In Picasa, in the Properties List, there is a property labeled "Unique Id".

In the Exif spec, there is a field called ImageUniqueId.

However, I have tried several Exif libraries/command line tools (e.g. exiftool), and none of them show ImageUniqueId.

So — is the Picasa "Unique ID" something that is stored in the image metadata, or is it something that Picasa computes on the fly?

If the latter, what is the algorithm that Picasa uses?


It is in the exif specification:


0xa420  42016   Photo   Exif.Photo.ImageUniqueID    Ascii   This tag indicates an identifier assigned uniquely to each image. It is recorded as an ASCII string equivalent to hexadecimal notation and 128-bit fixed length.

It is based on the timestamp and camera serial number, in an encryption manner.

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