1

I'm trying to understand how sidecar files are logically associated with their respective photo files.

As far as I can tell, both Apple and Adobe save nondestructive editing information in XMP files, except Apple uses the .AAE file extension, whereas Adobe uses .XMP. (I use a Mac, and I currently use both Apple Photos and Adobe Lightroom with the expectation to abandon one of them in the future. Occasionally and carefully, I may also use the Finder Desktop or third-party utilities for managing my files. My photos often originate from my iPhone Camera app.)

What I'm not clear about is how photo-editing software and/or operating systems understand that photo file X.jpeg is associated with the sidecar file X.aae, and photo file Y.aae is associated with the sidecar file Y.aae.

Is this file-association information intelligently contained somewhere, maybe using some sort of unique IDs for the photo files, or are these relationships dumb, i.e., simply associated because the (base) filenames are the same?

I'm asking because:

  1. I'd like to understand how renaming a photo file via the shell has to be handled when a sidecar exists. (Presumably, photo-editing programs like Photos or Lightroom take care of this automatically.)
  2. I'd like to understand if/how to find and manage orphaned sidecar files. For example, I'd like to determine a sidecar's original parent photo and re-associate the two. Likewise, I'd like to determine with confidence that an orphan sidecar does not have a parent photo in my library and then delete accordingly.
  3. I'd like to know if I can find out if a photo file should have a sidecar file (or once did), but the system thinks it is missing.

I've looked for documentation describing the technical ins-and-outs of this subject, and I've found nary a thing.

2

That part is pretty simple. The same file name (in the same folder) associates them (the two files with the same file name, but with the two different extensions). For example a Nikon file in Adobe:

DSC_0512.NEF is the Nikon raw data, perhaps 30 MB.
DSC_0512.XMP is the Adobe meta file containing the list of edits, perhaps 8 KB.

When the raw app renames the file name, it renames both of them. When it deletes or moves the file name, it deletes or moves both of them. If you delete (or otherwise remove) just the one XMP file, you simply delete the list of all past edits, so the state of the raw file becomes unedited then.

  • I'll add that Corel Aftershot even puts the original file extension into the sidecar file name. So it would be DSC_0512.NEF.xmp – Zenit Feb 2 '18 at 22:57
  • I get that an app knows that they are associated, and that an app will rename both the same way, but how do you know that the software is solely using the filename to associate them? – EJ Mak Feb 3 '18 at 0:27
  • 1
    I don't understand the concern. Why would anything else be necessary? The file name takes you directly to it, and the match in the same folder ensures exclusivity. – WayneF Feb 3 '18 at 2:16

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