XMP Metadata¶
This document is meant as a short 4D Developer’s Guide to XMP Metadata.
The authoritative reference documentation on XMP can be found at Adobe’s XMP Developer Center. Wikipedia also has a good article on XMP, worth reading to get familiar with the platform. So, what is XMP anyway?
- XMP = Extensible Metadata Platform;
- Initially developed by Adobe, now ISO standard;
- The Esperando of metadata: accommodates traditional metadata (Exif, JPEG/TIFF comments, IPTC) in a uniform and extensible format;
- Agnostic of platform and image format.
The XMP Packet¶
XMP metadata in their raw form are organized in a packet. The XMP packet is actually an XML instance (RDF, specifically).
The XMP packet is presented to the application as a hierarchy of properties. The raw form of the packet is also available as RDF text.
XMP Properties¶
XMP properties have the following attributes:
- Namespace
- Name
- Type
Namespace¶
Namespaces are used in XMP just like in XML: to group properties into schemas (Dublin Core, Exif, etc.), and to avoid name collisions.
Each namespace has a unique identifier in the form of a URI, and
a prefix. The use of namespace prefixes makes it much easier to address properties;
it is easier to use "dc:" instead of "http://purl.org/dc/elements/1.1/:".
Name, Prefix and Full Name¶
The property name prefixed with the property’s namespace prefix form its full name.
For example, the full name of the "author" property in the Dublin Core namespace is
"dc:author".
Type¶
The type of a XMP property can be one of these:
| Simple | A simple textual value. Simple properties are always the leaves in the property hierarchy tree. |
| Structure | A collection of properties indexed by name. Similar to 4D’s C_OBJECT‘s. |
| Array | An ordered collection of properties indexed by number (the position in the collection). Similar to 4D arrays. |
| Bag | An unordered array. |
| LangAlt | A collection of text entries indexed by their RFC 3066 language code. Used to store multi-language text values. The |
The root of the property hierarchy is always a structure.
Properties at the root of the hierarchy are addressed by their full name. For example,
use "exif:UserComment" to access the Exif comment property.
Properties nested in the hierarchy are addressed by their full path. For example,
use "exif:Flash/exif:Fired" to access the Exif flash fired property.
Properties contained in Array and Bag containers are addressed by appending the property
index in square brackets to the container’s full name. For example, use
"exif:SubjectLocation[1]" to address the first element of the "exif:SubjectLocation" array.
XMP Schemas¶
Metadata properties are grouped into schemas, identified by their namespace.
The XMP specification includes definitions for a number of schemas commonly used with images and other document kinds. Applications are free to define and implement custom schemas.
A schema defines its properties, their types and containment structure, and how simple properties should be interpreted (metadata on metadata).
A schema classifies the values of its properties into simple, derived, and complex value types. The most common of these types are listed below. For details, see “Part 1, Data model, Serialization, and Core Properties” at Adobe’s XMP Developer Center.
Furthermore, a schema specifies whether a property is external (read editable) or not.
Common Simple Value Types¶
| Text | A possibly empty Unicode string. |
| Boolean | "True" or "False". |
| Integer | A signed or unsigned numeric string used as an integer number
representation. The string consists of an arbitrary length
decimal numeric string with an optional leading "+" or "–" sign. |
| Real | A simple text value denoting a floating-point numeric value, written
using decimal notation of an optional sign followed by an integer part
and a fraction part. Either the integer part or the fraction part,
but not both, may be omitted. The sign, if present, is "+"
or "-". The integer part, if present, is a sequence of one
or more decimal digits. The fraction,
if present, is a decimal point (".") followed by a sequence
of one or more decimal digits. |
| Rational | A simple text value denoting a floating-point numeric value in the
form of a rational number: nominator/denominator. |
| Date | A date-time value in the ISO 8601 format:
Where:
The time zone designator need not be present in XMP. When not present, the time zone is unknown, and the application should not assume anything about the missing time zone. Local time-zone designators |
Common Derived Value Types¶
| Choice | A value chosen from a vocabulary of values. Vocabularies provide a means of specifying a limited and possibly extensible set of values for a property. A choice can be open or closed:
A vocabulary may be just a list of values to be used in the property, for example
the A vocabulary may also contain values that are looked up in a list of more descriptive
texts. As an example, the |
| AgentName | The name of an XMP processor (application). The recommended format is
Organization Software_name Version (token;token;...). For example,
"Adobe Acrobat 9.0 (Mac OS X 10.5)". |
| GUID | A string representing a globally unique identifier. |
| ProperName | A simple text value denoting the name of a person or organization. |
| URI | An Internet Uniform Resource Identifier as defined in RFC 3986. |
| URL | An Internet Uniform Resource Locator. |
Complex Value Types¶
These are collections of properties grouped as members of structure properties, thus defining the hierarchical organization of the schema.
As an example, the "exif:Flash" property is a complex property (a structure)
containing the following simple properties:
"exif:Function" |
Boolean |
"exif:Fired" |
Boolean |
"exif:RedEyeMode" |
Boolean |
"exif:Mode" |
|
"exif:Return" |
|
XMP in Q2Pix¶
Q2Pix utilizes Adobe’s XMP Toolkit SDK combined with facilities provided by system frameworks on each platform, and offers a detailed API and other services for reading, examining, manipulating, and writing XMP metadata in image files.
The plug-in provides methods for:
- Registering XMP namespaces;
- Reading and writing XMP data from/to image files and RDF text;
- Inspecting the XMP property tree;
- Inserting, removing, and modifying XMP properties of any type;
- Copying property subtrees;
- Applying a template XMP data object to another;
- Parsing and converting XMP dates.
Additionally, the Q2XMPLib component provides a generic and extensible XMP editor dialog that can be invoked with a single line of code. The editor is driven by schema definition files (JSON) bundled within the component.
See the Working with XMP metadata section in the Programming Guide for more details, and the documentation for the XMP methods in the plug-in and component API reference.
