BUG: Reset smallestError per pixel in IterativeInverseDisplacementField

[physwkim]

Addresses item B31 of #6575.

IterativeInverseDisplacementFieldImageFilter declares the coordinate-descent residual bar smallestError outside the per-pixel loop and reassigns it only when a pixel's initial mapped point lies inside the input buffer. A pixel whose initial mapped point falls outside the buffer therefore inherits the previous pixel's residual as the value its probes must beat: probes that land back inside the buffer are accepted or rejected based on how well the previous pixel converged, so the inverse field's values at such pixels depend on raster order.

The fix moves the declaration inside the per-pixel loop, initialized to NumericTraits<double>::max(): the bar a probe must beat is the error of this pixel's current guess when that guess is evaluable (the existing inside-buffer branch still sets it), and "unknown — accept any evaluable probe" otherwise. Behavior for pixels whose initial guess is evaluable is unchanged.

The new regression test builds a 24×24 field where the pixel before the affected one in raster order converges to a zero residual (poisoning the stale bar) and the affected pixel's first guess maps just outside the interpolation buffer, with an in-buffer probe available one step away. Unfixed, the probe loses to the stale zero bar and the pixel never moves off its zero-iteration first guess; fixed, the descent accepts the probe and refines the estimate.

Verified locally (Linux, GCC 13, Release): the new test fails on unmodified main (moved by 0) and passes with the fix; all 24 ITKDisplacementField and 12 ITKRegionGrowing tests pass with the fix applied.

PR Checklist

• No API changes were made (or the changes have been approved)
• No major design changes were made (or the changes have been approved)
• Added test (or behavior not changed)
• Updated API documentation (or API not changed)
• Added license to new files (if any)
• Added Python wrapping to new files (if any) as described in ITK Software Guide Section 9.5
• Added ITK examples for all new major features (if any)

AI assistance

• Tool: Claude Code (claude-fable-5, with claude-opus-4-8 subagents)
• Role: implemented the fix and regression test from the analysis previously filed as item B31 of BUG: Findings collected while porting ~60 filters — reported together in one issue #6575; hand-traced the failing scenario before implementation
• Testing: built locally and verified the regression test fails before / passes after the fix; full ITKDisplacementField + ITKRegionGrowing test suites pass with the fix
• Note: clang-format 19.1.7 was not available locally; formatting was hand-matched to ITK style and may need the CI formatter's touch-up

[greptile-apps]

Greptile Summary

This PR fixes per-pixel residual handling in IterativeInverseDisplacementFieldImageFilter. The main changes are:

• Resets smallestError inside the per-pixel loop.
• Uses NumericTraits<double>::max() when the initial mapped point is outside the input buffer.
• Adds a GoogleTest case for the outside-buffer first-guess refinement path.
• Registers the new displacement-field GoogleTest driver in CMake.

Confidence Score: 5/5

Safe to merge with minimal risk.

The code change is narrow and keeps the existing inside-buffer residual path unchanged. The new tests cover the fixed outside-buffer behavior.

No files require special attention.

T-Rex Logs

What T-Rex did

• T-Rex ran the requested verification and completed the general contract validation, but local artifact references were not uploaded.

_{Ran code and verified through T-Rex}

Important Files Changed

Filename	Overview
Modules/Filtering/DisplacementField/include/itkIterativeInverseDisplacementFieldImageFilter.hxx	Moves smallestError into the per-pixel refinement loop with a max initial value so outside-buffer initial guesses do not reuse the previous pixel's residual.
Modules/Filtering/DisplacementField/test/CMakeLists.txt	Adds a GoogleTest driver for the new displacement-field regression test source.
Modules/Filtering/DisplacementField/test/itkIterativeInverseDisplacementFieldImageFilterGTest.cxx	Adds tests for outside-buffer first guesses and a zero-field sanity check.

Sequence Diagram

%%{init: {'theme': 'neutral'}}%%
sequenceDiagram
participant PixelLoop as Per-pixel loop
participant Interp as Input field interpolator
participant Descent as Coordinate descent
participant Output as Output pixel

PixelLoop->>PixelLoop: "Initialize smallestError = max()"
PixelLoop->>Interp: Check initial mappedPoint
alt Initial guess inside buffer
    Interp-->>PixelLoop: forwardVector
    PixelLoop->>PixelLoop: Set smallestError to current residual
else Initial guess outside buffer
    PixelLoop->>PixelLoop: Keep max() so first evaluable probe can win
end
PixelLoop->>Descent: Probe +/- step per dimension
Descent->>Interp: Evaluate in-buffer probes
Interp-->>Descent: probe residuals
Descent->>PixelLoop: "Accept probe when residual < smallestError"
PixelLoop->>Output: Store refined inverse displacement

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sequenceDiagram
participant PixelLoop as Per-pixel loop
participant Interp as Input field interpolator
participant Descent as Coordinate descent
participant Output as Output pixel

PixelLoop->>PixelLoop: "Initialize smallestError = max()"
PixelLoop->>Interp: Check initial mappedPoint
alt Initial guess inside buffer
    Interp-->>PixelLoop: forwardVector
    PixelLoop->>PixelLoop: Set smallestError to current residual
else Initial guess outside buffer
    PixelLoop->>PixelLoop: Keep max() so first evaluable probe can win
end
PixelLoop->>Descent: Probe +/- step per dimension
Descent->>Interp: Evaluate in-buffer probes
Interp-->>Descent: probe residuals
Descent->>PixelLoop: "Accept probe when residual < smallestError"
PixelLoop->>Output: Store refined inverse displacement

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