lepton VS libzim

JPEG XL can losslessy transcode JPEG into a smaller format. JPEG2000 (or WebP or anything but Lepton[0]) didn't offer that. Besides, we have gif and png for approximately the same space. gif still isn't gone. Displacement isn't necessary for a new format to become useful.

[0] https://github.com/dropbox/lepton

We also use https://github.com/dropbox/lepton to save disk storage. It give 20% saving in our experience.

It may not be directly related to your question, but Dropbox has developed a great open source software called Lepton. It is able to compress JPEGs losslessly and saves on average about 22% of storage space. The files are not readable after compression without decompression, but since your focus seems to be on archiving anyway, it might be handy. And for larger collections of high-resolution images, it can actually save a decent amount of storage space.

Just wondering, is there a "lossless" conversion from PNG/JPEG/etc to AVIF? Kinda like Lepton[1] but just re-compressing without further loss of details.

[1] https://github.com/dropbox/lepton

I meant the Kiwix dump (https://download.kiwix.org/zim/wikipedia_en_all_nopic.zim – careful, 60GB!).

At a first glance, the Wikimedia XML dump does not look substantially different from what Kiwix/ZIM does with compressed HTML: They're both compressed (bz2 for the Wikimedia dump, zstd or LZMA for Kiwix/ZIM), and both compress multiple files at once, so inter-file redundancy should hopefully be significantly reduced.

HTML seems a bit more verbose than the Mediawiki syntax (plus the XML header for each article), but I'd be surprised if that actually accounted for a 3x difference in size.

Then again, Kiwix seems to have experimented with shared dictionary brotli compression, which supposedly yields an >2x improvement: https://github.com/openzim/libzim/issues/144

I wonder if their current zstd implementation also uses shared dictionaries. If not, that might just be the reason: If ZIM compression chunks are much smaller than the bz2 streams of the Wikimedia dumps, there would still be a lot of redundancy between chunks.

This is low-level stuff, but the new release of the backend that powers Kiwix clients, libzim 8.1.0 (changelog), now targets WebAssembly (a.k.a. WASM). This is big because it potentially brings the full powers of the Kiwix backend to Web apps such as Kiwix JS and Kiwix JS PWA/Electron. Up till now, these apps have relied on a custom JS port of parts of libzim, but it lacked full features, such as Full Text search. So, you will begin to see us integrating new features into the Web versions soon!

I'm doing my part o7

It's seriously a very interesting and useful dataset that you can do a lot of fun stuff with, if you grab one of the zims without pictures it's of very manageable size too of just a few dozen gigabytes compressed, and there are reasonably good library support in many languages.

Last point doesn't go for Java. Only one I could find for that was this <https://github.com/openzim/libzim>, it's antique and extremely poorly optimized and lacks support for newer compression schemes. I have fixed the performance and added support for zstd compression, but not published the code as it's extremely not finished and major features in the original codebase are very broken. I'll get around to sharing the code some day but right now it's basically permanently mid surgery as I've only patched so far as to get it to extract all or specific files. If anyone wants a copy of this code regardless of state, give me a holler.