CODEWITHSUNDEEP
command-linecompression7ziplzma
Shilpi Agrawal
Shilpi Agrawal

Reputation: 615

Get Maximum Compression from 7zip compression algorithm

I am trying to compress some of my large document files. But most of files are getting compresses by only 10% maximum. I am using 7zip Terminal Commands.

7z a filename.7z -m0=LZMA -mx=9 -mmt=on -aoa -mfb=64 filename.pptx

Any suggestion on changing parameters. I need at least 30% compression ratio.

Upvotes: 4

Views: 52637

Answers (4)

durette
durette

Reputation: 424

PPTX, XLSX, and DOCX files can indeed be compressed effectively if there are many of them. By unzipping each of them into their directories, an archiver can find commonalities between them, deduplicating the boilerplate XML as well as any common text between them.

If you must use the ZIP format, first create a zero-compression "store" archive containing all of them, then ZIP that. This is necessary because each file in a ZIP archive is compressed from scratch without taking advantage of redundancies across different files.

By taking advantage of boilerplate deduplication, 30% should be a piece of cake.

Upvotes: 1

Nadu
Nadu

Reputation: 2501

Well just an idea to max compressing is

  1. 'recompress' these .zip archives(the .docx, .pptx, jar...) using -m0 (storing = NoCompression) and then
  2. apply lzma2 on them

lzma2 is petty good - however if the file contains many jpg's consider to give the opensource packer peazip or more specify paq8o a try. Paq8 has a build in Jpeg compressor and supports range compression. So it will also come along with jpg's the are inside some other file. Winzip's zipx in contrast to this will require pure jpg files and is useless in this case.

But again to make PAQ effectively working/compressing your target file you'll need to 'null' the zip/deflate compression, turn it into an uncompressed zip.

Well PAQ is probably a little exotic, however it's in my eye's more honest and clear than zipx. PAQ is unsupport so it's as always a good idea to just google for what don't have/know and you will find something. Zipx in contrast may appears a little intrigious since it looks like a normal zip and files are listed properly in Winrar or 7zip but when you like to extract jpg's it will fail so if the user is not experienced it may seem like the zip corrupted. It'll be much harder to find out that is a zipx that so far only winzip or The Unarchiver(unar.exe) can handle properly.

Upvotes: 2

CodesInChaos
CodesInChaos

Reputation: 108790

If you really need that compression, you could use the fact that a pptx is just a fancy zip file:

Unzip the pptx, then compress it with 7zip. To recover an equivalent (but not identical) pptx decompress with 7zip and recompress with zip.

There are probably some complications, for example with epub there is a certain file that must be stored uncompressed as first file in the archive at a certain offset from the start. I'm not familiar with pptx, but it might have similar requirements.

I think it's unlikely that the small reduction in file size is worth the trouble, but it's the only approach I can think of.

Depending on what's responsible for the size of the pptx you could also try to compress the contained files. For example by recompressing png files with a better compressor, stripping unnecessary data (e.g. meta-data or change histories) or applying lossy compression with lower quality settings for jpeg files.

Upvotes: 6

MC ND
MC ND

Reputation: 70923

.pptx files or .docx files are internally .zip archives. You can not expect a lot of compression on an already compressed file.

Documentation states lzma2 handles better data that can not be compressed, so you can try with 

7z a -m0=lzma2 -mx filename.7z filename.pptx

But the required 30% is almost unreachable.

Upvotes: 8

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