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Tag Name Syntax

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Robin Mills edited this page Aug 27, 2021 · 8 revisions

Tags in Exiv2 are identified by a 3 part tuple in the syntax Family.Group.TagName. For Example Exif.Image.Make.

The Exif tags are in a hierarchy. The .Group. indicated the IFD in which the tag was found. At the top are tags defined by Adobe in the Tiff_EP specification. The are of the style: Exif.Image.Tag.

Exif defines a tag for the Image section called ExifTag (0x8769). This introduces an IFD in the group Exif.Photo.Tag.

Exif defines a tag called MakerNote (0x927c) which introduces the maker's IFD. Exif.Nikon.TagName are tags in the Nikon MakerNote. There are binary tags in the maker note. For example: Exif.NikonPc.TagName are in the PictureControl" record stored in the Nikon MakerNote.

ExifTagNames

Exif Metadata is stored in a Tiff formatted sub-file of the image. The arrangements for the storage of this sub-file are defined by the file format. The Exif metadata is independent of the file format. The following drawing illustrates the design of a Tiff file.

Tiff File Layout

The IPTC tags are simpler. IPTC a two level structure. There are 9 "sections" such as Envelope and Application2 for which the spec defines the tags.

XMP is a heirarchical tree because it's XML. The syntax of XML tags is handled by (and documented by) the Adobe XMP SDK.

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