XMP instanceID - when to change?

[Mac2]

The XMP documentation says "InstanceID should change on every save operation".

I could not find any conclusive info if changing metadata (e.g. adding a new keyword or title to the XMP data or making changes to EXIF or classic IPTC) would be considered a save operation and thus would require me to update the XMP instanceID as well.

But if changing metadata in a file would require the instanceID to be updated, would ExifTool not do this automatically?

[Phil Harvey]

The simple beauty of ExifTool is that it does only what you tell it to do.  If you want to change InstanceID, then you can do this:

exiftool 'instanceid<uuid:${newguid}' FILE

(single quotes as above for Mac/Linux, or double quotes instead for Windows)

- Phil

[Mac2]

Hi, Phil

I'm aware of that I can instruct ExifTool do update the instanceID. My problem is rather when to do it. Since ExifTool does not do it automatically, it seems to be optional on metadata changes. E.g. the IPTCDigest update is not optional and ExifTool does it automatically.

I was seeking some input from more experienced users about when to update the instanceID. Only when I modify the file contents or also when I just add a new keyword...    The XMP documentation is not clear (to me) in this matter.

[Phil Harvey]

E.g. the IPTCDigest update is not optional and ExifTool does it automatically.

No it doesn't.  However, if you are talking about the MWG option I'll give you a partial concession because writing the Composite MWG tags does also write the IPTC digest, but this is a special case:  The MWG tags are a separate set of Composite tags, and the user must specifically implement this module to incur this behaviour.

I was seeking some input from more experienced users about when to update the instanceID. Only when I modify the file contents or also when I just add a new keyword...    The XMP documentation is not clear (to me) in this matter.

The July 10, 2010 XMP spec is fairly clear that xmpMM InstanceID should change with every save operation.

- Phil

[Mac2]

Thanks, Phil

I will then update the instanceID every time I write data back via ExifTool. I will also add an explicit digest update when the MWG option is disabled in my app.