Quick question on working with JPEG's

10/26/2003 09:06:16 PM · #1

If I'm editing a jpeg image and want to save it as a jpeg again (for emailing reasons etc), what is the best method for preserving it's quality? Am I better to save the image as a TIFF first and then do the editing etc and save as a jpeg last or would it be just the same to edit it whilst it's a jpeg and then save it as a jpeg when I'm finished. In other words will I be wasting time converting it to a TIFF if it's only going to end up as a jpeg again?

Thanks,

Jax

timj351

Sony DSC-F707

10/26/2003 09:35:32 PM · #2

First of all work on a copy of your original image. I first view my images in a viewer (ACDSee) and as I am iewing my images I save copies of the ones I want to work on into another folder named Edited. Then I use my image editor to work on these images which, in my case, are also jpegs and than save them again as jpegs at a very low compression and they look great. So there is no problem doing what you are asking especially for smaller sized images for emailing. Since I have my originals in a diffeent folder from my edited version, I can always go back and re-edit a particular image if I plan to do something special to it like make a poster. In some cases if I know ahead of time that a photo is going to be enlarged I will then save it as a tiff when I first edit it but usually I find that high qualities jpegs are just fine as long as I resave only once.

T

Canon EF 24-105mm f/4.0L IS

10/26/2003 10:00:03 PM · #3

Thanks Tim for clearing that up.

Natator

Canon EOS-5D

10/26/2003 10:18:12 PM · #4

Tim ... You use the term "resave only once".

How does the jpeg compression work when you save a file, have closed it and then reopen it at a later time and then save it again.

Is it applying the compression again and again, and thus quality gets worse with EVERY save?

cvt_

Canon EOS-20D

Canon EF-S 17-55mm f/2.8 IS USM

10/26/2003 10:33:01 PM · #5

in reply to natator:
if you apply the same compression, it shouldn't change the image data, because of the way it compresses the files (looks for patterns).
when you open the file for editing it decodes the patterns to individual pixels.
if it decodes correct, when you re-encode its going to find the same patterns and resave the same.
if you do work on the resaved copy, you will lose data, because you started with a clean file, compressed it into slight patterns, altered the pixels, hence the patters will be changed, and not as accurate.

while working on the file, there is no difference if it was a tiff or jpeg, because it works pixel for pixel. it only matters if you save it and reopen it for editing again, then doing that temporary save as a tiff is a good idea, but if you save a jpeg to tiff just to edit it, it makes no difference at all.

edit: I lost 2 k resaving at 4 quality, from 286k, at 10 quality I lost 4k from 3M
IMHO its not worth worrying about, as mentioned above is true, resaving a few times wouldn't hurt if not re-editing it, but I don;t know why you'd do that anyway :)

Message edited by author 2003-10-27 06:50:02.

timj351

Sony DSC-F707

10/26/2003 10:42:41 PM · #6

Originally posted by natator:

Good question. The file is saved once when it is first saved in the camera and then you need to re-save it again after you have edited it (say that ten times fast). I was suggesting that you only save it or re-save it only once as a jpeg to avoid degradation in quality. If, after working on your file, you simply hit Save than I "think" it saves it at the same compression it was first saved at so, to avoid this, and have more control, I either select Save As and manually select a jpeg compression or I use the Save To Web feature to get even more specific. I hope that helps.

T

e301

10/27/2003 05:51:23 AM · #7

I find it helpful to think of the original files (almost always JPEG at high quality from the camera) as negatives: the archive consists of these files. Anything that's worked on is copied elsewhere and then messed with, as Tim suggests. If I'm especially happy with an end result that is also saved, as are those that are the results of very complex post-processing (such as my Future shot), which are saved as PSP files with all the necessary source images - so I often end up with five or six versions of an image stored somewhere.

Storage is cheap!

Ed

10/27/2003 09:11:24 AM · #8

This is the way I understand it. Every time you save a JPEG to a file you lose a little something. You can minimize the amount of loss by using the same compression all the time. If you open and save the same file over and over again at say 95% compression you will eventually (if you do it enough times) lose detail. Where JPEGs compression may affect us here at DPC the most is in the editing process. If you have a JPEG saved at 95 percent and you edit a portion of that picture and save it, the edited portion of the image will be compressed again resulting in a loss of data for that area. If then you edited a smaller area inside the original edited area, you compress the new edit even more resulting in MORE loss. Every time you edit the shot and save it the edit gets recompressed!

TIFF files on the other hand (to the best of my understanding) are not lossy. You could open and save the same file over and over again and it would be the same the 1000 time as the first time. They also are much bigger files. The advantage for us would be that you can make as many edits as you want without a loss of data. This means you could work on the image over time without loss of quality! After all editing is complete you resave as a JPEG and voila, nice clean image!

I did some research before joining the sight... Didn't know it would come in so handy here! Anyone else have more info???

Message edited by author 2003-10-27 09:17:37.

e301

10/27/2003 09:17:22 AM · #9

But if you just open and close that file you lose nothing - so if you keep your original files and only work on copies you have no problem.

ed

Canon 12x IS USM (S3 IS built-in Lens) -- 36-432mm (35mm EQ)

10/27/2003 09:19:54 AM · #10

Originally posted by e301:

But if you just open and close that file you lose nothing - so if you keep your original files and only work on copies you have no problem.

ed

That is why I did the research I did. I had someone in film photography try and tell me that every time you OPENED a JPEG you lost data and I knew from my computer science experience that that was IMPOSSIBLE! So I had to prove it!

Message edited by author 2003-10-27 09:20:24.

GeneralE

Canon PowerShot S3 IS

10/27/2003 10:06:38 AM · #11

I use the method first mentioned with even more redundancy. I after I open the original JPEG from the camera, I resave it in Photoshop format for editing, which will allow me to retain all (adjustment) layers for future modification.

When I'm done editing I use Save As (Copy) to create a TIFF file. This is the stage I reduce the dimensions if it's a DPC entry and perform any sharpening on the final composite image. I will then save a copy at various compression levels until I get below the file size limit.

One of the reasons I do this is so I have the full-size edited image still available for further adjustment or as a base image for a print file.

Message edited by author 2003-10-27 10:08:12.

10/27/2003 02:59:53 PM · #12

Bump-a-roo for the new...

Canon 12x IS USM (S3 IS built-in Lens) -- 36-432mm (35mm EQ)

10/27/2003 10:44:45 PM · #13

So just to confirm, it's ok to open the JPEG straight from the camera, edit and then save a PSD or TIFF? I don't need to open the JPEG, convert to TIFF/PSD and then edit? In other words, quality won't be lost if I edit the image WHILST it's still a JPEG, just as long as I save it a TIFF afterwards?

I will get my head around this one day:)

Thanks.

Originally posted by GeneralE:

GeneralE

Canon PowerShot S3 IS

10/27/2003 11:01:40 PM · #14

Originally posted by Jaxson:

Right ... until you execute a Save command, the original file is intact on the disk, with Photoshop putting the data into scratch memory in RAM and/or on your hard drive. PS wants free space on your disk, about 5x the size of the file you're opening, as a scratch pad.

But, if you do anything and then accidentally save, you will have over-written the original. One way I keep from doing this (besides saving to another format first, and burning originals to CD) is to create an adjustment layer. It's then impossible to save in JPEG (or almost any other format) as they don't support layers.

10/27/2003 11:06:05 PM · #15

Thanks

Originally posted by GeneralE:

Originally posted by Jaxson:
So just to confirm, it's ok to open the JPEG straight from the camera, edit and then save a PSD or TIFF? I don't need to open the JPEG, convert to TIFF/PSD and then edit? In other words, quality won't be lost if I edit the image WHILST it's still a JPEG, just as long as I save it a TIFF afterwards?

I will get my head around this one day:)

Thanks.

Right ... until you execute a Save command, the original file is intact on the disk, with Photoshop putting the data into scratch memory in RAM and/or on your hard drive. PS wants free space on your disk, about 5x the size of the file you're opening, as a scratch pad.

But, if you do anything and then accidentally save, you will have over-written the original. One way I keep from doing this (besides saving to another format first, and burning originals to CD) is to create an adjustment layer. It's then impossible to save in JPEG (or almost any other format) as they don't support layers.