But now If I allow LR3 to "automatically write changes into XMP" -- will this WRITE these develop module changes (e.g. the contrast change) to the original JPEG?
You only write the instructions, not the rendering with Auto write XMP (or command/control S in a manual fashion). So if you open such a document in say Photoshop, you will NOT see the color any differently than before you wrote those metadata edits. You have to actually render out an iteration (a new JPG in this case) to see the edits applied in the new document.
In both cases (DNG and JPEG), LR - if instructed so by "automatically write ..." or Ctrl-S - writes metadata into the so called XMP section of the appropriate files, and does not alter the image data itself.
In the case of a DNG, this written data includes metadata such as keywords, title, caption etc. as well as develop settings.
In the case of a JPEG, this includes metadata such as keywords, title, caption etc., and develop settings only if "Include Develop settings in metadata inside JPEG, TIFF, and PSD files" is checked in the catalog settings.
The develop settings written into the XMP section can only be seen if you open the image with a tool that recognizes and interpets this data, i.e. open in PS/PSE via ACR or a re-import into LR.
You would see the effect of the XMP data in the JPG if your PS is new enough and you have PS set to use ACR on the way in for JPG files that either have settings or always, but you see those changes because PS has called on ACR to render them, not because they are in rendered into the file, already.
So if you open such a document in say Photoshop, you will NOT see the color any differently than before you wrote those metadata edits. You have to actually render out an iteration (a new JPG in this case) to see the edits applied in the new document.
Actually, if you write the xmp adjustments into a JPEG, I'm pretty sure Bridge will see them (if you have the ACR Prefs set to open JPEGs with settings) and you can always open the JPEG using Camera Raw which deffinately will see the adjustments if the ACR prefs are set to use Camera Raw to open JPEGs with settings.
So just so I have this correct with JPEGs:
1, If I make changes in LR3 to a JPEG (e.g. contrast), this contrast change is saved in the LR3 catalog.
2. I can ADDITIONALLY save metadata (keywords, etc) in XMP data into a JPEG if I have the "automatically write xmp." settings checked or if I save it manually to the file
3. I can ADDITIONALLY save the develop settings (e.g. the contrast change I made in #1) by including the develop data in the JPEG in the same dialogue box.
But now if I opened this keyword, added contrast-adjusted JPEG in another program, only "XMP-aware" programs would allow me to see these keywords and contrast changes. Is this a correct assessment?
And lastly, if I do save the develop settings to the JPEG itself, doesn't that violate the "non-destructive" editing concept of LR3?