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Old Nov 7, 2002, 4:29 AM   #1
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Default Another Question on .JPG's

Awhile ago I asked a question about doing a "save-as" and percentages of saving the photos, like 25% or 75% and the losses resulting from this.
I got some very good helpful information and I did find out every time I did another "save-as" I lost detail and the photo looked worse and worse each time I saved it again.
To all I thank you very much, I learned a lot.
Now to my other question which I should have asked before.
All my photos on my computer are .JPG's, none are .BMP's because of HD space, with that in mind, Will I lose data in the photo by keep "opening" up the photo to "view" it?
I have a lot of vacation photos, family, etc. and a lot of times I use some of them for wallpaper on my desktop.
I know what was told to me about JPG's being a loss type of format, but what about looking at the photos, just viewing them, will I lose detail by keep opening them?
Yes I do have these JPG's backed up on a CD, but these photos were taken years ago, before I knew about saving right from the camera.
Thanks, sorry about the long winded post.
Bill
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Old Nov 7, 2002, 4:37 AM   #2
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You can view jpegs or any other format as many times as you wish without any effect on the image quality. You can also copy the file as many times as you wish and still no deterioration. It is only when you open the jpeg in an image editing program and then resave it that you will lose quality every time. This of course is not the case with lossless formats like TIFF or PNG which you can open and resave as much as you like.
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Old Nov 7, 2002, 4:41 AM   #3
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In my opinion, i feel that you won't be losing any details just by opening the .jpg photos for viewing them.
however, I would wait for other member's opinion as well.
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Old Nov 7, 2002, 7:19 AM   #4
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I download my images to an archive folder. I then copy the original images to another folder to edit them. I started doing this because I destroyed an original and was left with nothing. When the archive file reaches 600mb, I archive it to disk.
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Old Nov 7, 2002, 7:25 AM   #5
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You can view JPGs as often as you like. The file will not be altered. Furthermore, once you save it at the desired image quality the very first time, after that you should always save it at 100% image quality thereafter, and no further loss will result even if you do editing or whatever else you want.

You can take my word for it--I'm a software developer by trade. I can't believe other people responded to this question saying you will lose image quality every time you resave it, as if an image format could become as popular as JPG has if it continually degraded over edits! Come on, people, let's use some common sense here!

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Old Nov 7, 2002, 8:01 AM   #6
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WOW! I'm gonna just sit back and wait for the responses to the above post (server) Should be some good ones.
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Old Nov 7, 2002, 12:03 PM   #7
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How about some knowledge instead of common sense? Let me quote Chris Cox from Adobe Photoshop team: JPEG is a compression algorithm which is implemented differently in different programs; there's a "loseless" JPEG compression, but it's rarely used. In other words: most programs don't give you an option to "save it at 100% image quality" in JPEG format - you have to use some other format like TIFF.

Every time you save your image as JPEG it's compressed.
Depending on a program you use there's more or less control over the extent of compression in the range between big-losses/big-compression and little-losses/little-compression.
View, copy or move operations doesn't involve any changes in file content.
Sergey
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Old Nov 7, 2002, 12:16 PM   #8
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WOW is the word!- Of course my question this time was about viewing the JPG, but I will say this, after hearing the answers about losing quality each time I re-saved that JPG, I made this test: what I tried was, I took a JPG (1024X768) and opened it in my editor (MGI Photo Suite II) I kept re-saving it at 100% as Kids1.jpg, Kids2.jpg, Kids3.jpg, and so forth, I kept this up and each time the picture got worse when I zoomed in on it with the magnfying glass in the program, by the time I got to Kids4.jpg you could not even recognise their faces!, so I am no expert and a newbe at this, I hate to say it but I must disagree with server....
I learned just recently on this forum that you DO lose quality at each re-save a JPG, funny thing I have been editing my photos for years and re-saving them always as JPG's, and never knew that you could lose quality by doing a re-save.
Some of my photos are scanned photos I had done before I even had a digital camera, always brightning up the photos, rotating them, sharpining them, I do the same with my digital camera photos because my inside shots are always dark and need to be brightend up, no wonder my pictures don't look as good to me as my cousins photos and she has the same camera I have, but she does not edit them like I do, if they are dark she leaves it that way.
I hope I did not stirr up a hornets nest here-- :lol:
Bill
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Old Nov 7, 2002, 2:09 PM   #9
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Well I might as well join in on this!

I'm not a software engineer, but I do know something about testing MPEG CODECS. The most common method offered by manufacturers to test a coding algorithm is to iteratively code and decode. After some number of iterations, degradation will become obvious and deteriation follows more quickly afterwards, for each code-decode. There are many failings with this approach - the types of test pic used for a start. BUT we all like measures in single numbers don't we?

We mustn't fool ourselves, in my example above, I said CODEC, assumed to be compatible coder and decode algorithms, usually optimised by the manufacturer. JPEG isn't my forte, but usually each vendor is allowed 'patent' on their coding algorithm and a whilst a generic or vanilla decoder will work, the best solution for both coding and decoding, usually comes from the same manufacturer!
So two different software coders or decoders, whilst 'working' as the standard says they should, won't necessarily give the same optimised results in test where they are transposed.

Back to PC coding, I think the confusion is really over what 100%, or any %, means in quality terms. Remember that the older JPG compression routines concentrated on output file size, less on the meaning of quality in the actual process. I think this is still a hangover.

So, if you really want to be sure, you need to allways keep a cam original. Import into a photo editor and save as an uncompressed bitmap, output a JPEG once only at the quality level you need if it's a jpeg you really want. For any subsequent editing or changes to compressed (re-saved) JPEGS, go back to the intermediate bitmap version.

From this example you will see that, as with analogue editing, if you do all your edits in one session with one save, and limit the amount of processing, you will be OK, but do several saves over several sessions and you will be in trouble. The same applies to file type conversion and interpolation. DO IT ONCE ONLY AND AVOID CONCATENATION.

Here's the solution I'm waiting for from the software writers, and we have a few contributing to this Forum!

All photoeditors track changes in the undo list. If that list could be exported as an edit decision list (edl) along with your edited pic, then when you wanted to re-tweak an edited pic, you pulled in the cam original file, run the script and further process from there with only one compression iteration for the edited output.

Another technique, which has been used in MPEG is to record along with the first JPEG compression, a table of what the compressor did with the pic. in the cam. When the pic file is then decoded, the small coder data file is used to give precise instructions to the decoder, allowing more iterations without impairment. Unfortunately though, this technique ends up being proprietory to individual manufacturers.
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Old Nov 7, 2002, 4:34 PM   #10
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Well, this being a photography board and the original poster saying that he a "a program" that allowed him to pick different quality levels of the JPG format, I naturally assumed that program was Photoshop. Furthermore, I also assumed that any program that allowed one to control the quality with which a JPG saves would allow the lossless quality save at 100% quality.

Apparently, I was wrong about that. Still, though, this is not a problem with the JPG format--it's a problem of software. Any program that claims to let you control the range of quality with which to save a JPG and then only lets you choose from a deteriorated subset of the quality spectrum is not exactly making a good choice.

So, the answer is, if you have image editing software that does the bare minimum of what it should with respect to JPGs, then you shouldn't have this problem. Alternatively, you can convert every image into a file format that is lossless and keep those as originals. Hard disks are cheap now, so this is a good option.

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