PC help – zipping, unzipping & "quality"

I'm a bit thick, see….

does zipping leave files completely unaltered when unzipped again (photos, music etc) or is there a residual effect from compression?

it should.

unless they get corrupted.

Yep. Leaves them unaltered.
No residual effect – zipping is "lossless" compression.

does zipping leave files completely unaltered when unzipped again (photos, music etc) or is there a residual effect from compression?

Yes.

Examples of lossy compression include MP3 for music and JPEG for pictures. Both of these use techniques to remove information which we generally cannot detect or that doesn't affect the perceived picture or sound quality.

Yes.

What a great answer to a question with two possible options!

What a great answer to a question with two possible options!

I'm grateful for whatever I get !

(maybe could've put the question better too)

tell me more about corruption – likely ? big risk of failure for batch process with LOTS of files ?

Reasonably unlikely but unzipping does not affect the zipped copy, if unzipping produces corrupt files you can try unzipping again. The only time I've had corrupt zip files is when they've been corrupted during transmission.

Also worth noting that pictures (jpgs) and music (mp3s) are already pretty well compressed so zipping them often doesn't actually gain you very much.

OK lets get this straight, GrahamS was correct, zipping is lossless compression – you'd be a bit fecked if your application or important data lost info during compression. Certain file types that store things compressed (the aforementioned jpg, mp3 etc) sample the image and "merge" nearby parts that are the same, effectively just simplifying the data and re-using parts.

If you use a JPG format to store a picture you'll lose quality over the original. If you use ZIP to store JPGs the files will be identical when unzipped. The problem being that, also as stated above, zipping into one file leaves the whole batch vulnerable if any one section gets damaged during transmission – damage some parts of your zip and the whole lot are gone. I've had quite a bit of trouble with some of the free zipping tools for windows corrupting archives so try never to use zips anymore if possible. As graham says, you'll get 1-3% compression on jpgs and mp3s as the format tends to use the same sorts of techniques on top of the original lossy compression technique to further shrink files.

Yeh, noticed the low "gain" with MP3 etc (& assumed that was why 8) )

I'm not zipping it all into 1 big zip file – it's lots of little ones (syncback backup thingy – a free "zipping tool" I suppose) so, presumably, corruption would be limited to single/few files rather than all ?

Trouble is, I've already run this thing & it took nearly 24hrs all in. Have opened & checked some of the pics but obviously not all. Loath to do it all again, but ….

Lots of little zip files ? That sounds a bit errmmm…pointless? The point of zips is to keep it all in one file. Yes, less chance that way though, youre right.

I just dont understand why you're zipping them TBH, out of curiosity rather than judgement.

Zipping because I thought it might use less space on the backup disc (1tb but that backs up 3 pcs inc pics & vids)

It was just a tickbox in the options (could've gone for 1 big zip but that just didn't seem good to me at the time)

Probably did help then if there's more than just pics and vids. Worry not, your files are safe and somewhat shrunken.