XMP Specification September 2005 21 3 XMP Storage Model This chapter describes how XMP metadata that conforms to the data model discussed in the previous chapter is stored (serialized) in files. ● XMP properties are serialized as XML, specifically RDF (see “Serializing XMP”, below). ● The serialized data is wrapped in packets for embedding in files. “XMP Packet Wrapper” on page 31 describes the structure and capabilities of these packets. ● Packets are stored in files in a natural manner for each file format specific file formats are discussed in Chapter 5, “Embedding XMP Metadata in Application Files”. ● “External Storage of Metadata” on page 36 describes how to store XMP data in a separate file from the document with which it is associated. All of the XMP about a resource should be stored in a single XMP packet. The XMP Specification does not describe any notion of merging multiple packets about the same resource. A single XMP packet contains information about only one resource. The XMP about distinct resources must be in separate packets. The connection between the XMP and the resource that it is about is typically physical, given by the embedding of the XMP within a file. The XMP specification does not define a general mechanism for making this connection where the XMP is not embedded. Serializing XMP In order to represent the metadata properties associated with a document (that is, to serialize it in a file), XMP makes use of the Resource Description Framework (RDF) standard, which is based on XML. By adopting the RDF standard, XMP benefits from the documentation, tools, and shared implementation experience that come with an open W3C standard. RDF is described in the W3C document Resource Description Framework (RDF) Model and Syntax Specification. The sections below describe the high-level structure of the XML content in an XMP Packet: ● The outermost text is the XML processing instructions and whitespace comprising the XMP Packet wrapper. ● The outermost XML element is optionally an x:xmpmeta element, which contains a single rdf:RDF element (or the rdf:RDF element can be outermost). ● The rdf:RDF element contains one or more rdf:Description elements ● Each Description element contains one or more XMP Properties.