Metadata for Describing Multimedia Content (Interactive 3D Content Standards)

Metadata can be used to describe contents and characteristics of data resources. Metadata descriptions permit to more efficiently organize, search, access, and interpret content, and therefore are critical to facilitate content reuse. Different types of metadata may be used for different types of content and for different application areas.

Numerous metadata standards have been developed to enable description of multimedia content [23, 50]. These include metadata for describing still images such as Exif [40], DIG35 [18], and NISO Z39.87 [45]; metadata for describing audio content such as ID3 [43]; and metadata for describing audio-visual content such as P_Meta [46]. There are also generic multimedia metadata standards such as XMP [66] and MPEG-7 [36], which can be used to describe different types of multimedia content. Moreover, there are general purpose metadata description schemes such as Dublin Core [19].

From these standards, only a few are suitable for describing 3D content. These include the general purpose Dublin Core standard as well as the XMP and MPEG-7 standards. In this section, these metadata standards are presented.

Dublin Core

Dublin Core (DC) metadata schema consists of a collection of elements designed to foster consensus across disciplines for discovery-oriented description of diverse resources in an electronic environment. The first element set—Dublin Core Metadata Element Set (DCMES) [61]—was created during metadata workshop, held in Dublin, Ohio USA [21]. Originally, the DCMES was created to enhance searching of document-like objects on the Web. Nevertheless, it can be applied to any kind of electronic resources, e.g., graphics, sounds, and video files. Due to its generic nature and portability, DCMES quickly became a widely used metadata standard for digital library applications. Currently, work on DCMES specification and extensions is lead by Dublin Core Metadata Initiative (DCMI) [20].

The Dublin Core Metadata Element Set [19] has been formally endorsed by ISO Standard 15836-2003 [39] and ANSI/NISO Standard Z39.85-2007 [44]. DCMES is composed of fifteen elements describing basic properties of content (Table 2.4). The DCMES specification defines only URIs, names, and definitions of the metadata elements. It does not provide information about structure of element values nor their syntax.

Since DCMES is intended to support cross-discipline resource discovery, it does not satisfy all metadata requirements of different application domains. Therefore, in most applications it is used in connection with more advanced, domain-specific metadata standards. DC elements are usually a subset of resource metadata and are used as a minimal metadata for data exchange and discovery.

Table 2.4 Dublin Core Metadata Element Set reference (version 1.1)





An entity responsible for making contributions to the resource.



The spatial or temporal topic of the resource, the spatial applicability of the resource, or the jurisdiction under which the resource is relevant.



An entity primarily responsible for making the resource.



A point or period of time associated with an event in the lifecycle of the resource.



An account of the resource.



The file format, physical medium, or dimensions of the resource.



An unambiguous reference to the resource within a given context.



A language of the resource.



An entity responsible for making the resource available.



A related resource.



Information about rights held in and over the resource.



The resource from which the described resource is derived.



The topic of the resource.



A name given to the resource.



The nature or genre of the resource.


XMP—Extensible Metadata Platform—specification [66] has been created by Adobe Systems Inc. [3] to improve management, search and retrieval, reuse, and consumption of multimedia assets. The XMP specification consists of three main components: data model, storage model, and schema definitions. The data model is derived from RDF and it has been designed as a subset of the RDF data model. The data model provides support for metadata properties attached to a resource. Properties have property values, which can be simple or complex (structure or array). Properties also may have properties offering information about the property value. The storage model describes serialization of metadata to the XML format. Metadata serialized in such a way can be attached to various types of digital resources and processed across different systems and applications. A schema is a collection of predefined sets of metadata property definitions. There are some predefined schemas in XMP such as Dublin Core schema, basic schema, rights management schema, and media management schema. The collection of schemas is extensible—it is possible to extend existing schemas and to define new ones.


MPEG-7 [42] is an ISO standard [36] developed by the Moving Picture Experts Group (MPEG) [55]. The MPEG-7 standard, formally named “Multimedia Content Description Interface,” does not target a particular type of multimedia data, but is intended for describing any kind of multimedia content. MPEG-7 standardizes tools that are used to create metadata. These tools include: Descriptors, Description Schemes, Description Definition Language (DDL), and a number of Systems tools.

Descriptor is the basic element of the MPEG-7 metadata. It is used to represent specific features of content. Descriptor specification defines syntax and semantics of feature representation. Examples of descriptors are: a time code for representing duration of a movie, a color histogram for representing image color, and a character string for representing title.

Description Scheme is a structural component of MPEG-7. It defines structure and semantics of relationships between its components, which may be both Descriptors and Description Schemes. With Description Schemes it is possible to describe multimedia content structure and semantics. For example, it is possible to describe a movie, divided into shots, including semantic textual descriptors at the movie level, and technical descriptors such as color, motion, and audio amplitude at the shot level.

Both description tools presented above are represented by using Description Definition Language (DDL). DDL enables creation of new Description Schemes and Descriptors. It permits extending and modifying existing description tools. MPEG-7 DLL is based on W3C XML Schema [58] recommendation. It uses all XML Schema tools and constructs, and adds a few more, such as array or matrix data types.

Systems tools are not related directly to metadata descriptions. They cover the areas of storage and transfer of descriptions, as well as issues related to intellectual property rights.

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