Railfan Technology > JPG Myth, JPG Handling

It has been stated here on T.O. that when you copy a .jpg from one source to another, say from your hard drive to your back up drive, the picture degrades. Not true because nothing is changed in of the file (except possibly some exif data).

I have just done my research on .jpg degradation. There are several good articles out there on the net. Here's what I found.

The quality of a .jpg picture does degrade when you save it from your screen. So say you bring up a .jpg, crop it with a photo editor, and put it back with a different name. The version that was uncompressed, cropped, and recompressed looses some quality. Why, because during file compression some information is lost. If you bring up the two files on a good monitor you should be able to see the degradation.

So what does this mean for a railfan's normal .jpg file moving and editing activity.

1. When you copy a .jpg from your camera to your hard drive and then to a backup drive, all versions are of the same quality. Likewise, .jpg files on your hard drive from your scanner can be copied to a backup drive without degradation.

2. If you want to modify a .jpg on your hard drive from your camera or scanner, keep the original file unchanged and save the modified file under another name. Never modify the original file.

3. Set your camera for the highest resolution possible and your scanner for at least 3600 dpi. That should give you good "original" .jpgs for any possible future use.

4. If, for example, if you want to post a picture to T.O., load the original into you file editing software, if it is crooked, rotate it, crop it if you want to, then resize it to 1000 pixels wide if it is a horizontal (landscape) picture and maybe 800 pixels tall if it is a vertical (portrait) picture. Then do the color correction, contrast correction, etc. that need doing. Both digital cameras and scanners introduce noise to the photo. You will have to remove the noise. Also, because you have played with it, it will need sharpening.

5. When saving a .jpg file, always save it with the minimum compression your software allows.

6. If after you have saved this modified version you want to further adjust it, open it, make the adjustments with your software, sharpen it slightly, and save it. I find this slight sharpening seems to eliminate the .jpg degradation due to the previous file compression.


As always, comments welcome.
Marty Bernard



Edited 1 time(s). Last edit at 01/20/15 17:21 by MartyBernard.

I've never heard or read the theory that copying a JPEG from one drive to another degrades it. That's a new one on me.

trainjunkie Wrote:
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> I've never heard or read the theory that copying a
> JPEG from one drive to another degrades it. That's
> a new one on me.

Right - it's total rubbish.

Ray

Marty,

This is a good set of guidelines. Thanks for posting.

JPEG is a lossy codec (compression / decompression method) which uses the DCT algorithm (Discrete Cosine Transform) to achieve its amazing compression ratios. If one re-compresses a file, the image loses a small amount of its original resolution for a large number of the first re-compressing iterations, but then this stabilizes to where the 'lossiness' reaches a maximum. To most casual viewers of high resolution photography, it is my opinion that this is nearly imperceptible.

But for archiving, it might be very meaningful. As you've seen, there is a large and robust amount of documentation on the web on this subject matter, not to mention massive phone-book thick text books. I guess it depends on what you intend on using the photo for.

A direct byte-for-byte transfer of a file (writing to media such as a hard drive or thumb drive or DVD, emailing / etc.) should never affect quality. Its only choosing to re-compress the file that invokes another conversion through the DCT and more degradation.

Happy shooting,

Alan

MartyBernard Wrote:
-------------------------------------------------------

>
> 5. When saving a .jpg file, always save it with
> the minimum compression your software allows.
>

I never bother with jpeg except for publishing on the web.
I'd recommend shooting in whatever RAW format your camera
allows, edit your RAW image with an image editor that can
handle the 12 - 14 bit image and then saving in a lossless
format. For the web, I save at jpeg quality 85 which provides
lots of detail... in fact, once the quality gets high enough
(I like 85) increasing the quality seems to do little more
than increasing the file size.

Long ago I noticed that editing jpegs out of the camera resulted
in a degraded image... Simple resizing resulted in more
'jaggies' than I wanted to see in a photo, and sharpening seemed
to add to the mess ;-)

HTH.

John.

Without rehashing the specifics, Marty's points are well taken.
I think a lot of "problems" with jpgs was people early on using auto Save for the WEB, or editing and resaving again and again jpgs, or simply saving at too small a compression.
It is like any tool, used properly and in the right place there should be no problem.

Bob

Correct. Direct copy of file is not an issue. Saving multiple times is. You can see it in the histagram.

RWJ


robj Wrote:
-------------------------------------------------------
> Without rehashing the specifics, Marty's points
> are well taken.
> I think a lot of "problems" with jpgs was people
> early on using auto Save for the WEB, or editing
> and resaving again and again jpgs, or simply
> saving at too small a compression.
> It is like any tool, used properly and in the
> right place there should be no problem.
>
> Bob