A small Windows utility that walks a directory tree and generates JPEG thumbnails for every recognized image file it finds, dropping each one into a per-directory output folder. The original UI is implemented in Borland C++ Builder 6 (VCL); this repository pairs that legacy Windows shell with a portable, unit-tested C++ core.

• Clean separation of concerns. The pure logic (extension whitelist, output-path derivation) lives in a portable static library that has no dependency on Windows, GDI+, or the VCL framework. The Windows UI is a thin shell on top of it.
• Two parallel build paths. The portable core builds and tests with CMake on any platform, while the Windows executable is still buildable with the original Borland C++ Builder 6 project file (Thumbgen.bpr).
• Hermetic unit tests. The portable core is exercised by GoogleTest cases that cover extension matching, case-insensitivity, multi-dot filenames, mixed separators, and missing extensions — none of which require a filesystem or a Windows runtime.
• Small, deliberate Doxygen surface. Only the public API (include/) is documented; internal helpers stay anonymous and self-named.

cmake -S . -B build
cmake --build build
ctest --test-dir build --output-on-failure

This builds the thumbgen_core static library and the thumbgen_tests GoogleTest executable. On Windows it additionally builds the thumbgen_engine static library that links against GDI+.

1. Open Thumbgen.bpr in C++ Builder 6.
2. The project's include path is configured for include/ and form/.
3. Press F9 to build and run.

The resulting Thumbgen.exe is the GUI shell: drive picker, directory list, width/height spinners, output-folder name, and a Generate button.

doxygen docs/Doxyfile
# Output: docs/build/html/index.html

Thumbgen/
├── CMakeLists.txt              CMake build for portable core and tests
├── include/                    Public headers (Doxygen-annotated)
│   ├── image_filter.h          Supported-extension predicate
│   ├── thumbnail_path.h        Output-path derivation
│   └── thumbnail_engine.h      GDI+ JPEG encoder wrapper (Windows-only)
├── src/                        Implementations
│   ├── image_filter.cpp        Portable
│   ├── thumbnail_path.cpp      Portable
│   ├── thumbnail_engine.cpp    Windows / GDI+
│   └── main.cpp                VCL form glue (Windows-only)
├── tests/                      GoogleTest suites for the portable core
│   ├── test_image_filter.cpp
│   └── test_thumbnail_path.cpp
├── form/                       VCL form artifacts (Windows-only)
│   ├── main.h                  TForm1 declaration
│   ├── main.dfm                Form layout
│   └── main.ddp                C++ Builder diagram metadata
├── docs/Doxyfile               Doxygen configuration
├── Thumbgen.bpr                C++ Builder 6 project file
├── Thumbgen.cpp                WinMain entry point
└── Thumbgen.res                Compiled Win32 resources

bmp, jpg, jpeg, gif, png, tif, tiff, emf (case-insensitive). Output is always JPEG.

Provided as-is for educational and portfolio purposes.