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We used this experimental project in the development of the PNG-EXIF ("eXIf") specification, back in 2017. The project evolved together with the draft specification, which was finalized on 2017-Jun-15 and approved by the PNG Group on 2017-Jul-13. The EXIF specification, outside of the scope of PNG and libpng, is quite complex. The libpng implementation cannot grow too much beyond performing basic integrity checks on top of serialization. In order to create and manipulate PNG-EXIF image files, the use of external libraries and tools such as ExifTool is necessary. Now, with the addition of contrib/pngexif to the libpng repository, offline tasks like metadata inspection and linting can be performed without importing external dependencies.
This "contrib" directory contains contributions which are not necessarily under the libpng license, although all are open source. They are not part of libpng proper and are not used for building the library, although some are used for testing the library via "make check".