Hayo,
PM currently does not recognize that .heic files can have .xmp files with (additional) metadata. Instead it shows the proxy for the .xmp files.
Please allow for .heic files to have accompanying .xmp files (e.g. ACR creates a .xmp when editing the .heic).
As an option: treat .heic just as any other raw file type, also with regards to embedding metadata – if that ever gets implemented, that is.
This is going to become rather problematic now that the Canon 1Dx Mark III is out. It can generate a CR3 and a HIF file (heif). If XMP sidecar files are used, to which image file does an XMP sidecar file belong? HEIF/HEIC files can have metadata embedded in them (and should!) but there is no library out in the community for doing so. It should theoretically be easy to add new 'chunks' to a HEIF file, but we're seeing already that some creators are not following the rules and their offsets to other chunks would be broken by adding new chunks to the file. It's a dilemma for sure. One that we're not happy about at all.
Hmm, dilemma indeed. Yes, the file format is actually very capable of storing embedded metadata (heck, the file format is actually quite straightforward – once you figure out the basics). It's a real pity (even?) Adobe does not embed the metadata in the file, but in a sidecar (I guess this is because this is their approach with ALL “raw” files). But to answer your question “to which image file does an XMP sidecar file belong”, I guess the simple answer is probably: to both; if you open either in e.g. ACR, it will (very likely) make use of the XMP sidecar (I have no R3/HIF files to confirm this, but it sounds plausible this is what will happen).
An advantage of using the XMP for the metadata is that implementing metadata writing is a lot simpler for you (for reading the embedded metadata you should e.g. be able to use the library I developed for you last year), without the risk of corrupting files that other (non-standard-conforming) developers write.
Cheers,
Hayo
