Description Definition Language
Part of the MPEG-7 content standard
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Introduction
DDL is not a modeling language, such as the Unified Modeling Language (UML), but a schematic language to represent the audiovisual data results, which must conform to the descriptors, the description schemes and the MPEG-7 descriptions.[3]
MPEG-7 DDL Must:
- Be able to express relationships of elements within a DS or between two DS, whether they are structural, spatial, temporal, conceptual or hierarchical.
- Give a rich set of unions and references between one or more descriptions and the data they describe.
- Be application and platform independent.
- Be able to specify the descriptors data type, whether they are primary (integers, text, time, ...) or derived (enumerated, ...).[3]
History
In 1999, the team in charge of MPEG-7 DDL was comparing and evaluating proposals in the MPEG-7 AHG Test And Evaluation Meeting held in Lancaster. The main agreement was that DDL had to use the XML syntax[2], support object-oriented semantics, as well as being able to validate structural, relational and data typing constraints.[1]
Although no proposal satisfied the requirements, the DSTC proposal was used as a starting point, extending it with the additions of ideas and components from other proposals and contributors.[1] Moreover, the strategy was to keep tracking and influencing the W3C community, specially the XML Schema, XLink, XPath and XPointer working groups.[1]
At the 51st MPEG meeting, the adoption of the XML Schema syntax with specific MPEG-7 extensions was decided.[3]
Requirements
As defined in the MPEG-7 requirements document:
- Compositional capabilities: DDL shall allow new DSs and Ds to be created and existing DSs to be modified or extended.
- Unique Identification: A unique identifier should be allowed for each D or DS.
- Primitive Data Types: A set of primitive data type, such as integer, real, date, ..., shall be provided.
- Composite Data Types: Composite data types, as color histograms, RGB values, enumerated types, ..., shall be allowed to be described.
- Multiple Media Types: DDL shall provide a mechanism to relate Ds to data of multiple media types, not necessarily of the same nature.
- Various types of DS instantiations: Full, partial, full-mandatory and partial-mandatory instantiations should be allowed.
- Relationships within a DS and between DSs: DDL shall be able to express spatial, temporal, structural and conceptual relationships between the elements of a DS or between different DSs.
- Relationships between description and data: A rich model for links and references between one or more descriptions and the described data shall be supplied.
- Link to ontologies: A link mechanism between a description and several ontologies shall be supplied too.
- Platform independent: DDL shall be platform and application independent.
- Grammar: DDL's grammar shall be unambiguous and easily parsed.
- Validation of constraints: DDL's parser shall be able to validate: values of properties, structures, related classes and values of properties of related classes.
- Intellectual property management: A mechanism for the expression of Intellectual Property Management and Protection (IPMP), that is what digital rights management (DRM) is known as in the MPEG world, for DSs and Ds.
- Human readability: DDL shall allow DSs and Ds to be read by humans.
MPEG-7 Specific Extensions To XML Schema
They are basic to satisfy the MPEG-7 DDL requirements
- Distribution and datatype matrix
- Written references
- Enumerated datatypes, such as MimeType, CountryCode, RegionCode, Character-SetCode, ...