WIP: Fix FFTWF-only build on release-5.4 (CI-validation commit temporary)

[hjmjohnson]

WIP — do not merge yet. Fixes the FFTWF-only (single-precision FFTW) build on release-5.4, and temporarily carries a CI-only commit (17509f0) forcing the Pixi leg to build single-precision-only so the matrix actually exercises the fix. The WIP commit will be dropped once CI is green. Stacked on release-5.4. Companion to #6330 (same fixes on main).

The two defects (masked by the default dual-precision build)

1. CMake/itkExternal_FFTW.cmake set FFTW_INCLUDE from the double-precision FFTW3_INCLUDE_DIRS inside the ITK_USE_FFTWF block, leaving it empty in single-precision-only builds (fftw3.h not found).
2. The FFTW factory instantiated both precisions. FFTImageFilterFactory<FFTWxxx> now consults per-precision predicates (FFTImageFilterEnableFloat/Double) guarded by if constexpr. Each FFTW filter header specializes the predicate to false_type for the absent precision, so the factory does not instantiate fftw::ComplexToComplexProxy<double> (undefined when ITK_USE_FFTWD=OFF).

The specializations live in each FFTW filter header (next to FFTImageFilterTraits<FFTWxxx>), not in a .cxx, so every translation unit that instantiates the factory — including itkTestDriverIncludeRequiredFactories.cxx — sees them. (This is the fix learned from #6330, where the .cxx-only specializations left other TUs broken.)

Local verification

Built ITKTestKernel on macOS arm64 with ITK_USE_FFTWF=ON / ITK_USE_FFTWD=OFF / ITK_USE_SYSTEM_FFTW=OFF against release-5.4: staged FFTW built its own fftw3.h, and itkTestDriverIncludeRequiredFactories.cxx compiles and links.

[greptile-apps]

Greptile Summary

Fixes a CMake variable typo in the ITK_USE_FFTWF block where FFTW_INCLUDE was mistakenly assigned from FFTW3_INCLUDE_DIRS (the double-precision variable, only populated in the ITK_USE_FFTWD block) instead of FFTW3f_INCLUDE_DIRS, leaving the staged fftw3.h include path empty in single-precision-only builds.

• Root cause: FFTW3_INCLUDE_DIRS is set only when ITK_USE_FFTWD=ON; when only ITK_USE_FFTWF=ON, it is unset, so FFTW_INCLUDE evaluated to an empty string and the compiler could not locate fftw3.h.
• Fix: One-line change to reference FFTW3f_INCLUDE_DIRS, which is always set within the same ITK_USE_FFTWF block, correctly pointing to ${FFTW_STAGED_INSTALL_PREFIX}/include.

Confidence Score: 5/5

Safe to merge — the one-line change corrects a variable reference that was wrong only in single-precision-only builds and has no effect when both precisions are enabled.

The change is a single variable rename inside a well-bounded conditional block. FFTW3f_INCLUDE_DIRS is set on the line immediately above in the same block, so the replacement is always valid. The bug was genuinely masked in dual-precision builds, explaining why it reached main. No other code paths are affected.

No files require special attention.

Important Files Changed

Filename	Overview
CMake/itkExternal_FFTW.cmake	Single variable name fix: FFTW3_INCLUDE_DIRS → FFTW3f_INCLUDE_DIRS in the FFTWF block so FFTW_INCLUDE is populated in single-precision-only builds.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[CMake configure] --> B{ITK_USE_SYSTEM_FFTW?}
    B -- YES --> Z[Use system-installed FFTW]
    B -- NO --> C{ITK_USE_FFTWF?}
    C -- YES --> D[ExternalProject_Add fftwf]
    D --> E[set FFTW3f_INCLUDE_DIRS = FFTW_STAGED_INSTALL_PREFIX/include]
    E --> F["set FFTW_INCLUDE = FFTW3f_INCLUDE_DIRS ✅ (was: FFTW3_INCLUDE_DIRS ❌)"]
    C -- NO --> G{ITK_USE_FFTWD?}
    F --> G
    G -- YES --> H[ExternalProject_Add fftwd]
    H --> I[set FFTW3_INCLUDE_DIRS = FFTW_STAGED_INSTALL_PREFIX/include]
    I --> J[set FFTW_INCLUDE = FFTW3_INCLUDE_DIRS]
    G -- NO --> K[FFTW_INCLUDE used by ITKFFT targets]
    J --> K

Loading

_{Reviews (1): Last reviewed commit: "BUG: Set FFTW_INCLUDE from FFTW3f_INCLUD..." | Re-trigger Greptile}

[hjmjohnson]

Companion CI-validation PR #6330 (WIP, targets main) exercises the FFTWF-only build path that this fix addresses — it forces ITK_USE_FFTWF=ON / ITK_USE_FFTWD=OFF on the Pixi-Cxx leg, which the default dual-precision matrix never covers.

[hjmjohnson]

@dzenanz Build a 3rd-party app against this, identified an issue with placement of code in .cxx that needs to be in .h files.

Adding more CI testing (temporarily) to investigate the completeness of the solution.