ISO standard
The Extensible Metadata Platform (XMP) is an ISO standard, originally created by Adobe Systems Inc., for the creation, processing and interchange of standardized and custom metadata for digital documents and data sets.
XMP standardizes a data model, a serialization format and core properties for the definition and processing of extensible metadata. It also provides guidelines for embedding XMP information into popular image, video and document file formats, such as JPEG and PDF, without breaking their readability by applications that do not support XMP. Therefore, the non-XMP metadata have to be reconciled with the XMP properties. Although metadata can alternatively be stored in a sidecar file, embedding metadata avoids problems that occur when metadata is stored separately.
The XMP data model, serialization format and core properties is published by the International Organization for Standardization as ISO 16684-1:2012 standard.
Data model.
The defined XMP data model can be used to store any set of metadata properties. These can be simple name/value pairs, structured values or lists of values. The data can be nested as well. The XMP standard also defines particular namespaces for defined sets of core properties (e.g. a namespace for the Dublin Core Metadata Element Set). Custom namespaces can be used to extend the data model.
An instance of the XMP data model is called an XMP packet. Adding properties to a packet does not affect existing properties. Software to add or modify properties in an XMP packet should leave properties that are unknown to it untouched.
For example, it is useful for recording the history of a resource as it passes through multiple processing steps, from being photographed, scanned, or authored as text, through photo editing steps (such as cropping or color adjustment), to assemble into a final document. XMP allows each software program or device along the workflow to add its own information to a digital resource, which carries its metadata along. The prerequisite is that all involved editors either actively support XMP, or at least do not delete it from the resource.
Serialization.
The abstract XMP data model needs a concrete representation when it is stored or embedded into a file. As serialization format, a subset of the W3C RDF/XML syntax is most commonly used. It is a syntax to express a Resource Description Framework graph in XML. There are various equivalent ways to serialize the same XMP packet in RDF/XML.
The most common metadata tags recorded in XMP data are those from the Dublin Core Metadata Initiative, which include things like title, description, creator, and so on. The standard is designed to be extensible, allowing users to add their own custom types of metadata into the XMP data. XMP generally does not allow binary data types to be embedded. This means that any binary data one wants to carry in XMP, such as thumbnail images, must be encoded in some XML-friendly format, such as Base64.
XMP metadata can describe a document as a whole (the "main" metadata), but can also describe parts of a document, such as pages or included images. This architecture makes it possible to retain authorship and rights information about, for example, images included in a published document. Similarly, it permits documents created from several smaller documents to retain the original metadata associated with the parts.
Example.
This is an example XML document for serialized XMP metadata in a JPEG photo:
 ;?xpacket begin="?" id="W5M0MpCehiHzreSzNTczkc9d"? ;
 ;x:xmpmeta xmlns:x="adobe:ns:meta/" x:xmptk="Adobe XMP Core 5.4-c002 1.000000, 0000/00/00-00:00:00 " ;
 ;rdf:RDF xmlns:rdf="[URL] ;
 ;rdf:Description rdf:about=""
 xmlns:xmp="[URL] ;
 ;xmp:CreatorTool ;Picasa ;/xmp:CreatorTool ;
 ;/rdf:Description ;
 ;rdf:Description rdf:about=""
 xmlns:mwg-rs="[URL]
 xmlns:stDim="[URL]
 xmlns:stArea="[URL] ;
 ;mwg-rs:Regions rdf:parseType="Resource" ;
 ;mwg-rs:AppliedToDimensions rdf:parseType="Resource" ;
 ;stDim:w ;912 ;/stDim:w ;
 ;stDim:h ;687 ;/stDim:h ;
 ;stDim:unit ;pixel ;/stDim:unit ;
 ;/mwg-rs:AppliedToDimensions ;
 ;mwg-rs:RegionList ;
 ;rdf:Bag ;
 ;rdf:li rdf:parseType="Resource" ;
 ;mwg-rs:Type ; ;/mwg-rs:Type ;
 ;mwg-rs:Area rdf:parseType="Resource" ;
 ;stArea:x ;0.680921052631579 ;/stArea:x ;
 ;stArea:y ;0.3537117903930131 ;/stArea:y ;
 ;stArea:h ;0.4264919941775837 ;/stArea:h ;
 ;stArea:w ;0.32127192982456143 ;/stArea:w ;
 ;stArea:unit ;normalized ;/stArea:unit ;
 ;/mwg-rs:Area ;
 ;/rdf:li ;
 ;/rdf:Bag ;
 ;/mwg-rs:RegionList ;
 ;/mwg-rs:Regions ;
 ;/rdf:Description ;
 ;rdf:Description rdf:about=""
 xmlns:exif="[URL] ;
 ;exif:PixelXDimension ;912 ;/exif:PixelXDimension ;
 ;exif:PixelYDimension ;687 ;/exif:PixelYDimension ;
 ;exif:ExifVersion ;0220 ;/exif:ExifVersion ;
 ;/rdf:Description ;
 ;/rdf:RDF ;
 ;/x:xmpmeta ;
 ;!-- whitespace padding -- ;
 ;?xpacket end="w"? ;
This metadata describes various properties of the image like the creator tool, image dimension or a face region within the image.
Embedding.
Embedding metadata in files allows easy sharing and transfer of files across products, vendors, platforms, without metadata getting lost. Embedding avoids a multitude of problems coming from proprietary vendor-specific metadata databases.
XMP can be used in several file formats such as PDF, JPEG, JPEG 2000, JPEG XR, JPEG XL, GIF, PNG, WebP, HTML, TIFF, Adobe Illustrator, PSD, MP3, MP4, Audio Video Interleave, WAV, RF64, Audio Interchange File Format, PostScript, Encapsulated PostScript, and proposed for DjVu. In a typical edited JPEG file, XMP information is typically included alongside Exif and IPTC Information Interchange Model data.
Location in file types.
"For more details, the XMP Specification, Part 3 Storage in Files listed below has details on embedding in specific file formats."
Support and acceptance.
XMP Toolkit.
The XMP Toolkit implements metadata handling in two libraries:
Adobe provides the XMP Toolkit free of charge under a BSD license. The Toolkit includes specification and usage documents (PDFs), API documentation (doxygen/javadoc), C++ source code (XMPCore and XMPFiles) and Java source code (currently only XMPCore). XMPFiles is currently available as a C++/Java implementation in Windows, Mac OS, Unix/Linux.
Proprietary tools (read/write support).
The mainstream IPTC Information Interchange Model editing tools also support editing of XMP data.
Licensing.
XMP is a registered trademark of Adobe Systems Incorporated.
The XMP specification became an ISO standard and is not proprietary anymore.
Initially, Adobe released source code for the XMP SDK under a license called the "ADOBE SYSTEMS INCORPORATED — OPEN SOURCE LICENSE". The compatibility of this license with the GNU General Public License has been questioned. The license is not listed on the list maintained by the Open Source Initiative and is different from the licenses for most of their open source software.
On May 14, 2007, Adobe released the XMP Toolkit SDK under a standard BSD license.
On August 28, 2008, Adobe posted a public patent license for the XMP.As of  2016,[ [update]] Adobe continues to distribute these documents under the "XMP Specification Public Patent License".
History.
XMP was first introduced by Adobe in April 2001 as part of the Adobe Acrobat 5.0 software product.
Before that, it was called XAP (Extensible Authoring and Publishing) as internal code name.
On June 21, 2004, Adobe announced its collaboration with the International Press Telecommunications Council. In July 2004, a working group led by Adobe Systems' Gunar Penikis and IPTC's Michael Steidl was set up, and volunteers were recruited from AFP (Agence France-Presse), Associated Press, ControlledVocabulary.com, IDEAlliance, Mainichi Shimbun, Reuters, and others, to develop the new schema.
The "IPTC Core Schema for XMP" version 1.0 specification was released publicly on March 21, 2005. A set of custom panels for Adobe Photoshop CS can be downloaded from the IPTC. The package includes a User's Guide, example photos with embedded XMP information, the specification document, and an implementation guide for developers. The "User's Guide to the IPTC Core" goes into detail about how each of the fields should be used and is also available directly as a PDF. The next version of the Adobe Creative Suite (CS2) included these custom panels as part of its default set.
The Windows Photo Gallery, released with Windows Vista, offers support for the XMP standard, the first time Microsoft has released metadata compatibility beyond Exif.
References.
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