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Old Jul 16, 2003, 2:21 AM   #1
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Default saving changed images - degrades quality ?

Hi All

Am I right in thinking when one makes changes to an image in an editor and then saves the new version that each change and save operation degrades the image quality ?

If so anyone any approx ballpark figure for level of degridation ?

I mean are we talking 3-7 % or are we talking 20 %.

Thanks for any feedback

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Old Jul 16, 2003, 3:05 AM   #2
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You've just asked an incredibly broad question, the answer to which depends on a lot of variables.

It firstly depends on what file format you're working in - some are lossless, some are not and will degrade. It depends on what software you're using, but applications like PhotoShop and Paint Shop Pro have their own proprietary formats that can retain a lot of your working information like layers and selections and are good if you want to return to work and perhaps undo some changes etc., these are good for work in progress and don't lose anything when you re-save. Formats like TIFF are also loss-less, but can't retain some data like layers and formats like JPEG/JPG are lossless and how much the image degrades each save depends on the compression ratio you've selected.

What I do and would advocate anyone does, is to keep the file out of the camera totally in-tact, untouched and as it was taken, all the time and only ever work on a copy - either copy the file itself before opening, or once open, 'save as' and choose an appropriate format. This way you always have your original to return to if you louse up any changes and any work you've done that you may want to return to can be kept in a loss-less format like tiff for further work.

Most of my work is for web use, so when I work on a file, if I want to return to it or the work is extensive, I save a full-sized version in PSP's own format with any layers etc. preserved and then resample and sharpen etc. for the version to be published and export/save as a JPEG at the right size and compression for web use - meaning I end up potentially with up to three copies of the photo - the original, a working full-size copy and the published version. If I then want to say print an enlargement commercially at 8 x 10, I'd open the full-size working copy, crop it to the right proportions and save as a minimal compression JPEG to send to the printer. etc. etc. You end up with several copies (and perhaps other intermediates with diferent work done on them) but each one is just right for the job in question and with CDs and hard drives so cheap, it isn't worth your time re-working images for the few pennies it costs to store the addtional files.

If you open your photo and it was originally saved by the camera as a JPEG, you work on it and keep saving it (at whatever is the default compression for your software, it will degrade visibly with each save and is to be highly recommended against - you can never get that data back, it simply can't be restored once lost.
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Old Jul 16, 2003, 3:50 AM   #3
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Default Re: saving changed images - degrades quality ?

Originally Posted by stefank
Am I right in thinking when one makes changes to an image in an editor and then saves the new version that each change and save operation degrades the image quality ?
I agree with everything Boo says, but would just add one non-obvious point when working with jpeg files.

When you are working in Paint Shop Pro, for example, when you click 'Save' it will overwrite the original image, so of course should use 'Save as..." and change the name as a first step.

Thereafter, the image you are working on is retained in full, without compression, in PSP's working memory, and each time you press 'Save' you'll overwrite the file with a slightly degraded jpg image (if that's what you save), but you'll only degrade it once (the final time) in any one successive set of saves. You can use the 'undo' facility to go back a few steps.

However, if you do the safe thing, and close a version you like and reload it, and then resave under another name, and then reload to continue work now or later, you WILL rejpeg your jpegs several times with a risk of degradation.

Nevertheless, if you save with very high jpeg quality, the degradation will be little, but the file size will still be small relative to lossless formats, and you retain the advantages of rapid reads & writes to&from disk of many versions without clogging your hard disk and backup CDs.

Have fun!
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Old Jul 16, 2003, 4:49 AM   #4
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When you first copy your camera originals to your hard drive, highlight all the new files, select properties and check the write protect option.

At least you'll get a warning if you tried to save back to the camera original! VOX
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Old Oct 2, 2003, 6:25 AM   #5
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thanks all

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Old Oct 2, 2003, 11:39 AM   #6
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I find the easiest thing to do is open the file, then when saving it just delete the file opened and write it again, this way you don't re-compress...providing its jpg. Lossless format's it does not really matter. Also take into consideration that you are looking at the original quality when saving, you need a package that can show you the optimised image preview when re-saving so that you know how much compression you are adding.
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