I too have been very confused by this discussion. However, after digging around a bit, here is what I suspect is happening.
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All of the external programs/apps mentioned above read and write the metadata for raw files to sidecar, usually with an extension xmp. The format of this file is an industry standard ( Since early 2012, the XMP specification part one, which defines the XMP data model and core namespaces, is an open, international ISO standard (16684-1).) This is how Photoshop, Lightroom, etc. handle metadata. I use Photo Mechanic 6 for my DAM and have the preferences set so that the metadata for raw files are stored in such a sidecar. It is possible, but not a good idea, to embed the metadata in the file. If you do this, it is my understanding that PL3 will not process the file. Metadata for non-raw files (e.g., jpg, tif) is always embedded in the image.
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If the preferences are set, PL3 creates a sidecar with the extension .dop. This file turns out to also be an xmp-formated file. You can verify this by using Phil Harvey’s command line tool Exiftool.
(the simple command to display the xmp data in a file is ‘exiftool filename.ext’. This file is thus a second, separate, and DxO proprietary xmp file. (I note the both files are, in fact, txt files, so you can read them in any text editor.) -
What is happening in the discussion started by Brian is that when he goes to IMatch and changes the number of stars, the information is stored in the industry-standard xmp file described in (1) above, while PL is reading their xmp file described in (2). Hence, PL does not see the change. And, vice-versa, when he changes the stars in PL, IMatch cannot see the change.
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The consequences of this disparity are significant. For those who use a cataloging program external to PL (e.g., Neofinder, Photo Mechanic 6 Plus, …), to the best of my knowledge they index only the xmp files described in (1). I have not looked in detail at how the metadata that one enters in PL is handled, but some of these data are set up to transfer to Lightroom, but certainly not all. Also, if you need to store IPTC data, as far as I can tell, the is not done in a dop file, but is fully supported by the programs that write xmp files. Caveate–I may misunderstand some of these details and certainly may stand to be corrected.
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For my personal workflow, I turn off the creation of the dop sidecar and rely on saving all of my parametric edits in the PL database. I do all of my classification and cataloging in Photo Mechanic 6 Plus (still in beta test as of this writing, but available to all customers). I can then find the images I want quickly and easily. Many readers of this forum do the same thing using other DAM systems as well. Once sorted and selected, I create an empty project in PL3. Then, in Photo Mechanic, if you right click on one of the selected images, you get a choice to ‘Edit Photos With …’. I have set up DxOPhotoLab3 as one of my choices (you get a pull-down, so you can add as many choices as you wish). One click, and all of the images show up in the project perfectly. As noted elsewhere in this forum, you may be able to drag and drop the selected images in your DAM onto an empty Project in PL3, but I have not tried this.
Since I back up my work nightly from my working drive to two separate external drives (a primary and a secondary backup), I also want to backup all of the parametric data in the PL database. Thus, I also copy this database (as noted elsewhere in the forum, there are a variable number of database files–anywhere from 3 to 5 have been documented) to a folder on all three of my hard drives. (My working drive only has images and their derivatives–all of my programs, apps, word processing, etc. are yet on a separate hard drive.)
Hope some of this helps in understanding what appears to be happening with PL and metadata.