Mmm, and a technical question

[kelliem]

Someone brought muffins in to work for everyone to share today. Chocolate ones. I am vainly trying to pretend that these are actually nutritious food and not just giant cupcakes. :-)

Also, I have one of those stupid techno questions that will have everyone rolling their eyes at the ignoramus. It's about file compression. For years I've downloaded and opened files that have been zipped or rar-ed, and all this time I have assumed that the files had been zipped or rared in order to make them smaller and easier to upload/download. With this idea in mind, I decided that since I had a large file I wanted to upload, I would rar or zip it first so it wouldn't take so long to upload and be easier to download by its recipient. So I do the rar thing and the final product is almost exactly the same size as the original file. Confused, I then try to zip the original file, with the same result. The 'compressed' file appears to be virtually the same size as the original. Am I doing something wrong or have I been mistakenly assuming all these years that zipping or raring a file will make it smaller? (And if it doesn't, why do people bother?)

Comments

[elke-tanzer.livejournal.com]

It depends on what kind of file you're trying to compress, and what compression you're trying to use. Some file formats compress better than others.

One reason many folks compress digital video files is to force people to download (save-as) rather than streaming it off their server. It may not make the file much smaller (or maybe it will, depending on the file) but it makes the file unstreamable.

[kelliem.livejournal.com]

Aha. Okay, that makes sense. I can understand why you wouldn't want someone streaming off your server. So if the file I have is an .avi, then what should I try, or should I just not bother? My main reason for wanting to do it was because it can take several hours to upload a file to yousendit or one of the other filesharing sites, and I was trying to cut that down. Downloading seems to go much faster for some reason.

[elke-tanzer.livejournal.com]

You're doing this from a home DSL or cable modem connection, right? If so, your upstream transmission rate (from your home computer up through your ISP to the internet) is going to be much slower than your downstream transmission rate (from the internet to your ISP to your home computer). The DSL and cable modem protocols are tuned that way because the majority of users are downloading a lot more information than they're uploading.

If you're trying to transfer an .avi... well, if a lot of the frames of the image are all one color, or multiple frames of the video are a lot alike, then it may compress down to a slightly smaller file size, because compression depends (mostly, and I'm generalizing here a lot) on similarities in a file being able to be mooshed (not a technical term, I know) together to save space. If the file is lots of very different frames, lots of motion and changes, it probably won't compress much.

Willya look at that... I'm actually using stuff I learned in my networking class a couple weeks ago... amazing.

[kelliem.livejournal.com]

Right doing it from home over a cable modem. That's excellent information. Frustrating, but at least now I know why it does what it does. Sounds like I shouldn't bother trying to compress the file, then, and just set it to upload when I go to bed so it won't be annoying me by sitting there uploading for hours on end. At least if I'm asleep, I won't care. :)

[highnez.livejournal.com]

Sometimes the files are smaller after they are zipped and sometimes they aren't. I'll leave the explanation to someone else, because I can't remember the details at the moment.

As to why you would zip a file: Because of virus scares, some services will not allow you to post anything with a .exe, .chm, or several other extensions that could transmit viruses. You have to put the files in a zip file in order to send them to someone.

[kelliem.livejournal.com]

Thanks! That makes sense. Elke did manage to explain some of the technical details, too.

[shiningmoon.livejournal.com]

For the most part, the reason they aren't smaller is because they started out in a compressed file format in the first place, such as JPG. One can only compress a file down so much. :)

[kelliem.livejournal.com]

Ahhh! Thanks!

[ardent-muses.livejournal.com]

Elke explained it the way I understand it, only much better. So I really have nothing to add, except ::::waves:::: :)

[daughtershade.livejournal.com]

Also too with video files there are a lot of video and audio codecs and when downloading sometimes the information gets corrupted during the transfer. See you're getting (or sending) by splitting up the big thing into lots of little pieces. By zipping something, you're breaking up the big file into pieces beforehand so there's less chance of the transfer process doing it wrong. So I know a lot of people zip stuff to protect the file.

[kelliem.livejournal.com]

Thanks! (and a Blazing Saddles icon?? Cool!!)

[brynwulf]

I know that text based or heavy files seem to compress more than artwork and graphics. Also, I zip stuff to send to myself at home or work if it is an .exe file because my spam and virus blockers won't let me download executables, which is a good thing, but sometimes a pain.

Glad you got your answers.

[kelliem.livejournal.com]

Yeah, the .exe thing makes much sense.

[isilya]

You usually save about 4-6MB in zipping up a 350MB avi. Not much, and it takes both you and the user time to zip and unzip.

Where zip archives come into their own is in:

a) forcing the file to download rather than stream
b) concealing the nature of the file: you can convert "House Episode 2.12" as "cake.zip", and your host will never know what it is without downloading and unzipping. If you simply *rename* the file, the file data can still be ascertained from the file header.
c) you can password protect zip archives, so that should your host wish to download your files to look at what you've uploaded, they'll need a password to unzip.

The downsides of zip archives:

a) they take time for the uploader to zip
b) they take time downloader to unzip
c) the downloader needs more than twice the actual file size of free disk space
d) they leave mess behind
e) partial downloads will not be playable (I can play avis missing MB, I can't unzip a zip file missing any bytes at all)

The advantages of rar files are similar to those of zip files, however the convenience of being able to chop the files up into easily uploadable/downloadable chunks is added.

The disadvantages of rar files are the same as the those of zip files, but additionally you also have to explain the unraring process every time and unraring requires extra software.

[kelliem.livejournal.com]

I knew I could count on my flist to enlighten me! Thank you, this is all excellent information and I am now much less ignorant. :)

[zebra363.livejournal.com]

Muffins vs cupcakes: it's all in the nomenclature!