear matrices via successive matrix multiplications instead of setting multiple off-diagonal entries directly. 2D now composes two rank-3 shears; 3D composes six elementary shears in a loop. Public API and error behavior unchanged. Tests updated: adjusted expected matrices for RandAffine/RandAffined outputs and added/updated tests verifying determinant ≈1 and sequential-equivalence of composed shears.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

🚥 Pre-merge checks | ✅ 1 | ❌ 2

❌ Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	Description is minimal and lacks required template sections (issue reference, types of changes, test/doc updates).	Expand description to include issue reference, selection of change types, and checks for tests/docs as per template.
Docstring Coverage	⚠️ Warning	Docstring coverage is 57.14% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (1 passed)

Check name	Status	Explanation
Title check	✅ Passed	Title clearly summarizes the main change: fixing shear transform by composing individual matrices instead of direct assignment.

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✨ Finishing Touches

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[ericspod]

Thanks @aymuos15 I think this looks fine, I'd like @atbenmurray to have a look as well if possible. We need to merge a fix shortly then can come back to this one.

[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@monai/transforms/utils.py`:
- Around line 958-966: The Affine/AffineGrid documentation still describes
shear_params as direct off-diagonal assignments; update the Affine/AffineGrid
docstring (the Affine class and/or AffineGrid transform) to match the
composed-shear behavior: state that shear_params are composed shearing factors
(tuple of 2 floats for 2D, tuple of 6 for 3D), explain the multiplication order
used to compose single-axis shear matrices into the final affine (and that this
yields determinant 1), and give the same example matrix form (e.g., for 2D (Sx,
Sy) produce the composed matrix shown in the shear utils doc) so users
understand how parameters map to the resulting affine.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

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Run ID: a3776a96-a3fc-41e5-bfaa-ba25d25359b4

📥 Commits

Reviewing files that changed from the base of the PR and between 37564b6 and 4f1be32.

📒 Files selected for processing (1)

• monai/transforms/utils.py