If the scope of this were to unvendor just these four (libjpeg, libpng, zlib, giflib), I think it is enough to start, but it brings up the question of whether or not JPEG, PNG, and GIF support are hard dependencies in GDAL afterward. They're always available no matter how you build now, so would we make them hard dependencies or relax them to optional after unvendoring? I assume zlib would move to a hard dependency of GDAL.
I would let libjpeg, libpng, giflib, liblerc as optional dependencies. Our CMake scripts already handle them as optional. Someone doing a GDAL build only for vector might want to skip them.
But strongly recommended at least for libjpeg and libpng, since beyond being standalone formats, they are also used for example for GeoPackage, MBTiles, MRF, OGCAPI/WMS/WMTS, etc.
Yes zlib would move to a hard dependency, like PROJ (and if you build PROJ with libtiff or libcurl support, you must certainly already have zlib as it is a canonical dependency of those). There are lots of places in the code base where we assume deflate compression/decompression to be available.
-- http://www.spatialys.com My software is free, but my time generally not.
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