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Page history last edited by GregReser 12 years, 6 months ago


How is metadata stored in image Files?

Each digital file format (JPEG, TIFF, DNG etc.) has its own rules for embedding metadata. 

Within a single format there are several metadata segments and each of those can have multiple containers, each with a unique encoding standard.  Understanding how these standards function with applications and devices is critical to creating interoperable embedded metadata. more 


What are the advantages?

Embedded metadata makes digital images independent, self-defining, sharable and flexible.  

XMP, IPTC and Exif could be a great way to enhance the transfer of metadata, mostly between supplier and end user, but also between large institutional databases.  Because these technologies have such widespread use and backing from camera manufacturers and software designers, they offer us a mechanism to share rich metadata that can be easily (automatically?) read and used.

Benefits for cultural heritage image users:

  • Create image files that are searchable with desktop file browsers and photo applications.
  • Share these tagged images with colleagues and students who can search them too.
  • Download and upload images to a shared workspace while retaining the metadata.
  • Automate user contributions by extracting embedded metadata directly to a database.



What are the disadvantages? 

Embedded metadata has several disadvantages compared to a separate, central database.

  • Maintenance/synchronization of embedded data with external metadata sources.  example
    • Are tools available to easily run and validate batch updates?
    • Different metadata support, different metadata formats at various “levels” of description (collection, series, object, file)
    • Multiple sources of metadata to draw from
    • Complete absence of item-level (or file-level) metadata
    • Unavailability of metadata at time of creation
    • Inability to draw metadata from source
    • Workflow issues: how often to update? How do we know to update embedded metadata if external metadata and image files (and any embedded metadata they contain) are maintained by different custodial groups? 
  • Data corruption
    • Can embedded data be updated without affecting the integrity of the image data itself?
  • Accidental deletion
    • If image files are migrated to a new file format, can seamless metadata migration be guaranteed?
    • If images are not migrated to a new file format and the existing file can’t be opened, then any embedded metadata will likely be inaccessible as well
    • Metadata is dropped during derivative creation or other maintenance activities


What is Exif?

From http://en.wikipedia.org/wiki/EXIF

The Exif tag structure is taken from that of TIFF files. On several image specific properties, there is a large overlap between the tags defined in theTIFF, Exif, TIFF/EP, and DCF standards. For descriptive metadata, there is an overlap between Exif and IPTC Information Interchange Model info, which also can be embedded in a JPEG file.

When Exif is employed for JPEG files, the Exif data is stored in one of JPEG's defined utility Application Segments, the APP1 (segment marker 0xFFE1), which in effect holds an entire TIFF file within. When Exif is employed in TIFF files (also when used as "an embedded TIFF file" mentioned earlier), the TIFF Private Tag 0x8769 defines a sub-Image File Directory (IFD) that holds the Exif specified TIFF Tags. In addition, Exif also defines a GPS sub-IFD using the TIFF Private Tag 0x8825, obviously holding location information, and a "Interoperability IFD" specified within the Exif sub-IFD, using the Exif tag 0xA005.

The derivation of Exif from the TIFF file structure using offset pointers in the files means that data can be spread anywhere within a file, which means that software is likely to corrupt any pointers or corresponding data that it doesn't decode/encode. For this reason most image editors damage or remove the Exif metadata to some extent upon saving.

Exif metadata is restricted in size to 64 kB in JPEG images because according to the specification this information must be contained within a singleJPEG APP1 segment. Although the FlashPix extensions allow information to span multiple JPEG APP2 segments, these extensions are not commonly used.  more


What is XMP?

Adobe's http://www.adobe.com/products/xmp/ Extensible Metadata Platform (XMP) is a labeling technology that allows you to embed data about a file, known as metadata, into the file itself. With XMP, desktop applications and back-end publishing systems gain a common method for capturing, sharing, and leveraging this valuable metadata — opening the door for more efficient job processing, workflow automation, and rights management, among many other possibilities. more


What is IPTC?

IPTC is the photo metadata standard developed by The International Press Telecommunications Council, a consortium of the world's major news agencies, news publishers and news industry vendors.  Adobe added embedded metadata capability to Photoshop in 1991 and included IPTC’s IIM schema in its File Info panels.  In 2004, IPTC worked with Adobe to update IPTC’s schema, for use with Adobe's Extensible Metadata Platform (XMP). This updated version of the schema is called IPTC Core and is the most widely used schema in photo metadata today.


What are sidecar files?

Sidecar “.xmp” Files Stores the settings in a separate file, in the same folder as the camera raw file, with the same base name and an .xmp extension. This option is useful for long-term archiving of raw files with their associated settings, and for the exchange of camera raw files with associated settings in multiuser workflows. These same sidecar XMP files can store IPTC (International Press Telecommunications Council) data or other metadata associated with a camera raw image file. If you open files from a read‑only volume such as a CD or DVD, be sure to copy the files to your hard disk before opening them. The Camera Raw plug‑in cannot write an XMP file to a read‑only volume and writes the settings to the Camera Raw database file instead. You can view XMP files in Adobe Bridge by choosing View > Show Hidden Files.

Important: If you are using a revision control system to manage your files and are storing settings in sidecar XMP files, keep in mind that you must check your sidecar files in and out to make changes to camera raw images; similarly, you must manage (e.g., rename, move, delete) XMP sidecar files together with their camera raw files. Adobe Bridge, Photoshop, After Effects, and Camera Raw take care of this file synchronization when you work with files locally.


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