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03/03/2011 11:43:15 PM · #1 |
I had this idea from a discussion that came up in this thread that  bvy started. The general wisdom is to not save an image as a JPEG while in-work, as everytime you open and save the same image as JPEG, there is some quality lost.
I decided to see how much effect this actually had if that JPEG were saved at the highest possible JPEG quality (lowest amount of compression). I chose an image that had some good details and high contrast edges, which is where compression artifacts will normally show first.
Here are 100% screenshots of the original and the test file after being opened and overwritten 20 times (You will have to click on "view full sized image" to see differences):
Original..................................After 20 saves
 
At this 1:1 view, I can see the some very faint softness compared to the original and an extremely light halo around the edge of the building against the sky. (Of course, if the compression level were significantly higher, then you would certainly see major problems long before this point.) My conclusion is that saving at a very low compression level as JPEG is acceptable for general usage, especially if you are only opening and resaving once or twice.
Here are the full 15Mp resolution images, for any one that wishes to pixel peep for themselves:
 
Message edited by author 2011-03-03 23:44:27.
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03/04/2011 12:09:55 AM · #2 |
| It seems that the loss of quality after the image has been overwritten 20 times is fairly negligible. Perhaps some of the clarity could be recovered with some sharpening edits? |
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03/04/2011 12:15:39 AM · #3 |
Originally posted by scooter97:
It seems that the loss of quality after the image has been overwritten 20 times is fairly negligible. Perhaps some of the clarity could be recovered with some sharpening edits? |
I would expect so. At a normal view of the whole scene, I could not discern any difference.
I may try this again another time, but using a modest amount of compression.
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03/04/2011 07:42:06 AM · #4 |
Interesting, Steve. Thanks for doing this. It had to have been tedious, but I know it would have eaten at you until you did the experiment (!).
A couple conjectures: The smaller the image, the more rapid the degradation will be. Seems obvious, but I'm suggesting that it's a nonlinear sort of relationship; that is, I wouldn't be surprised if an image half the size showed degradation four times faster.
Also, I wonder how an image with a lot more texture (a concrete wall) and virtually no texture (a mostly clear sky) would be affected. I agree that images with high contrast contours will show degradation quickly, but sometimes images with a lot of fine detail do a good job of hiding artifacts. Conversely, it might be more obvious with an image having subtle color gradients.
When I rotate JPG's (accidentally) with Windows' image viewer, I notice that the file size drops by some margin. So another experiment might be, how many times can you rotate an image before artifacts appear? It might be equivalent to what you’re already doing. I might try this myself later.
Useful stuff in any case. Thanks again.
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03/04/2011 08:00:37 AM · #5 |
The degradation is definitely minor for this shot and this compression level. You might try the same experiment with a shot containing fine detail like branches or leaves against a blue sky. The results may be somewhat different. At high quality levels, I still don't expect that you will see drastic artifacts.
The real concern is that many folks edit their only copies of shots, and thus every time they re-save they are degrading their masters. |
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03/04/2011 08:44:18 AM · #6 |
| I think the results would be significantly different if they were saved at 65-75%. It would be interesting to see. |
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03/04/2011 08:45:21 AM · #7 |
I experimented with this some years ago.
Posted:
After 100 rotations:
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