A question about jpeg stability during duplication.

[Karin Sue]

I went to our local PC Users Group last night and we had a presentation on graphics. The presenter is an artist and our computer guru. He said that he has found jpegs to degrade simply from copying the files. Not saving and resaving.

Apparently over the course of upgrading to different computers--copying his files sequentially to 5 different harddrives, his jpeg images had deteriorated. He now saves all of his originals as bmps.

I have never heard of this. Has anyone else experienced or heard anything like this?

I have always assumed that a digital copy is identical to the original.

[Byron Gale]

Um.... hogwash. (and I'd re-think that "guru" ranking, too!)


IMO. :p

[dj_paige]

I agree with Byron. Codswallop!

And yes, a copy is identical to the original.

[Karin Sue]

He really is very good at most things computer. I have disagreed with him before on a few specifics but he generally knows what he is talking about and has a much greater breadth and depth of knowledge than I or the rest of our group has. He has actually assembled computers from parts. He is the expert in our amateur group, not a professional. He is a very good speaker and willing to volunteer the time to put together presentations.

I can see how some files may be corrupted during a copy, but I don't see why a jpeg would be any more vulnerable than a bmp. I know sometimes I need to reload a graphic to my website that had been okay in the past but has somehow become corrupted.

I think his experience is more an argument for having multiple backup copies on a variety of media than that jpegs are unsafe.

Any more comments?

[Edmund]

Karen Sue, I think the key is like you pointed out saving and re-saving.
I think this is what happens but if wrong the guru's will come along and correct me. When saving a JPEG it can be be possible to deteriate the file by resaving the saved file numerous times. In other words the original file is saved by compression (which discards data). Now go back and redo some more stuff and save the saved image again it is now compressed again and data is lost . Do this enough and eventually you will be able to see the difference. The first few times will not be noticable to the eye. Like you said, I don't believe copying a image will degrade anything as you are making a mirror image pixel for pixel. IMHO
Eddie:)

[genevh]

Copying a file is supposed to be a bit for bit exact duplication of the original file, with absolutely no degradation or loss. If there is, its a problem with the machine or S/W that is actually doing the copying, not generally the file itself. Corruption is another issue altogether and can happen at almost any time for a variety of reasons. We deal with it all the time in my line of work. Unfortunately for us though, when it happens, we have a service provider with a system down because of it.

And no disrespect to your friend, but being able to slap together a PC from a pile of parts is not that hard with some basic understanding of the box itself and how it goes together. Plug and play has been a great benefit in that regard. Its how I learned most of what I know about PCs. :) The real trick is once the parts are all together and functioning properly, is keeping it running once you start loading all that fancy software and stuff and working with it for any length of time. I get by and offer advice from time to time, but in no way would really consider myself an expert. People start to expect too much then! :)

[Wendy]

Oh my ...

Sometimes I just have to laugh at some of the things that are put forward by "experts"

:D:D:D

Wendy

[jonknee]

Apparently over the course of upgrading to different computers--copying his files sequentially to 5 different harddrives, his jpeg images had deteriorated. He now saves all of his originals as bmps.

I have never heard of this. Has anyone else experienced or heard anything like this?

I have always assumed that a digital copy is identical to the original.

He is absolutely wrong. Copied files are bit-for-bit duplicates and that much can be proven with hashes (MD5, SHA1, you name it). There is no way to tell if a file is an original or a copy, they are identical. If copies changed bits, your computer would quickly degrade into random data.

Also if you think about it for a bit, almost every image you have is a copy. Either from the web or the memory card of your camera. Saving a JPEG of a JPEG will result in marginal quality loss, but that's logical. JPEG throws out data, so if you're making a JPEG of a file that is already compressed it's only going to come out more compressed. But you can safely copy any file (JPEG/BMP/RAW/MP3/etc) without issue.

So, you have nothing to worry about. If you'd like to give him directions to prove this via hashing, let me know.

[GaryK]

Karin

The only way I could see this being an issue would be if he used a program to batch copy them....
AND....
this program was actually resaving them without him knowing it. Possibly he let the program rename the files and in doing so it actually resaved them.
All a bit of a stretch with a lot of maybes and ifs.:D

[JonE]

And saving all originals as bmps????? OK give the person the benefit of the doubt and maybe he said psds!

[Karin Sue]

Thanks everyone for your assurances.

I guess one of his transfer procedures messed up his jpegs without his being aware of it. Doesn't sound like he referred to them often. If he was following his own advice he should have had backups somewhere. He encourages frequent backups.

If they have gone through that many upgrades his original backups may be on those old giant floppies. I forget what they are called. I think I still have a few floating around here and there even though there is no computer around here that can read them anymore.

[JonE]

The old floppies were called floppies and they were. The new floppies were called that too but they weren't. You couldn't even get one regular size image on one of the old floppies - 512K as I recall. I still have one machine in the house that can read them but it goes to electronic recycling Monday morning. Kind of sad but they are useless.