Please to make smaller file!
May 10, 2010 11:23 AM   Subscribe

Why doesn't the compress feature on my Mac actually compress the files? It creates a .zip file but it's usually only a couple hundred k smaller than the original.

Does the compression utility work on the Mac? Basically, you right-click on a folder or group of files and can choose "compress" from one of the drop-down options. How handy! However, I've noticed that it doesn't really compress anything though it does make a .zip file. Today, I took a folder containing 77 mb of info (jpegs) and did the little "compress" thing and it compressed it to 76.5 mb. Not exactly what I'm going for. Searching help files isn't telling me much. Is it better to use a stand-alone utility or is there some setting somewhere that I need to alter or is my fundamental understanding of compression not right?
posted by amanda to Computers & Internet (17 answers total) 1 user marked this as a favorite
[...] or is my fundamental understanding of compression not right?

In the specific case you gave as an example, a bunch of jpeg files, this is the problem. Jpegs are already compressed, so they can't be made significantly smaller by compressing them again.

If this were not the case, you could compress any file down to 1 byte by compressing it enough times.
posted by FishBike at 11:28 AM on May 10, 2010 [1 favorite]

JPEGs are already highly compressed using an algorithm that is much better at compressing photos than ZIP is. The same is likely true for most other large files you are likely to have on your system, like videos and MP3s.
posted by Good Brain at 11:29 AM on May 10, 2010

JPEGs are already compressed with a compression scheme optimized for images. Compressing them again into a .zip will make them a little bit smaller, but each time you do will be like squeezing blood from a stone. Other files that usually don't compress well because they're already compressed with schemes optimized for their content are MP3/MP4/M4A files, video files, etc.
posted by zsazsa at 11:29 AM on May 10, 2010

JPEGs are already pretty highly compressed. ZIP looks for redundancy within files and then simplifies from there. JPEGs are pretty unique within themselves, so there isn't a lot of simplification to do.

on preview, what FishBike said
posted by Xoder at 11:29 AM on May 10, 2010

JPEGS are already compressed. Blame Claude Shannon.
posted by GuyZero at 11:29 AM on May 10, 2010 [4 favorites]

jpgs are already compressed. It's like trying to compress another .zip file. Try zipping up a bunch of Word documents and you'll see the file size will shrink a lot more.
posted by bondcliff at 11:29 AM on May 10, 2010

Certain file types compress better than others. File types that are generally already compressed, like .jpgs, aren't going to compress all that much.
posted by Nattie at 11:29 AM on May 10, 2010

In this case, it's the latter - jpegs are already massively compressed; generating a zip file isn't going to do much more than get them all into one file. Try compressing something with text, like a book, and you'll see a much more dramatic reduction in size.
posted by Tomorrowful at 11:31 AM on May 10, 2010

Hmm... okay. So I just tried to compress a folder containing a variety of items -- word docs, pdf, text files, gifs. 5.8 mb --> 5.3 mb.

I feel like back in the day, on a PC, I could magically compress the hell out of things.
posted by amanda at 11:31 AM on May 10, 2010

Are you calling me an old fogie?! You whippersnapper hipster!! Get off my highly compressed lawn!
posted by amanda at 11:35 AM on May 10, 2010

Yeah, files like jpegs, mp3s, and most movie files aren't going to be very compressible at all because they're already compressed.

Good candidates for compression are things like text files, raw media files, etc.
posted by kmz at 11:35 AM on May 10, 2010

The ZIP compression algorithm should be more or less the same, whether on a PC or a MAC. Some things compress better than others, some things are already compressed. No way round it.

Some years ago, computers weren't fast enough to uncompress say a video on the fly, while it was playing. Thus files were uncompressed and big. Today, computers are faster (yay!) and thus the file itself can be compressed and smaller. And cannot be compressed further.
posted by oxit at 11:35 AM on May 10, 2010

You could go get other archiving apps (e.g. Stuffit) and then compare how they zip a set of files with Mac OS X.
posted by teedee2000 at 11:35 AM on May 10, 2010

Okay, my fogie comment doesn't make much sense when the comment that preceded it was deleted. In any case, I thought your answer was funny, GuyZero.

So, it looks like I am a bit of a fogie not realizing how happily compressed most things are.

Doing a compression of just .doc files worked great. Compressing vector files (.ai), not so much.

Fundamental understanding has now been updated. Thanks all!
posted by amanda at 11:40 AM on May 10, 2010

or you could save all the JPEGs as JPEG2000s from Preview. That would be much smaller.
posted by scruss at 3:09 PM on May 10, 2010

For what it's worth, there's a bit of a trend in new file formats toward using Zip as a container format. For example, the Open Document Text (.odt) and related formats used by the suite, the .docx and related formats used by Microsoft Office versions since 2007, Java .jar files and various others are actually just Zip files with a specific internal structure of files and subfolders.

This is good because you can open these things up with a standard Zip archive utility and poke around inside them to get useful stuff (like embedded graphics and whatnot) out. It also means that they're already compressed pretty well, so they don't waste disk space.

The flip side is if you put a bunch of .odt documents in a folder and then Zip that folder, you won't see the same kind of enormous compressions that you're accustomed to seeing with plain text or Word documents in .doc or .rtf formats.
posted by flabdablet at 7:06 PM on May 10, 2010 [1 favorite]

Compressing vector files (.ai), not so much.

Depends on how you save them. AI does some pretty hefty internal file compression already (on by default), so there's not a lot for further compression algorithms to work with.
posted by nathan_teske at 9:47 PM on May 11, 2010

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