Comment by joshvm

I've been diving into OpenCV recently. It's a very popular library and generally pretty readable (although the code can get quite messy because they use vectorisation a lot). Since it's about as close as we have to a standard, fast, vision library, it's interesting to see how stuff gets implemented.

There are loads of little things that could be improved, so it's also a nice codebase for contributing to.

In general I think you should pick something that you use every day. There's no point reading OpenBSD for the sake of reading OpenBSD, you'll get far more value out of something familiar.