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DPChallenge Forums >> Tips, Tricks, and Q&A >> Saving as a jpg again and again and again
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 11/28/2005 01:03:44 PM · #1
Ok, when you save as a jpeg, I know that you lose some data due to the compression. Here is my question:

If I have PICTURE-A open in PS and save it, then do an edit and save it again, does the quality decrease each time? I know its easy to speculate, but does anyone know for sure?

My thinking is that of course if you save it, re-open it later and save again it will degrade each time, BUT if you save multiple versions having only opened it once, wont all the saves have the same quality (which is original + one save)?

Does this make sense? I ask because I'll take a shot, edit it to an 11x14 and save it, then crop it to an 8x10 and save it, then a 5x7 etc. etc. and then close it when I'm done.
 11/28/2005 01:10:55 PM · #2
Think of it as successively making copies on a Xerox -- you copy the copy, then copy that, then copy the third-generation, etc.

Yes, you will get degradation of the image. Whether it's visible or bothersome will depend on the image content and the viewer.

If you have Photoshop, why are you not saving your "master" image in .PSD format, maintaining layers and non-destructive editing with lossless compression, and then using the SaveAs command to export JPEGs as needed?
 11/28/2005 01:13:24 PM · #3
For any image I print, I'll end up with the following files (minimum):

@ Original (JPEG)
@ Edited (PSD)
@ Edited composite (TIFF)
@ Print image (JPEG)
 11/28/2005 01:13:59 PM · #4
What the general said. You should NEVER be doing ANY editing on jpg images in photoshop; keep a master psd file and clone the resized images off that as needed.

R.
 11/28/2005 01:14:38 PM · #5
So when you save a jpeg in PS, the actual picture on the screen degrades instantly - not just the saved file?
 11/28/2005 01:14:42 PM · #6
Yes!

Make all your edits except for re-sizing and cropping for print size and save the file as a .tif or .psd (uncompressed formats, does not change the image)

Then crop and resize the tif/psd file and save as a .jpg to print.

This way you only save a .jpg once and you do not have to re-edit the photo for each print size.
 11/28/2005 01:14:48 PM · #7
Originally posted by GeneralE:
For any image I print, I'll end up with the following files (minimum):

@ Original (JPEG)
@ Edited (PSD)
@ Edited composite (TIFF)
@ Print image (JPEG)

What's the edited composite one for, or from?
 11/28/2005 01:20:21 PM · #8
Originally posted by eslaydog:
If I have PICTURE-A open in PS and save it, then do an edit and save it again, does the quality decrease each time?


I do the same thing all the time, and I think it works exactly as you speculated. If you perform several steps and save as JPEG, you can back out a step or two before the save in the History palette and it will still show the JPEG file name. I think it's clear from this that Photoshop is not reading from the already-saved JPEG file while you have the image open. This is NOT the same as repeatedly opening and saving a JPEG.

Message edited by author 2005-11-28 13:21:20.
 11/28/2005 01:20:36 PM · #9
Originally posted by GeneralE:
For any image I print, I'll end up with the following files (minimum):

@ Original (JPEG)
@ Edited (PSD)
@ Edited composite (TIFF)
@ Print image (JPEG)


Can you give some details on the benefits of saving the Edited TIFF? Is it just format protection away from photoshop or is there something else to it?

Right now I have two paths for digital :-

RAW -> Various JPG's for printing (different crop/size).
RAW -> TIFF* -> PSD -> Various JPG's for printing (different crop/size).

* - I need this cause I have an older PS version and cannot get directly at RAW from PS. Some spare cash would avoid this step :)

Edit: Sorry for the dupe question - typing at the same time.

Message edited by author 2005-11-28 13:23:36.
 11/28/2005 01:21:15 PM · #10
if you save it with a different name each time, the degradation does not occur as badly as each instance was saved only once.
i.e. - Open Original, Save as Copy1, go back to original, save as Copy 2, etc. It is key to go back to the original each time.

If you save it with the same name over and over degradation will occur.
 11/28/2005 01:21:30 PM · #11
That composite is the PSD file "flattened" into a single layer, so that I can run the UnSharp Mask (or other) filter on all the data at once.

Otherwise the filter would only affect the active layer in the Photoshop file. This would only matter if there was a layer containing pixel data, as in one where I've done retouching or cloning.

After I run any filters I use the "SaveAs" command to save the JPEG and leave the TIFF in its earlier, unsharpened state. That way, if the sharpening is bad or I have to resize the image, I can start over there.

In the earlier post I should have said to use the "SaveAsCopy" command to make a JPEG from the Photoshop file.
 11/28/2005 01:22:15 PM · #12
i'm pretty sure both photoshop and paint shop pro cache the current image so if you do multiple saves (without closing and re-opening the original) you shouldn't get any higher compressions.
Once you save then close it (flushing the cache), then re-open and start your editing again then resave it will incur higher compression.

someone please correct me if im wrong.

--edit--
well i see i was beat to the punch.

Message edited by author 2005-11-28 13:23:06.
 11/28/2005 01:25:30 PM · #13
Originally posted by GeneralE:
That composite is the PSD file "flattened" into a single layer, so that I can run the UnSharp Mask (or other) filter on all the data at once.


You can accomplish the same thing with an Unsharp Mask adjustment layer on the top level of your image. It's non-destructive, so you can go back and make modifications later, and eliminates the need for an extra file.

Message edited by author 2005-11-28 13:25:40.
 11/28/2005 01:25:31 PM · #14
Generally, executing a "Save" command writes the current data to disk.
 11/28/2005 01:27:28 PM · #15
Originally posted by scalvert:
Originally posted by GeneralE:
That composite is the PSD file "flattened" into a single layer, so that I can run the UnSharp Mask (or other) filter on all the data at once.


You can accomplish the same thing with an Unsharp Mask adjustment layer ...

Where do I find that? I usually use PS v5.0 ...
 11/28/2005 01:30:14 PM · #16
Originally posted by GeneralE:
Where do I find that? I usually use PS v5.0 ...


Eeeep! I don't remember that far back. It should be a little yin-yang symbol at the bottom of the layers palette, but you might have to ask an archaeologist. ;-)

Message edited by author 2005-11-28 13:31:13.
 11/28/2005 01:31:01 PM · #17
Originally posted by scalvert:
Originally posted by eslaydog:
If I have PICTURE-A open in PS and save it, then do an edit and save it again, does the quality decrease each time?


I do the same thing all the time, and I think it works exactly as you speculated. If you perform several steps and save as JPEG, you can back out a step or two before the save in the History palette and it will still show the JPEG file name. I think it's clear from this that Photoshop is not reading from the already-saved JPEG file while you have the image open. This is NOT the same as repeatedly opening and saving a JPEG.


Thats what I thought too Scalvert. I wish there was a way to know for sure. It makes sense that the opened file isnt changed/compressed each time it is saved, just once per save.

So if I had an image opened and saved it 100 times without closing it, all hundred copies would be of the same quality/degradation level (original, + degradation of one save).

Does this sound right?
 11/28/2005 01:31:57 PM · #18
Originally posted by eslaydog:
Does this sound right?


Yep, and I just confirmed it. Open a file in PS and save it as a JPEG (quality 1), then save another copy as a JPEG (quality 11). The second file will be MUCH better looking than the first, so PS is definitely still working from "original" data. Compression is not an issue until you close and reopen a saved JPEG.

Message edited by author 2005-11-28 13:52:41.
 11/28/2005 01:35:00 PM · #19
Why don't you avoid the question by opening the JPEG, SaveAs in TIFF, edit/save/edit/save ... and then resave the final as JPEG; then delete the TIFF. I just find disk space too cheap to be worth taking a chance.
 11/28/2005 01:42:07 PM · #20
With all this being said, you likely could resave a JPG file on high many times before the average person could even come close to seeing a difference...
 11/28/2005 01:47:03 PM · #21
Re: the general's "composite" tiff, the way I do it is save-as "filename_flat" a flattened version, then I make a duplicate layer from BG and do my sharpening on that. This becomes my print master; it's still a psd file and i can resize this at will and adjust sharpening on it as needed for the specific print size and paper.

R.
 11/28/2005 01:49:04 PM · #22
agreed, but in general ppl will be saving then editing and saving files for dpc, trying to reduce it to a 150k size. thus the average compression setting is usually around 9 instead of the 11. but in fact, this statement means nothing in regard to the original post since the working image is not the processed image.
 11/28/2005 02:10:09 PM · #23
To again confirm what Shannon already posted, you can open a JPEG, save it as many times as you wish, then close it, and you've lost ONE generation due to recompression. Each time the file is opened, the JPEG file is decompressed, with loss incurred in the last save of course. Your editing then takes place in an uncompressed form, and each time you save during the editing session, you are re-compressing from the UNCOMPRESSED form. Any degradation compared to the previous generation is due to:
1.) The loss incurred in the process of compression during the *last save performed* prior to closing the file after the previous editing session
2.) The losses incurred due to editing steps taken during the current edit. These losses are completely separate from JPEG compression induced losses.
The most conservative workflow is to use TIFF or PSD (non-lossy formats) as working documents. If, however, your editing workflow usually involves a RAW conversion followed by a single editing session (or perhaps two sessions) and saving of a high-quality JPEG output file, there is really no need for the intermediate format. If you shoot in JPEG, you incur one more generation of loss, namely the loss that occurs when the camera compresses the RAW data to JPEG in-camera.
 11/28/2005 02:38:32 PM · #24
A friend at work wrote a simple Matlab script that saved a JPEG copy, opened the copy and compared it pixel-for-pixel to the original, plotted the sum-total difference, then repeated the whole thing in a loop. The resulting plot was clearly logarithmic. In other words, the biggest loss was between the 1st and 2nd generations, significantly bigger than the one between 2nd and 3rd, and so on. After 10 to 15 generation there was no measurable further loss. In fact, when using something like 95% quality JPEG, it was very hard to tell the difference between any of the generations. (This was years ago so I don't have the exact numbers).

The motto of the story is that if for some reason you have to make 2nd or later generation JPEGs, don't worry about it. So long as you use a high-quality setting, the new generation will look extremely similar to the last one (most of the quality loss has already happened).
 11/28/2005 03:22:36 PM · #25
Originally posted by GeneralE:
Originally posted by scalvert:
Originally posted by GeneralE:
That composite is the PSD file "flattened" into a single layer, so that I can run the UnSharp Mask (or other) filter on all the data at once.


You can accomplish the same thing with an Unsharp Mask adjustment layer ...

Where do I find that? I usually use PS v5.0 ...

I don't know about an adjustment layer, as USM can only be applied to pixel data -- but there is a way around this that I believe worked in PS 5. To do this create a new layer on top of all others and stamp it with all visible layers (Ctrl-Alt-Shift-E, or look in the help file for 'stamp visible'), the USM can then be applied to that layer and blended as desired. Granted, if you changed any of the adjustment layers that were active when it was stamped it would have to be recreated -- but the same applies to the composite file.

Just curious though, why Tiff? Png is lossless and generally produces much smaller files. Disk space is cheap, but why incur a larger resourse penalty than needed?

David

Message edited by author 2005-11-28 15:23:10.
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