3/10/2023 0 Comments Actionally dictionary![]() However, if you know that your files contain repeating data at long distances, you can use a third-party filter. In order to eliminate repeating data, there is nothing you can do except increasing dictionary size. LZMA2 has certain restrictions on dictionary size, so if you specify 513 MB dictionary, 768 MB will be requested upon extraction (but only 513 MB will be actually used). With LZMA, you can specify exactly 760 MB dictionary, and 7-Zip will request only so much memory (however, 7-Zip File Manager may display it rounded to 768 MB). So, if you select a 1 GB dictionary with 760 MB of files, 7-Zip will request 1 GB from the system (and will fail if there is not enough virtual memory), but only 760 MB (and several megabytes for other purposes) will be really used. To extract the data, you need to put the dictionary into memory. However, replacing the data with such references costs more with bigger dictionaries, so sometimes you can increase the dictionary and get worse compression. That's why increasing dictionary size usually enhances compression. If you have two identical files, then 7-Zip can store the data of the second file as a reference to the first one and compress it into just several bytes, but the dictionary size must be at least the size of the first file (plus the size of any other files that are compressed between those two). The dictionary size means the maximal distance where 7-Zip can search for redundant data. 768 MB or even 1 GB), but the rest of the dictionary will not be used. If your files occupy 760 MB, then you can have dictionaries up to those 760 MB. It is created dynamically from the uncompressed file data. Lempel-Ziv family of compression methods use a so-called sliding dictionary, which is not stored in the archive. It seems that you misunderstand the meaning of the dictionary. ![]() For example, when you see LZMA2:23 in 7-Zip File Manager, this means that you have a 2 23=8 MB dictionary. It is possible to view dictionary size for most compression methods that use dictionary at all. I figured the file must be mostly dictionary so I compressed it once more with a 8MB dictionary and got a 9.8 MB file.Ĭlearly the 6.9 MB file I got doesn't contain a dictionary larger than itself so what's going on here? Does this have something to do with LZMA2 having more memory to work with when set to use a larger sized dictionary? If so, is there anything I can do that would allow it to get those kinds of compression results without specifying such larger dictionary sizes? Because right now I'm looking at that 6.9MB file and wonder if it could get any smaller with a 2GB dictionary which is kind of obsurd lol This time it only compressed it to 22.8 MBs. Since there doesn't seem to be a way to view the dictionary size of a compressed archive, I compressed the file again but with a 6MB dictionary this time. That made me curious how small the actually dictionary must be. Since that was such a good result I decided to see what would happen if I maxed out the dictionary size. There was one file I ran into that is 760 MBs uncompressed that shrunk to 188 MBs after being compressed with a 64KB dictionary. By that, I mean how small could I get them using smaller dictionary sizes on single files since an actual game wouldn't want to allocate over 1GB of memory for a dictionary out of the no where. ![]() ![]() I've been playing around with compressing game files with LZMA2 to see how small they could realistically make them.
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