Damage from re-saving jpg

[toriaj]

I've heard that every time a jpg file is saved, damage is done to the file. I think I noticed it once in a jpg I edited, saved, edited, saved, about 20 times. Here's my question: if a 16x24" print of a jpg that was saved twice was compared to a 16x24" print of the same jpg saved four or five times, would the resulting damage be visible?

[kgosden]

Well, the answer depends on far more variables than you have thought about. The output (print) size doesn't tell us what the source resolution is. It also depends on the actual image content itself.

(The following is a simplification) The basic JPEG process works in part by looking at neighboring pixels and when the color difference between them is small it averages them to a single value. This is done on small groups of pixels. The amount of this averaging is controlled by a 'quality' setting (in Photoshop a scale from 1-12). I cannot recall if the number of pixels in the group varies with this setting as well. Basically, if the setting is high then the nearby pixels need to be closer in color to be averaged. This is why the 'damage' often shows up most in large areas of similar color such as the sky or skin in portraits. Fine details which have high contrast and color differences from pixel to pixel are impacted less obviously.

[toriaj]

Yes, I understand that, I was hoping to remove the variables by comparing two identical prints (other than the saving.) Same print size, same resolution, etc. The only difference being the number of saves.

Does that help?

[erichlund]

The problem is that the original image quality and size have an impact on the degradation rate for a given print size. If it's a fairly low dynamic range image from a high pixel count camera, then it may not degrade visibly as fast as an image that is using the full dynamic range of the format and has a low pixel count. But the basic answer is that every time you save a jpg with compression (hint: you can turn compression off or at least reduce it by going for the highest quality jpg), it will degrade to some degree. This is because jpg uses a lossy compression algorithm.

[leaftye]

Why haven't camera manufacturers switched over to a lossless format like PNG? It's still quite small, but without the risk in quality.

[erichlund]

An object in motion tends to stay in motion until acted upon by an outside force. You really should take physics. :p;)

Actually, since the entire industry uses jpg, it's not likely to change unless there is a truly compelling reason. You get superior quality with RAW, so that is compelling, and a large subsegment of the industry does use RAW.

While png is lossless (I'll take your word for it), I don't know anything about the format. I may lack features like EXIF data that make it suitable for publishing work. I know nothing about the format, so I just cannot say.

OK: I looked it up. It does not support colorspaces. End of story. Not gonna happen. Toast.

[toriaj]

Hmm, I don't know if my question is unanswerable, or if I'm just not asking it well enough. Thanks for your tries :o I'll try again.

Let's say I have a 2000x3008 pixel image from my D50, relatively low dynamic range (meaning the camera was able to capture all the brightness levels in the image) taken in RAW, then saved as a jpg by my RAW converter at the highest quality setting. Then it was edited and saved one more time, again at the highest quality setting. It was printed at 16"x24" on a high-quality printer and paper.

Then I wanted to print it again, but I ended up saving it two more times (all at the highest quality setting) before re-printing it.

In real life, with my real eyes, would I be able to tell the difference in quality between the prints?

[erichlund]

The answer is probably not. But, no guarantees. Don't come back me a say, "Nah Nah, Neh, Naaaah, Nah. I can see the difference." The image does not degrade super fast, but whether you can or can't see a difference depends on so many factors that none of us can predict it for any particular photo.

[Ray Schnoor]

I think that you would be able to tell a difference between the 2 photos because I assume that you made some changes to the image if you had to resave it. If you didn't make any changes, why would you have to resave the image? If there were changes to the image, of course you would probably be able to see differences between the 2, but you probably wouldn't be able to see any degradation by eye in a print due to jpg artifacts after 2 additional saves at the highest quality.

If you are not making any changes to the photo, there is no need to resave the image. If you open a jpg and then close it without making any changes, there is no degradation, and would therefore not be any differences in the photos.

edit: OTOH, If I need to make any changes to an image, I would start over from the Raw file.

Ray.

[toriaj]

Thanks, Erich, and don't worry, I won't come after you :D
Ray, I know what you mean, I thought of it too, but I wanted to make a simple case. Maybe there was just one tiny thing I wanted to change, like I found a dust spot. Or I emailed the full-size jpg to a friend, who re-saved it on her computer. Anyway, thanks for the answers, I think I don't need to be super-paranoid about saving. Once or twice won't make a huge deal. *phew* just kidding:rolleyes:

[whubble]

Truly, I am not an accomplished photographer (yet!) but I am a computer geek. This appears to be more of a computer question than a photographer question, if I understand what you are asking. JPEG works the same in your pc as it does in your camera. Its primary job is to make the image smaller by

Basically, you shot an image with a quality camera in RAW format and converted to JPEG using a RAW converter at a higher setting. I haven't worked with RAW yet, but I am assuming there is less compression and thus less loss of color variables and gray scale then would be apparent with a higher compression setting. Then you opened it, edited it and saved it at the same higher quality setting, you used initially. What you want to know is, does further compression take place, even at the high quality setting, thus degrading the color and gray scale variables each time you save. Did I understand your question properly?

I read an article, older to say the least, but interesting all the same. This paragraph was in it. It might answer your question:

"The real disadvantage of lossy compression (JPEG) is that if you repeatedly
compress and decompress an image, you lose a little more quality each time."

I would say that, JPEG acts the same in your PC as it does in your camera that if you resave in JPEG even at the highest quality, your image will degrade slightly each time. However, it may not be a loss that you would see in per say a 4x6 image printed 5 or 6 saves apart. But in an expanded image, I don't see how it can be avoided.

I hope I was able to help.