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Scope of this International Standard

This International Standard specifies a metadata system for describing multimedia content. It specifies the Description Definition Language (DDL) that comprises part 2 of the standard (ISO/IEC 15938-2).

The goal of this part of the MPEG-7 International Standard is to specify a language that will enable MPEG-7 users and developers to:

• create valid MPEG-7 description schemes and descriptors;
• develop tools such as editors and parsers for processing descriptions, description schemes and descriptors;
• generate refinements, extensions and modifications to the DDL.

This International Standard describes the features of the DDL. It defines the syntax of the DDL constructs and datatypes and provides optional (informative) examples that illustrate the application of the DDL to the specification and instantiation of MPEG-7 descriptions.

Overview of Description Definition Language

This International Standard, known as the “Multimedia Content Description Interface”, aims at providing standardized core technologies allowing the description of audiovisual data content in multimedia environments. This is a challenging task given the broad spectrum of requirements and targeted multimedia applications, and the broad number of audiovisual features of importance in such a context. In order to achieve this broad goal, the standard specifies:

• Descriptors (D): representations of Features, that define the syntax and the semantics of each feature representation;
• Description Schemes (DS), that specify the structure and semantics of the relationships between their components, which may be both Ds and DSs;
• A Description Definition Language (DDL), to allow the creation of new DSs and, possibly, Ds and to allow the extension and modification of existing DSs;
• System tools, to support multiplexing of description, synchronization issues, transmission mechanisms, file format, etc.

The DDL forms a core part of the MPEG-7 standard. It provides the solid descriptive foundation through which users can create their own Description Schemes and Descriptors. The DDL defines the syntactic rules to express and combine Description Schemes and Descriptors. According to the definition in the MPEG-7 Requirements Document [1] the DDL is

'...a language that allows the creation of new Description Schemes and, possibly, Descriptors. It also allows the extension and modification of existing Description Schemes.'

The DDL is not a modelling language such as Unified Modelling Language (UML) but a schema language to represent the results of modelling audiovisual data, i.e., DSs and Ds.

The DDL must satisfy the MPEG-7 DDL requirements. It has to be able to express spatial, temporal, structural, and conceptual relationships between the elements of a DS, and between DSs. It must provide a rich model for lnks and references between one or more descriptions and the data that it describes. In addition, it must be platform and application independent and human- and machine-readable.

(Non-normative) DDL Parser applications will be required which are capable of validating description schemes (content and structure) and descriptor data types [both primitive (integer, text, date, time) and composite (histograms, enumerated types)], against the DDL. The DDL Parsers must also be capable of validating MPEG-7 descriptions or instantiations, against their corresponding validated MPEG-7 schemas (DSs and Ds).

The DDL design has been informed by numerous proposals and input documents submitted to MPEG-7 since the MPEG-7 Call for Proposals in October 1998 [2]. It has also been heavily influenced by W3C's XML Schema Language [3,4,5] and the Resource Description Framework (RDF) [6].

At the 51^st MPEG meeting in Noordwijkerhout in March 2000, it was decided to adopt XML Schema Language [3,4,5] as the MPEG-7 DDL. However because XML Schema language was not designed specifically for audiovisual content, certain extensions have been necessary in order to satisfy all of the MPEG-7 DDL requirements.

Hence the W3C's XML Schema Language Recommendations [3,4,5] together with the MPEG-7 extensions described in this document, constitute the normative reference for the MPEG-7 DDL. Overviews of the XML Schema Language structural components and datatypes are given in Annexes A and B for informative purposes only. If Annexes A and B contradict the normative references [3,4,5] then the normative references should be used as the definitive specification.

Structure of this document

The DDL can be broken down into the following logical normative components:

• The XML Schema structural language components;
• The XML Schema datatype language components;
• The MPEG-7 specific extensions.

The complete normative specifications for XML Schema structural and datatype components can be found in [4] and [5] respectively. This International Standard provides the normative specification of the MPEG-7-specific extensions as well as non-normative examples which provide the MPEG-7 community with a clear understanding of the DDL and assist with the development of valid description schemes and descriptors.

The structure of the remainder of this International Standard is as follows:

• Clause 5 describes the normative MPEG-7—specific extensions.
• Clause 6 describes those features of XML Schema which are currently not used by MPEG-7.
• Annex A provides an informative overview of the structural components of XML Schema Language.
• Annex B provides an informative overview of the datatyping mechanisms within XML Schema Language.
• Annex C provides the normative XML Schema which defines the MPEG-7 extensions.
• Annex D contains the Patent Statements.