Chapter 2: MPEG-7: The Multimedia Content Description Standard

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2.1 Introduction

MPEG [1], [2], [3], [4] is the ISO-IEC 15983 international standard from the Moving Picture Experts Group (MPEG) for describing multimedia content data. It provides core technologies allowing the description of audiovisual (AV) data content in multimedia environments. Audiovisual data content that has MPEG-7 data associated with it may include still pictures, graphics, three-dimensional models, audio, speech, video, and composition information about how these elements are combined in a multimedia presentation (scenarios).

MPEG-7 allows different granularity in its descriptions, offering the possibility of having different levels of discrimination. Even though the MPEG-7 description does not depend on the (coded) representation of the material, MPEG-7 can exploit the advantages provided by MPEG-4 coded content. For instance, if the material is encoded using MPEG-4, which provides the means to encode AV material as objects having certain relations in time (synchronization) and space (on the screen for video, or in the room for audio), it will be possible to attach descriptions to elements (objects) within the scene, such as audio and visual objects.

MPEG-7 data may be physically located with the associated AV material, in the same data stream or on the same multimedia database system, but the descriptions could also live somewhere else in the Internet. When the content and its descriptions are not colocated, mechanisms that link the multimedia material and their MPEG-7 descriptions are provided; these links have to work in both directions.

MPEG-7 addresses many different applications in many different environments, which means that it needs to provide a flexible and extensible framework for describing AV data. Therefore, MPEG-7 does not define a monolithic system for content description but rather a set of methods and tools for the different viewpoints of the description of AV content.

The MPEG-7 methods and tools are regrouped in eight parts (ISO-IEC 15938-1-8) and are organized as shown in Exhibit 2.1. The parts that are designed for creating a description and delivering it are shown on the righthand side of the figure. The first (Part 2) is the description definition language (DDL) that is the language for defining the syntax of the MPEG-7 description tools and for defining new description schemes. The descriptive elements are defined in Parts 3 to 5, separated by the level of genericity employed. MPEG-7 Visual (Part 3) contains the description tools dealing with (only) visual descriptions. MPEG-7 Audio (Part 4) contains the description tools dealing with (only) audio descriptions. MPEG-7 Multimedia Description Schemes (MDSs, Part 5) contains the description tools dealing with generic features and multimedia descriptions. MPEG-7 Systems (Part 1) provides the binary format for encoding MPEG-7 descriptions for delivery and the terminal architecture. It is drawn as the bottom layer in Exhibit 2.1 to point out its coding and delivery character.

Exhibit 2.1: General organization of MPEG-7.

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Parts 3 and 4 provide a lower abstraction level that may, for example, be shape, size, texture, color, movement (trajectory), and position ("where in the scene can the object be found?") for video, and key, mood, tempo, tempo changes, and position in sound space for audio. The highest level would give semantic information and may be found in Part 5, the MDS. It allows one to represent semantic content similar to, "A soccer video with the goalkeeper on the right and the ball with the player on the left." Note that intermediate levels of abstraction may also exist. In addition to semantic description tools, the MDS also provides a means for describing the creation and production processes of the content, usage, and context. [5]

Part 6, the Reference Software, encompasses the first five parts in the sense that it provides a software implementation of relevant parts of the MPEG-7 Standard with normative status and in that it is publicly available (for more details, see Section 2.9). The final two parts of MPEG-7 are Part 7, MPEG-7 Conformance Testing, which deals with guidelines and procedures for testing conformance of MPEG-7 implementations, and Part 8, MPEG-7 extraction and use of descriptions, which supplies informative material (in the form of a technical report) about the extraction and use of some of the description tools. All parts are international standard; Parts 1 to 6 were published in 2002, and the remaining parts were published in 2003.

MPEG-7 uses Extensible Markup Language (XML) as the language of choice for the textual representation of content description. XML Schema [6] has been the base for the DDL that is used for the syntactic definition of MPEG-7 description tools and that allows extensibility of description tools.

The main elements of the MPEG-7 standard are (see Exhibit 2.2):

  • Description tools: Descriptors (Ds) that define the syntax and the semantics of each feature (metadata element); and description schemes (DSs) that specify the structure and semantics of the relationships between their components that may be both descriptors and description schemes.

  • DDL: Defines the syntax of the MPEG-7 description tools and allows the creation of new description schemes and, possibly, descriptors and the extension and modification of existing description schemes.

  • Classification Schema (CS): Defines a list of typical terms used in many applications together with their meanings. For instance, it allows the definition of file formats in a standardized way. MPEG-7 provides many predefined CSs for characterizing roles, formats, and so forth. New CSs have to be registered.

  • Extensibility: Supported through MPEG-7 schema extensions mechanism (new DSs and Ds).

  • System tools: Support binary coded representation for efficient storage and transmission and provide the necessary transmission mechanisms.

Exhibit 2.2: Main elements of the MPEG-7 standard and their relationship.

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The following sections give a practical guide for the creation and usage of MPEG-7 documents in multimedia systems and databases. Section 2.2 details possible relationships to multimedia database management systems in general. Further examples of the usage of MPEG-7 may be found throughout the book (e.g., in Chapter 4 for retrieval and indexing, in Chapter 5 for distribution). Section 2.3 introduces the principles of creating an MPEG-7 document. Section 2.4 describes the fundamentals of the DDL and provides comprehensive examples on how to use the DDL. The core Section 2.5 guides the user step-by-step to the creation of meaningful MPEG-7 documents. Important parts of the MDS are explained and their conceptual models are presented. Section 2.6 shows how MPEG-7 may be extended to deal with application-specific CSs and DSs. Section 2.7 describes how MPEG-7 documents may be encoded for delivery, and examples are given. The system part of MPEG-7 is also described here. Section 2.8 introduces the audio parts of MPEG-7. Finally, Section 2.9 introduces tools supporting the multimedia annotation process with MPEG-7.

[1]Martínez, J.M., Overview of the MPEG-7 Standard. ISO/IEC JTC1/SC29/WG11 N4980 (Klagenfurt Meeting), July 2002, http://www.chiariglione.org/mpeg/.

[2]Koenen, R. and Pereira, F., MPEG-7 issues, Image Comm., 16(1 and 2) September, 2000.

[3]Chang, S.-F., Puri, A., Sikora, T., and Zhang, H., Special Issue on MPEG-7, IEEE Trans. Circuits Syst. Video Technol., 11, 2001.

[4]Manjunath, B.S., Salembier, P., and Sikora, T., Introduction to MPEG-7, John Wiley & Sons, New York, 2002.

[5]van Beek, P., Benitez, A.B., Heuer, J., Martinez, J., Salembier, P., Smith, J., and Walker, T., MPEG-7: Multimedia Description Schemes, ISO/IEC FDIS 15938-5:2001, International Standard Document, 2001.

[6]Fallside, D.C., XML Schema: W3C Recommendation. W3C Consortium, May 2001, http://www.w3.org/XML/Schema.



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Distributed Multimedia Database Technologies Supported by MPEG-7 and MPEG-21
Distributed Multimedia Database Technologies Supported by MPEG-7 and MPEG-21
ISBN: 0849318548
EAN: 2147483647
Year: 2003
Pages: 77
Authors: Harald Kosch

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