Rewrite jpg's to many times? Degrade Pictures?

buckethead · 06-07-2009, 07:50 PM

I have been searching, but can not find an answer that seems to be authoritative on either side here. Does resaving the same jpg file a few times degrade the image? I am not doing any changes to the picture itself, just adding tags, etc to the file.

I keep all pictures on two different laptops and actually will do tagging on each laptop, syncing them with Syncback through a WHS box. I need to make sure that each file is the same on all 3 places when done with it. Right now there does not seem to be a program that will allow me to catalog this cleanly so I am going to resort to this method - if it does not degrade the original photos.

Or maybe I do nothing and wait for someone to write a program that will allow the syncing of a catalog that has different file paths on multiple computers...

Quension · 06-07-2009, 08:12 PM

I think the answer to your question is going to depend on what you're using to tag the files.

MrApollinax · 06-07-2009, 08:17 PM

Originally Posted by buckethead

Or maybe I do nothing and wait for someone to write a program that will allow the syncing of a catalog that has different file paths on multiple computers...

Windows Live Mesh might be what you are looking for:

https://www.mesh.com/welcome/default.aspx

buckethead · 06-07-2009, 08:40 PM

Originally Posted by Quension

I think the answer to your question is going to depend on what you're using to tag the files.

That I am not sure on yet, been trying out iTag and XnView.

Originally Posted by MrApollinax

Windows Live Mesh might be what you are looking for:

https://www.mesh.com/welcome/default.aspx

Would not solve the different path - basically my WHS box is live mesh on my local LAN.

**Marc Sabatella** · 06-08-2009, 01:24 PM

Originally Posted by buckethead

I have been searching, but can not find an answer that seems to be authoritative on either side here. Does resaving the same jpg file a few times degrade the image? I am not doing any changes to the picture itself, just adding tags, etc to the file.

It would have to be an exceedingly poorly written program to decode and then recompress an image just to write some tags. If the program is anything that ever gets mentioned here, I'm sure it's fine.

*Wheatfield* · 06-08-2009, 01:26 PM

The problems with image degradation and jpegs is when the file is opened and resaved multiple times, I think slight changes have to be made to the file before saving for anything noticable to happen.
Copying and pasting files between drives, such as what is done during back-ups should have no effect on the files with regard to image degradation.

bymy141 · 06-09-2009, 02:40 AM

Originally Posted by Marc Sabatella

It would have to be an exceedingly poorly written program to decode and then recompress an image just to write some tags. If the program is anything that ever gets mentioned here, I'm sure it's fine.

Well, my experience with the code written I've seen over the last few decades, you can not be sure. It is not all quality that's out there

I'd say: it depends on the program you are using. Try: "Save as...", close and open the saved file, then save it again, 20 times.
Then pixel peep the first input file versus the last output file. Examen the result.

- Bert

**Marc Sabatella** · 06-09-2009, 09:37 AM

Originally Posted by bymy141

Well, my experience with the code written I've seen over the last few decades, you can not be sure. It is not all quality that's out there

True. But given that it's *more* work to go to all that trouble versus simply updating the EXIF, it's not just a question of quality - it's a matter of someone having to be deliberately stupid or mean to write a program that went out of its way to degrade an image when the simpler solution would *not*.

*graphicgr8s* · 06-09-2009, 01:53 PM

Anytime you save a jpeg you lose. JPGS are considered a "lossy" file format because of the way it compresses an image. How much it looses depends on the photo.

Professionally, in my work environment I never save a jpeg back to itself. It will always get saved as a tif with no compression if I feel I may need to work on it later. If I give it to a client I will keep the tif and save as a new jpeg.

bymy141 · 06-09-2009, 02:59 PM

Originally Posted by graphicgr8s

Anytime you save a jpeg you lose. JPGS are considered a "lossy" file format because of the way it compresses an image. How much it looses depends on the photo.

Professionally, in my work environment I never save a jpeg back to itself. It will always get saved as a tif with no compression if I feel I may need to work on it later. If I give it to a client I will keep the tif and save as a new jpeg.

You are right. It is so. However, the question is, if you'd only change the exif data of a picture and then save it, would the program regenerate the JPG data (and degenerate the picture some more) or would it not touch the picture data?

Marc Sabatella is absolutely right that any proper programmer would avoid the regen. And I believe that Marc very well understands technology in a wide range of aspects, reading his posts....

However, my argument is that many object oriented code these days is written as such that the guys using these objects go for convenience instead of efficiency.
No offence Marc, but where I live, they do not teach students proper programming anymore. Most of them have no clue what their compiler actually is doing....
The best support for my argument comes from our todays Windows, Office etc etc.
10 years ago my computers were 20 times as slow, but my todays superduper hardware boots and reacts slower and slower than the stuff we worked with then.
It's all about structural programming and supportability, not about code or program efficiency.

Try it and be sure.

- Bert

wll · 06-09-2009, 03:41 PM

You must rename it, or it will, over repeated openings and saves loose image quality.

EVERYTIME you save a JPEG rename it ... just a number or letter after the original name will do.

wll

emalvick · 06-09-2009, 04:40 PM

The question of what happens with metadata probably depends on the software.

Programs that focus on metadata / cataloging are less likely to mess with the actual image data than those that are predominantly pixel editors.

I know that a program like Exiftools works because it doesn't touch the image data. That allows it to access Exif, IPTC, etc in as many formats as possible since those headers are in roughly the same format in all file formats (with the caveat that maker notes will vary).

Of course, if you are legitimately concerned with file degredation, why not test a file out for various situations. For instance, repeatedly change the metadata on a copy of a file. Compare the repeatedly tested file with the original. You could even use Photoshop to subtract the modified image from the original in two layers so that the differences become obvious (if there are any).

You could also test an image in photoshop by repeatedly saving it (i.e. purposely recompressing the jpg file). See how that differs from the original.

The one thing I'd wonder is what Lightroom might do. While it specializes in cataloging, it is also the type of program that might take the easy way out and just completely resave an image thus causing degradation. I haven't worked with it enough or know enough about it to know what it might do.

My own choice of cataloging software, IMatch, doesn't access the pixel data when it edits metadata according to the author, and I haven't found reason to doubt it... I have repeatedly changed some metadata on early jpgs as I learned to use that particular program, and I've never noticed any differences.

It will be interesting if anyone does do a test of some of the more mainstream programs and what happens when you either change metadata or even just continually save a jpg that is changing pixels.... i.e. how fast does image degradation occur, if it occurs given that nothing else really changes. I imagine that once you actually start editing pixels be it curves, sharpening, etc that degradation will go a lot faster in those cases.

wlachan · 06-09-2009, 04:46 PM

Originally Posted by buckethead

I have been searching, but can not find an answer that seems to be authoritative on either side here. Does resaving the same jpg file a few times degrade the image? I am not doing any changes to the picture itself, just adding tags, etc to the file.

I tried this out years ago in Photoshop and yes, it would. It's not rocket science, just open any jpeg and resave it 10x and see for yourself. But then the jpeg quality setting does matter too.

Quension · 06-09-2009, 04:49 PM

Originally Posted by buckethead

Originally Posted by Quension

I think the answer to your question is going to depend on what you're using to tag the files.

That I am not sure on yet, been trying out iTag and XnView.

iTag only alters the metadata; it will never degrade pictures.

XnView depends on exactly what steps you take. For the tags, you open the image and then Edit->Metadata->Edit IPTC data, Keywords tab. If you alter the keywords and then use the Write button on that dialog, it only alters the metadata, and there will be no degradation.

If you merely Apply and then close that dialog, followed by any form of File->Save/Export, it will rewrite the image data as well, and degradation will occur.

Originally Posted by Marc Sabatella

But given that it's *more* work to go to all that trouble versus simply updating the EXIF, it's not just a question of quality - it's a matter of someone having to be deliberately stupid or mean to write a program that went out of its way to degrade an image when the simpler solution would *not*.

It's not more work, just different work. XnView is actually a good example; if the primary purpose of the app is viewing and conversion, then the image data and relevant metadata will be converted to in-memory formats on load and completely rewritten from scratch on save, according to the export/conversion format chosen.

Selectively modifying file metadata in-place for a specific format is a different task entirely, and is iTag's primary purpose. XnView accomplishes this by confining the task to the IPTC dialog itself. (IrfanView is similar.)

**Marc Sabatella** · 06-09-2009, 09:01 PM

OK, granted, there *would* be reasons aside from meanness or stupidity where an application might alter the mage data even though your intent was otherwise. I still say it's going to be a rare corner case - "most" programs won't do this unless you use them "incorrectly" (eg, opening an image for editing and then doing a save, as opposed to not even entering the main editor but making the changes directly to the metadata screens). But yes, by all means, do verify you are in fact doing it correctly, and that your program does in fact provide a way to do this. Again, "most" programs will provid a straightforward wa to accomplish this.

BTW, I've got *some* amount of knowledge on a variety of topcs, including photography, but my most extensive training and experience (including BS & MS degrees) is in software engineering. FWIW. So while I have no knowledge of the specific internals of any particular image processing application, I do have a pretty good feel for how things are programmed.