Peter, thanks for your response. A corresponding bug report was sent to Adobe.
Adobe used to have a technote titled "PDFs generated from InDesign do not show XMP metadata of placed image files in Acrobat (CS, CS2) (ID 332920, dated 01-04-2007) that laid out conditions when object-level XMP metadata was not passed on to exported PDFs; one of the listed conditions was "Flattened items contain transparency."
Unfortunately, I didn't save the whole technote (though I referenced it in a 2009 post), I can't find it now, and of course much may have changed since 2007. Or maybe stayed the same.
Thanks for sending this info and pointers to previous discussions, David!
Would be great to get hold of a copy of that technote, though. May still help, although it's a bit dated.
With reference to my original post, I checked a test version of ID CS6 today and found that the problems with certain effects and loss of XMP metadata after export to PDF persist.
Since there seems to be - at least some - interest in this topic, I'd like to give a brief summary of what from my experience with CS5.5 currently works ok and where some problems remain. Please take this with a grain of salt since it's mostly from memory, but I may provide further details, if needed.
- For placed images export to PDF generally preserves XMP metadata, independent of image format. There are issues with legacy IPTC metadata in case of IPTC format errors or lack of synchronisation between IPTC and XMP according to the IPTC core specification. Worst case export behaviour here is the replacement of original XMP with a new, rudimentary XMP dataset containing only some basic elements. There is another situation where XMP metadata is lost if certain effects are applied to a placed image. Please see my original post for reference.
- Object level metadata for placed vector graphics is supported since at least CS5 (compared to CS2). Don't know exactly which version introduced this feature though. There is a problem with"Illustrator 8" compatible EPS files - AFAIK these do not support XMP metadata at all. Vector files in native Illustrator format are also ok,
- Placed documents (PDF, EPS) are embedded as marked content sequences in the PDF content stream. Document level metadata is preserved and is referenced as a property of the marked content sequence. Object level metadata _inside_ the placed document should be preserved as well, although I haven't investigated this any further, since I'm currently only interested in the top level objects and their XMP data.
- I have witnessed fragmentation due to transparency flattening mostly for placed documents, which have not been "properly" prepared (e.g. exported to PDF-1.3 format or distilled from Postscript). In that case all objects in their original form are replaced by flattened image fragments. With the original objects their associated XMP metadata is of course lost as well.
So much for now.
I appreciate your summary very much: this kind of information is hard to find, and is rarely discussed here (or anywhere else).
I confess I haven't kept up with developments in InDesign's handling of object-level XMP: I continue to use IDCS4 for most work, still relegating IDCS5.5 to a test-bed status. However, a current project for distribution in PDF as well as print and would really benefit from preserving object-level metadata for AI files placed in ID: for my purposes, this single feature would justify upgrading ID on my production hardware.
As for the old technote #332920 (01-04-2007), all I preserved of the content seems well out of date, and I include it below only for its very limited historical interest :
Under the following conditions InDesign CS2, files with placed files do not support the pass through of XMP metadata to the resultant PDF:
-- Flattened items contain transparency
-- Placed files are non-raster file types
-- When exporting to PDF with PDF compatibility of 1.3 or earlier
-- When creating the PDF files by printing to PostScript and then converting with Acrobat Distiller
-- When placed PDF files were created with Photoshop, Illustrator, or GoLive versions earlier than CS
Google found no hits for the phrases I tried, though searching for the title "PDFs generated from Design do not show XMP metadata of placed image files in Acrobat (CS, CS2)" did turn up one reference to technote 332920 on a Korean site.