Originally Posted by
nanoflooder
SolVanDyck, maybe this will help:
Imagine a bunch of files, including dll's and exe's with embedded pictures, jpg's, png's and so on - when you try to compress them with an archiver, it doesn't compress all the precompressed data (such as jpg, png etc.), though the compression ratio of the algorythms used in those files is significantly worse than in most of the today's archivers - it's deflate, gzip, bzip2, instead of lzma, ppmd, proper bwt, you know. So what precomp does is decompresses all that precompressed data, so you can perform superior compression using your own, better archiver, and then afterwards decompress it and recompress all the jpg, gif, png back!
Compression: .jpg (contains metadata + deflated picture) -> [precomp] .jpg.pcf (contains metadata + decompressed picture) -> [7-Zip (for example)] .jpg.pcf.7z (contains well compressed metadata and the original picture)
Decompression: .jpg.pcf.7z -> [7-Zip] .jpg.pcf -> [precomp] .jpg