[physwkim]

The reinitialization term in RegionBasedLevelSetFunction::ComputeUpdate() subtracts a curvature that was only computed when the curvature-length term was active, so it silently degenerated to a bare Laplacian. Addresses B30 of #6575.

Root cause

itkRegionBasedLevelSetFunction.hxx:255-269 — curvature is populated inside the curvature-length guard, then read by the reinitialization term, which has its own independent activation condition:

if ((dh != 0.) && (this->m_CurvatureWeight != ScalarValueType{}))
{
  curvature = this->ComputeCurvature(it, offset, gd);      // only here
  curvature_term = this->m_CurvatureWeight * curvature * ... * dh;
}

if (this->m_ReinitializationSmoothingWeight != ScalarValueType{})
{
  laplacian_term = this->ComputeLaplacian(gd) - curvature; // but read here
}

Whenever CurvatureWeight == 0, or dh == 0 at a pixel, curvature stays at its zero initializer and the term collapses to Laplacian(phi) alone — coupling two regularization weights that the class documents (itkRegionBasedLevelSetFunction.h:445-448) as independent, and dropping the div(grad(phi)/|grad(phi)|) half of Laplacian(phi) - div(grad(phi)/|grad(phi)|). The dh factor belongs to the curvature-length term only; the reinitialization term has no dh in its formula.

ComputeCurvature() is now called once when either consumer needs it, and the two guards are separate.

Local validation

New GoogleTest itkRegionBasedLevelSetFunctionGTest.cxx, registered in ITKReviewGTests. phi = x^2 + y^2 gives curvature 1 and Laplacian 4 at the probed pixel, so the correct reinitialization contribution is 3.0 and the buggy one is 4.0. One test per half of the guard:

• ReinitializationTermSubtractsCurvatureWhenCurvatureWeightIsZero — fail-before 4.0, pass-after 3.0.
• ReinitializationTermSubtractsCurvatureWhenDerivativeIsZero — fail-before 4.0, pass-after 3.0.

No ExternalData baseline changes. All six ScalarChanAndVese*LevelSetImageFilter regression tests were run with baselines downloaded, before and after the fix: ImageError = 0 in every case. Test2_2 (CurvatureWeight=0, ReinitializationSmoothingWeight=1) is the one configuration that exercises the bug at every pixel, yet its output is unchanged — MultiphaseDenseFiniteDifferenceImageFilter::ApplyUpdate() re-runs a full SignedMaurerDistanceMap reinitialization every iteration (m_ReinitializeCounter defaults to 1), which rebuilds the level set from the sign of phi and discards the magnitude of the per-pixel correction.

AI assistance

• Tool: Claude Code
• Role: implementation and regression-test authoring from the B30 analysis in BUG: Findings collected while porting ~60 filters — reported together in one issue #6575
• All code was reviewed, built, and tested locally before committing

[greptile-apps]

Greptile Summary

This PR fixes curvature use in the region-based level set update path. The main changes are:

• Compute curvature when the reinitialization term needs it.
• Keep the curvature-length and reinitialization term guards separate.
• Add focused GoogleTests for zero curvature weight and zero Heaviside derivative cases.

Confidence Score: 5/5

This looks safe to merge.

No blocking issues found in the changed code.

No files need attention.

T-Rex Logs

What T-Rex did

• Tool availability was captured for the validation run, showing make, gcc, g++, and python3 present, while cmake, ctest, and ninja were missing.
• The narrow CMake configuration command was attempted but cmake was not found, causing the configure step to exit with an error.
• CTest discovery for RegionBasedLevelSetFunction was attempted but failed because ctest was not found.
• The ITKReviewGTestDriver build command was attempted but cmake was not found, so the build could not proceed.
• The filtered CTest run was attempted but ctest was not found, and no severity-bearing findings were reported because the tests could not be executed.

_{Ran code and verified through T-Rex}

Important Files Changed

Filename	Overview
Modules/Nonunit/Review/include/itkRegionBasedLevelSetFunction.hxx	Computes curvature for both term consumers while preserving each term's activation guard.
Modules/Nonunit/Review/test/CMakeLists.txt	Registers the new region-based level set GoogleTest source.
Modules/Nonunit/Review/test/itkRegionBasedLevelSetFunctionGTest.cxx	Adds regression tests for the reinitialization-only curvature subtraction cases.

_{Reviews (1): Last reviewed commit: "BUG: Compute curvature whenever the rein..." | Re-trigger Greptile}

[hjmjohnson]

First force-push = plain rebase on main; second = review fix only — the new GTest's helper classes are renamed (RegionBasedLevelSetReinitTestFunction/...SharedData) so they no longer share a qualified name with the different itk::RegionBasedLevelSetFunctionTestHelper in the sibling legacy test (latent ODR hazard). Verified locally that itkScalarChanAndVeseDenseLevelSetImageFilterTest2_* baseline comparisons still pass (8/8) — the CurvatureWeight=0 reinitialization change stays within the compare tolerance.

[hjmjohnson]

Force-push = plain rebase onto current main, folding the new GTest source into the GTests list that #6589 introduced for this module (two creategoogletestdriver calls in the CI merge build caused/would cause a duplicate-target configure error). Verified locally: configure clean, module tests pass.