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Author Topic: resaving jpegs  (Read 4758 times)

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ShadySue

« on: January 14, 2012, 12:30 »
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I've been trying to create a photo to show my beginners' class what happens when you resave jpegs.

I started with a full size crop of a RAW file which I saved out as Eye 1, then repeatedly 'Saved as' Eye2, Eye 3 etc down to 10 then put Eye 1 and Eye 10 side by side.

I don't think anyone other than an inspector would see any difference between them; certainly not a casual viewer. Does just repeately Saving As not do it? They have slightly different file sizes, Eye1 is 112Kb and Eye 10 is 116Kb.

I remember in the past seeing photos of jpegs saved only 4 or 5 times and the difference was obvious to any lay viewer.



« Reply #1 on: January 14, 2012, 12:33 »
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You need to close the file and re-open it otherwise you're saving repeatedly from the in-memory data which is fine. When you close the file the in-memory image is gone and it reads the file on disk. You might want to add a 1, 2, 3 in a corner each time as you save the file out to keep track of where you are.

ShadySue

« Reply #2 on: January 14, 2012, 12:37 »
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You need to close the file and re-open it otherwise you're saving repeatedly from the in-memory data which is fine. When you close the file the in-memory image is gone and it reads the file on disk. You might want to add a 1, 2, 3 in a corner each time as you save the file out to keep track of where you are.
Thanks Jo Anne!

ShadySue

« Reply #3 on: January 14, 2012, 12:52 »
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Hmmmm. Do I have to close Photoshop after each save too?
I closed,opened and resaved ten times, and I still don't see any significant difference.

« Reply #4 on: January 14, 2012, 14:07 »
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I'd have thought you'd need to make a change to the image data for anything to change.  As I understand it, JPEG represents the data in 8x8 pixel chunks, so only those chunks with changed pixels will be recalculated and can potentially lose quality.  If you don't change any of the image data and don't change compression, I wouldn't expect any loss.

« Reply #5 on: January 14, 2012, 14:58 »
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If you save as "12" in photoshop it actually is pretty good. If you did the same experiment saving as "6", I bet you'd see a lot more - also perhaps something needs to change to really see a difference (like if you resized up and down - which would cause its own problems), or maybe have something over white and shift it a few pixels before each save.

Merely opening and saving something as a jpeg a few times is not going to ruin it. Still, if you are planning lots of re-edits, save it as a .png or tiff.

« Reply #6 on: January 14, 2012, 15:03 »
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I've heard that the 'losses each time you Save' advice is basically a myth __ although one that is always being trotted out.

As others have suggested, try it yourself to prove or disprove the theory. Another clue might be decreasing file sizes with each Save (if stuff's being lost then there's not so much to save) but again that doesn't seem to happen.

ShadySue

« Reply #7 on: January 14, 2012, 15:07 »
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I don't think it's a real issue, then, for my purposes.
I'm lost at thinking of several significant, believable edits I would do to that pic (that wouldn't elicit, 'why on earth would you want to do that anyway?'); and my class will not be shooting isolations on white.

It's just for 'normal users' - nothing to do with stock.

Thanks, all.

« Reply #8 on: January 14, 2012, 15:21 »
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I think if the jpg was created at the highest quality setting, the compression is just about lossless and there is no significant degradation on repeated saves.

« Reply #9 on: January 14, 2012, 16:02 »
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Hmmmm. Do I have to close Photoshop after each save too?
I closed,opened and resaved ten times, and I still don't see any significant difference.

You can use "Save for web" and show them four different images immediately.

ShadySue

« Reply #10 on: January 14, 2012, 16:08 »
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Hmmmm. Do I have to close Photoshop after each save too?
I closed,opened and resaved ten times, and I still don't see any significant difference.

You can use "Save for web" and show them four different images immediately.

Oh yeah, I'd forgotten about that. Tx.

CD123

« Reply #11 on: January 14, 2012, 17:17 »
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Unless I misunderstand what you are saying, you are opening file 1 every time and then save it as file 2, open 1 again save as 3, etc.?

If this is the case I doubt if any difference will show, as the "master file" is the same every time. You should open file 1, save as 2, close 1 and open 2 and save as file 3, close 2 and open file 3, etc.. If there is any deterioration, only then will I think it be visible.

digitalexpressionimages

« Reply #12 on: January 14, 2012, 17:30 »
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Unless I misunderstand what you are saying, you are opening file 1 every time and then save it as file 2, open 1 again save as 3, etc.?

If this is the case I doubt if any difference will show, as the "master file" is the same every time. You should open file 1, save as 2, close 1 and open 2 and save as file 3, close 2 and open file 3, etc.. If there is any deterioration, only then will I think it be visible.

That's correct. It's not a myth that jpeg re-compresses with each save regardless of whether you make changes to the image. It's called "lossy" compression for a reason. If you use the highest quality setting it'll take a lot of re-saves to see visible loss in quality as it doesn't toss much out at the highest setting. I have personally seen images on art department file servers that were saved and re-saved repeatedly and you could no longer make out facial features on peoples faces.

Follow CD123's suggestion and use a lower quality setting if you want to see dramatic results.

ShadySue

« Reply #13 on: January 14, 2012, 17:42 »
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Unless I misunderstand what you are saying, you are opening file 1 every time and then save it as file 2, open 1 again save as 3, etc.?
That's what I did the first time

Quote
If this is the case I doubt if any difference will show, as the "master file" is the same every time. You should open file 1, save as 2, close 1 and open 2 and save as file 3, close 2 and open file 3, etc.. If there is any deterioration, only then will I think it be visible.
That's what I did the second time.

I've decided that it's not that much of an issue to bring up in the first total beginners' course.

jbarber873

« Reply #14 on: January 14, 2012, 17:53 »
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  Probably the best way to approach it is to teach them the advantages of the jpeg format, and caution them to save the  file at the highest quality setting, where the losses are minimal. I was taught that jpeg at the highest quality, for all practical purposes,and real world uses,  is identical to the original.

w7lwi

  • Those that don't stand up to evil enable evil.
« Reply #15 on: January 14, 2012, 18:00 »
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I'll throw another wrinkle into the mix.  Does the resolution of the original image enter into the equation?  This whole issue of re-saving JPEG images started way back with the earliest digital cameras.  Not uncommon for them to be 2, 3 or 4 mp.  Now we've got much higher resolution cameras with vastly more pixels.  If it's true that JPEG processes in blocks of 8 pixels, that would have a much greater effect on low resolution images as opposed to today's high resolution ones.

Thoughts?

« Reply #16 on: January 14, 2012, 18:14 »
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I think if the jpg was created at the highest quality setting, the compression is just about lossless and there is no significant degradation on repeated saves.

Yep.  It would take a lot of saves.  Try saving at 8 a few times instead.

ShadySue

« Reply #17 on: January 14, 2012, 18:18 »
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I'll throw another wrinkle into the mix.  Does the resolution of the original image enter into the equation?  This whole issue of re-saving JPEG images started way back with the earliest digital cameras.  Not uncommon for them to be 2, 3 or 4 mp.  Now we've got much higher resolution cameras with vastly more pixels.  If it's true that JPEG processes in blocks of 8 pixels, that would have a much greater effect on low resolution images as opposed to today's high resolution ones.

Thoughts?

The example above is a full size crop from a portrait, and is a small image. But I was saving it at 12.

I guess I'll wait to see the composition of the class. The last class I did, on Windows Live Photo Gallery, had some people who were neither camera nor computer literate, and I spent a lot of time explaining what click, double-click etc mean.  This class Ill definitely have one lady like that, but another is a former colleague who dragged me kicking and screaming onto a BBC Master in 1996! I'm just preparing one class until I find out what they're hoping to get out of the course.

« Reply #18 on: January 14, 2012, 18:43 »
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Take a look at this example; I think it illustrates what can go wrong if you resave JPEGs and use level 8 (worse if lower).

If you look at each of the copies it represents what would happen if you cloned something from the original, saved the file as a level 8 JPEG, then opened it up later, cloned again, and so on. The data you're cloning has been altered by the save and re-open, and then you subjected it to the same artifact-creating compression process.

If you look at 300% or 400% it is easiest to see, and look at the bloom along the bottom blue edge as well as around the flower stalks.

When I did the experiments saving at level 10 things were much less pronounced, so I did these at level 8, the lowest of the high-quality settings. I think the big takeway would be to save at the highest quality all the time while editing and you'll probably be OK.

ShadySue

« Reply #19 on: January 14, 2012, 19:12 »
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Thanks all; I'll know on Monday for sure, but I doubt if some artefacts visible at 300%+ will be of that much concern to my 'clientele', though of course this issue is important for stock.

« Reply #20 on: January 15, 2012, 08:05 »
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Let me add that you would see more problems if your image contained something like a sky or more tricky details in shadow areas. The well lit face won't show much artefacting. But you're right, it's not such a priority subject for a class.

red

« Reply #21 on: January 15, 2012, 11:03 »
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Just getting a "beginner class" to understand the difference between raw, tif,  jpeg or eps will be tricky (all options they will run across if they will be using any photo editing software). They have to get to this point before it will make a difference. Don't even start with photo editing and when you do, stick to the basics.

Are these beginners taking the class to simply learn how to take a "good" photo? I guess you have to make your teaching decisions based on the students who take the class. Is it important for them to learn camera basics (which could be anything from how to turn their cameras on and when to use or not use the zoom, what their particular camera settings mean, info about aperture, dof, iso, rule of thirds and when to break that rule, etc)? Do they need to learn the dif between photographing objects and people?

One trick I've used is to get them all to take the same photo of the same object or outdoor scene  from 4 dif angles or in 4 dif types of light and then compare them to point out photo principles. But, these days no one likes to be criticized. Teaching is tricky, good luck! You will learn as much about human nature as you teach about photography.

« Reply #22 on: January 18, 2012, 09:01 »
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It also depends very much on the software used to re-save the image... Photoshop 'correctly' chunks it's JPEGs so won't recompress unmodified chunks, but something like Microsoft Photo or any other terribly lazy piece of software will do a much worse job. It used to be a huge issue since the algorithms were quite complex but as hardware has gotten faster (over the last 10 years or so) it's become less of an issue.
It's still worth considering though as some agencies still run old image-processing software so it's always worth sending them highest possible quality you can so it does artifact when they resize it.


 

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