lightroom and jpeg question

So I am a total RAW convert at this point, but I've been processing some jpegs from back when I only took jpeg, and I'm wondering:

Is there still jpeg data loss from processing in Lightroom, when I'm not intentionally "saving" or resaving a file - just processing and moving on to the next file? When does the data loss occur, in Lightroom? Every time I shut Lightroom down?

Am I merely showing how ignorant I am about how jpeg processing actually works by asking this question?

I would think that it happens when the image is re-exported but as i shoot raw only, I'm guessing based on what i know about editing jpegs.

Yeah, I don't shoot jpegs either but I believe that the editing metadata is stored in the same way, in the Catalog database, until you do an Export. It will re-compress then, so don't do that until you need a final copy, and you should be OK. If you need to do an Edit in Photoshop, you can open it as a tiff or psd so that you avoid recompression, again until you need a final jpeg.

Interesting question... I just looked at a JPG that someone else posted on POTN. I downloaded it then imported it into LR and made some edits including cropping, vignetting, local adjustments, etc. In LR the image shows all those edits, but when I go to the folder containing that image and open it in another program it looks original and untouched. And yes, I am looking at the one and only version of the image on my system -- when I download something to edit I drop it into a watched folder so that LR auto moves and imports it.

So, it appears that LR treats JPGs as it does RAWs, and that your edits are written to the file only when you do an export.

Cool - thanks folks.

Rimmer wrote in post #12216249
So, it appears that LR treats JPGs as it does RAWs, and that your edits are written to the file only when you do an export.

This is correct. Nothing you do in lightroom to ANY image does permanent damage and can always be undone because you are not actually changing the original image. Your changes are only permanent in the exported image, which will be exported as a separate image from the original.

Even more than you know: RAW data straight from the sensor is linear in nature - double the intensity of the light coming into the sensor causes double voltage coming out and is digitalized as a doubled tone value. Part of the conversion to a jpg (or any other RGB format) is the application of a gamma curve. LR/ACR does the gamma correction at the very end of the workflow (not all converters do) and all the editing is done in linear space. When LR starts working on a jpg the first thing it does is apply an opposite curve to the data that sort of returns it to linear status. (It can't get it exactly back to RAW state because too much has been lost or skewered - compare it to a loaf of bread, you can dry it out and grind it to a fine powder, but it wouldn't turn back into flour although you could do with it many things that you can do with flour.) This allows LR to perform many of the RAW functions on jpgs also.

Boy, Elie -- one of these days I may come to understand some stuff about gamma curves and such but in the meantime it still tends to boggle my mind...

tonylong wrote in post #12220321
Boy, Elie -- one of these days I may come to understand some stuff about gamma curves and such but in the meantime it still tends to boggle my mind...

It's easy to understand. If you get too much of it you Hulk out so stay away.

Heh!

You know, even though I spent 15 years working in the world of digital imagery as part of the inkjet printer industry, the actual field of image science and technology was never my "thing". I rubbed shoulders with digital science folks, but my field was in providing and supporting software for testing/manufacturing those products. All the science/firmware stuff was already in place.

The nice thing was that things like scanners, printers, and eventually digital cameras were always available to use, as well as computers, monitors and laptops and such, since the company (HP) was making them all in various places. But, fortunately, terms like "gamma curve" never came around to haunt me!