BUG: Use itk::Math::Absolute instead of legacy itk::Math::abs in tests

[hjmjohnson]

Replace the deprecated itk::Math::abs shim with its non-deprecated equivalent itk::Math::Absolute in two test files, fixing the nightly build failures on all dashboard clients that set ITK_FUTURE_LEGACY_REMOVE.

Why the nightlies fail

itk::Math::abs is a backward-compatibility shim in itkMath.h, wrapped in #if !defined(ITK_FUTURE_LEGACY_REMOVE). Dashboard clients that build with ITK_FUTURE_LEGACY_REMOVE:BOOL=1 compile the shim out, so itk::Math::abs no longer resolves and these translation units fail with no member named 'abs' in namespace 'itk::Math'.

itk::Math::Absolute is the non-deprecated function the shim forwards to. For the double arguments used in both tests it has identical behavior.

Affected nightly builds (2026-07-17): RogueResearch Mac13/Mac14/Mac15/Mac10.15/Mac11 dbg-Universal, Mac26 ClangMain, Mac15 ASan/UBSan — every dashboard build-error on that day traced to this single site.

The second file is a latent breakage

The nightly reported only itkInverseDisplacementFieldImageFilterGTest.cxx, because the build stops at the first compile error. itkJointHistogramMutualInformationImageToImageMetricv4Test.cxx uses the same deprecated call and would fail as soon as the first is fixed, so both are corrected here in one commit.

Local verification

Replicated the RogueResearch24 Mac13.x-AppleClang-dbg-Universal cache locally (ITK_LEGACY_REMOVE=1, ITK_FUTURE_LEGACY_REMOVE=1, universal Debug).

• ITKDisplacementFieldGTestDriver + ITKMetricsv4TestDriver: 0 compile errors.
• InverseDisplacementFieldImageFilter GTest: 8/8 cases pass (including the two whose maxError assertions use the changed lines).
• itkJointHistogramMutualInformationImageToImageMetricv4Test: passes.
• pre-commit run --all-files: clean.

[greptile-apps]

Greptile Summary

This PR fixes nightly build failures on dashboard clients that set ITK_FUTURE_LEGACY_REMOVE=1 by replacing the deprecated itk::Math::abs shim (gated behind #if !defined(ITK_FUTURE_LEGACY_REMOVE) in itkMath.h) with its non-deprecated equivalent itk::Math::Absolute in two test files.

• itkInverseDisplacementFieldImageFilterGTest.cxx: two itk::Math::abs calls in maxError accumulation expressions replaced with itk::Math::Absolute.
• itkJointHistogramMutualInformationImageToImageMetricv4Test.cxx: three itk::Math::abs calls in the analytic/finite-difference ratio guard replaced with itk::Math::Absolute. The PR also proactively fixes the second file, which would have failed as soon as the first compile error was resolved.

Confidence Score: 5/5

Safe to merge — both changes are mechanical symbol substitutions with identical runtime semantics for the double arguments used.

All five changed call sites substitute itk::Math::Absolute for the deprecated itk::Math::abs shim, which is a direct forwarding wrapper. The argument types are double in both files, so there is no behavioral difference. A grep across the entire Modules/ tree confirms no remaining itk::Math::abs usages, and the second file was proactively included to avoid a follow-on failure.

No files require special attention.

Important Files Changed

Filename	Overview
Modules/Filtering/DisplacementField/test/itkInverseDisplacementFieldImageFilterGTest.cxx	Replaces two calls to deprecated itk::Math::abs with itk::Math::Absolute; behavior is identical for the double arguments used here.
Modules/Registration/Metricsv4/test/itkJointHistogramMutualInformationImageToImageMetricv4Test.cxx	Replaces three calls to deprecated itk::Math::abs with itk::Math::Absolute in the analytic/finite-difference comparison guard; behavior is identical for double arguments.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A["Compiler processes test TU"] --> B{ITK_FUTURE_LEGACY_REMOVE defined?}
    B -- No --> C["itk::Math::abs shim available\n(backwards-compat wrapper)"]
    B -- Yes --> D["itk::Math::abs shim compiled out\n→ compile error: 'no member abs'"]
    C --> E["itk::Math::Absolute called via shim"]
    D -.fix.-> F["itk::Math::Absolute called directly"]
    E --> G["double abs value returned"]
    F --> G

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flowchart TD
    A["Compiler processes test TU"] --> B{ITK_FUTURE_LEGACY_REMOVE defined?}
    B -- No --> C["itk::Math::abs shim available\n(backwards-compat wrapper)"]
    B -- Yes --> D["itk::Math::abs shim compiled out\n→ compile error: 'no member abs'"]
    C --> E["itk::Math::Absolute called via shim"]
    D -.fix.-> F["itk::Math::Absolute called directly"]
    E --> G["double abs value returned"]
    F --> G

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