fix(apps/nuclick): wire up unused min_area and do_reconstruction parameters

[qinxwew]

Fixes #9083.

Problem

Two documented parameters in monai/apps/nuclick/transforms.py were accepted and stored but never used, so setting them had no effect:

• SplitLabeld.min_area — documented as "the smallest allowable object size", never referenced outside __init__.
• PostFilterLabeld.do_reconstruction — documented as performing a morphological reconstruction, but post_processing neither accepted the flag nor received the nuc_points needed as reconstruction markers.

Root cause

Both behaviors existed in the original NuClick transforms addition (#4266) and were accidentally dropped in the later NuClick transform fixes (#5563):

• the original SplitLabeld._mask_relabeling filtered others components by stat.area > min_area;
• the original post_processing(..., do_reconstruction, nuc_points) regrew each mask from its click points via skimage.morphology.reconstruction.

This PR restores both behaviors, adapted to the current code structure.

Changes

SplitLabeld — after relabeling the others channel, connected components smaller than min_area pixels are discarded. Implemented with np.bincount rather than skimage.morphology.remove_small_objects on purpose: scikit-image 0.26 deprecated the min_size keyword and changed the threshold semantics from "smaller than" to "smaller than or equal to", so the bincount form keeps one well-defined behavior (discard area < min_area, matching the docstring "smallest allowable object size") on every skimage version.

PostFilterLabeld — post_processing again accepts do_reconstruction and nuc_points; when enabled, each instance mask is regrown from its click points (morphology.reconstruction), so only the mask component containing the user's click is kept. Two robustness improvements over the original implementation:

• the marker is intersected with the filtered mask, satisfying skimage's marker <= mask requirement by construction (the original relied on try/except BaseException for clicks outside the mask);
• an instance whose click point falls outside every mask component keeps its filtered mask instead of being silently emptied by an empty marker.

Docstrings for both parameters clarified (including that do_reconstruction consumes the nuc_points key).

Tests

• tests/apps/nuclick/test_nuclick_transforms.py: 5 new cases —
  • SplitLabeld: a 1-pixel others object is dropped with min_area=5 and kept with min_area=1;
  • PostFilterLabeld: without reconstruction both blobs survive (13 px); with reconstruction only the clicked blob remains (4 px); a click on background leaves the filtered mask unchanged (13 px).
• Full file: 24 passed (19 pre-existing + 5 new) with torch CPU + scikit-image.

Note for reviewers

• Unrelated pre-existing observation (not touched here): PostFilterLabeld.gen_instance_map is called as (masks, bounding_boxes, x=img_width, y=img_height) and indexes instance_map[bb[0]:bb[2], bb[1]:bb[3]], so non-square img_height/img_width inputs broadcast-fail — the existing tests only use square shapes. Happy to file a separate issue if you agree it's a bug.
• Also pre-existing: post_processing still calls remove_small_objects(min_size=...), which emits a FutureWarning and changed semantics under scikit-image 0.26 (see above). Can be addressed separately.

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📝 Walkthrough

Walkthrough

SplitLabeld now removes others components smaller than min_area. PostFilterLabeld now supports click-guided morphological reconstruction through nuc_points. Tests cover component filtering, clicked-component retention, and clicks outside all components.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: ⚪ Minimal · up to 74183

This change makes the documented NuClick area-filtering and click-guided reconstruction parameters functional, with regression coverage for the stated behaviors. No concrete merge-blocking risk remains in the supplied context.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 37.50% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 8 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	✅ Passed	The title clearly identifies the primary change: wiring up the previously unused min_area and do_reconstruction parameters in NuClick transforms.
Description check	✅ Passed	The description is detailed and covers the issue, root cause, implementation, tests, and known pre-existing observations. It omits the standard type-of-change checklist and leaves the new-tests item u…
Linked Issues check	✅ Passed	The implementation directly resolves issue #9083 by applying SplitLabeld.min_area filtering and adding optional PostFilterLabeld reconstruction using nuc_points. Regression tests cover both behaviors.
Out of Scope Changes check	✅ Passed	The code, documentation, and regression tests are directly related to issue #9083. The described unrelated observations are explicitly pre-existing and were not changed.

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Inline comments:
In `@monai/apps/nuclick/transforms.py`:
- Line 599: Update post_processing in monai/apps/nuclick/transforms.py (lines
599-599) with a Google-style docstring covering all parameters, including
reconstruction inputs, the returned mask, and any raised exceptions. Add
corresponding Google-style docstrings to the minimum-area test definition in
tests/apps/nuclick/test_nuclick_transforms.py (lines 309-309) and the
reconstruction test definition in tests/apps/nuclick/test_nuclick_transforms.py
(lines 338-338), documenting their inputs, assertions, return behavior, and
relevant exceptions.

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