ENH: Remove old bundled itkjpeg, use only libjpeg-turbo

[blowekamp]

Remove the old bundled libjpeg (JPEG-8b-based) source tree and the
ITK_USE_JPEG_TURBO option. libjpeg-turbo 3.0.4 is now the sole
bundled JPEG implementation. ITK_USE_SYSTEM_JPEG is unchanged.

The ITK_USE_JPEG_TURBO option was ON by default; this change
removes the dead code path and ~37,600 lines of the legacy source tree.

Changes

• Modules/ThirdParty/JPEG/CMakeLists.txt — removed option(ITK_USE_JPEG_TURBO ...)
• Modules/ThirdParty/JPEG/src/CMakeLists.txt — replaced if/else with unconditional add_subdirectory(itkjpeg-turbo)
• Modules/ThirdParty/JPEG/src/itk_jpeg.h.in — simplified three-way include to two-way (system vs. turbo)
• Modules/ThirdParty/JPEG/src/itkjpeg/ — deleted entire old source directory

AI assistance

• Tool: GitHub Copilot (Claude Sonnet 4.6)
• Role: identified all affected files, made edits, wrote commit message
• Local build verified: cmake --preset release --fresh configured without errors; cmake --build --preset default --target ITKIOJPEG compiled libjpeg-turbo sources and linked successfully

[dzenanz]

When was libjpeg-turbo added? Was it added during 5.x or only after we started 6.0 development?

[blowekamp]

When was libjpeg-turbo added? Was it added during 5.x or only after we started 6.0 development?

It was with 8181c07 in Nov 2025. So it has only been with v6.

There is the feature of supporting using both as system libraries, and then maintaining both in the third-party libraries. These can be considered independently.

[blowekamp]

@thewtex @hjmjohnson Any thoughts on removing the older, and non-updated jpeg library so we are only maintaining one and distributing libjpeg-turbo?

[thewtex]

+1 for just maintaining one, libjpeg-turbo.

[greptile-apps]

Greptile Summary

This PR removes the legacy JPEG-8b bundled source tree (itkjpeg/) and the ITK_USE_JPEG_TURBO CMake option, making libjpeg-turbo 3.0.4 the sole bundled JPEG implementation. The option was already ON by default, so the default build path is unchanged; this change eliminates ~37,600 lines of dead code.

• CMakeLists.txt: option(ITK_USE_JPEG_TURBO ...) removed; src/CMakeLists.txt collapses to an unconditional add_subdirectory(itkjpeg-turbo).
• itk_jpeg.h.in: The three-way #ifdef (system / turbo / legacy) is simplified to a two-way (system / turbo) include guard; the #cmakedefine ITK_USE_JPEG_TURBO line is gone.
• itkjpeg/: All 60 legacy source files are deleted.

Confidence Score: 5/5

Safe to merge — this is a straightforward dead-code removal with no functional change to the default build path.

The ITK_USE_JPEG_TURBO option defaulted to ON, so removing it and unconditionally selecting libjpeg-turbo leaves the default build path identical. All references to ITK_USE_JPEG_TURBO and the legacy itkjpeg/ source tree have been completely excised from the repository. The itk_jpeg.h.in two-way include is correct and simpler than before.

No files require special attention.

Important Files Changed

Filename	Overview
Modules/ThirdParty/JPEG/CMakeLists.txt	Removes the ITK_USE_JPEG_TURBO option; ITK_USE_SYSTEM_JPEG path is unchanged.
Modules/ThirdParty/JPEG/src/CMakeLists.txt	Replaces the if/else block with an unconditional add_subdirectory(itkjpeg-turbo).
Modules/ThirdParty/JPEG/src/itk_jpeg.h.in	Removes the ITK_USE_JPEG_TURBO cmakedefine and collapses the three-way include into a clean two-way (system vs turbo) branch.
Modules/ThirdParty/JPEG/src/itkjpeg/CMakeLists.txt	Deleted as part of the legacy itkjpeg source tree removal.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[CMake configure ITKJPEG] --> B{ITK_USE_SYSTEM_JPEG?}
    B -- Yes --> C[find_package JPEG\nsystem libjpeg]
    B -- No --> D[add_subdirectory itkjpeg-turbo\nbundled libjpeg-turbo 3.0.4]
    C --> E[itk_jpeg.h: include jpeglib.h / jerror.h]
    D --> F[itk_jpeg.h: include itkjpeg-turbo/jpeglib.h / jerror.h]

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