How does windows tag a file?

When a jpeg is tagged using something like MS photo gallery, does the file get updated or does windows store tagging information somewhere separate. The reason I ask is I am backing up photos and what to know the tagging info is backed up.

Thanks.

It depends on the program and the file format. A few common examples include the following:

• Some programs will put metadata in the file itself, assuming that the file format supports metadata fields.
  • The benefits include:
    • The metadata is self-contained and thus portable (goes with the file).
    • The metadata can be checked for validity since the file data is attached.
  • Some problems include:
    • Poorly designed programs will write metadata even to files that don’t support metadata and thus corrupt them.
    • The file is modified.
    • The file now has a new hash value and no longer matches the original and slips through duplicate detection (most programs can’t ignore metadata when comparing).
    • The timestamp of the file is updated so its original time is lost.
• Some programs will write metadata (and the filename and path) to a custom database file.
  • Benefits include:
    • Does not modify files.
    • Can work on any file-system, OS, and file type.
  • Disadvantages:
    • Database must be copied to retain metadata (copying metadata for only some files would require some sort of export function which is rare).
    • Most such programs get tripped up when files are renamed or moved.
• Some programs store metadata in ADS (Alternate Data Streams).
  • This has some advantages:
    • Does not modify files.
    • Can work for any file type.
  • Some disadvantages too:
    • Only works for NTFS volumes and usually only Windows.
    • Metadata can be stripped when copying or uploading files or moving to non-NTFS volumes.

In the case of picture files in Windows 7, adding metadata in Explorer will indeed modify the file and write the data directly to it (and of course update the timestamp). It only lets you tag formats that support a metadata field, but as Julian explained, some programs may not be able to read them, and older files of the formats may not take them well (e.g., a .jpg file from 1994 when the format was still new and metadata was not supported at all). It’s best to make a backup copy before writing a tag.

Photo Gallery specifically uses the IPTC "Keywords" meta tag.

Google Picassa also uses this. IPTC tags are an extension to the data that your digital camera uses to store meta information inside the photo file (like date, exposure, etc.).

So if you are using either of these tools or any of the other tools that make use of EXIF and IPTC (such as the excellent GeoSetter), the data will automatically be backed up if you back up the file.

However, a word of warning. Watch out for image editing utilities that do not understand this data. This used to be a common problem on the iPhone. They may loose the meta-data all together.

For reference, there is an alternative method for storing image meta data called "XMP", originally created by Adobe. With XMP, I believe that the data can either exist within the image file or be held in a matching external file with a .xmp extension.