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TITRE Adaptive Packet Video Streaming Over IP Networks - LaBRI

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A.3 Object Description Framework<br />

The purpose of the MPEG-4 object descriptor framework is to identify, describe and associate<br />

elementary streams with the various components of an audiovisual scene [22]. A “bootstrap”<br />

stream describes the streaming resources used in a virtual world. A media object is associated to its<br />

elementary stream resources via an object descriptor. The Figure A-4 shows the principle of MPEG-<br />

4 Object Descriptor.<br />

initial<br />

ObjectDescriptor<br />

ES_Descriptor<br />

:<br />

ES_Descriptor<br />

ES_ID<br />

BIFS Update (Replace Scene)<br />

Scene Description<br />

Scene Description Stream<br />

e.g. Audio<br />

Source<br />

e.g. <strong>Video</strong><br />

Object<br />

2D<br />

ES_ID<br />

ObjectDescriptorID<br />

ObjectDescriptorUpdate<br />

ObjectDescriptor<br />

Object Descriptor Stream<br />

Object<br />

Descriptor<br />

ES_D<br />

ES_D<br />

Object<br />

Descriptor<br />

... ...<br />

ES_D<br />

...<br />

...<br />

ES_Descriptor<br />

:<br />

ES_Descriptor<br />

ES_ID<br />

ES_ID<br />

Visual Stream (e.g. base layer)<br />

Visual Stream (e.g. temporal enhancement)<br />

Audio Stream<br />

Figure A-4: MPEG-4 object descriptor structure<br />

A.3.1 Object Descriptor<br />

An object descriptor is a collection of one or more Elementary Stream descriptors that<br />

provide configuration and other information for the streams that relate to a single object (media<br />

object or scene description. Each object descriptor is assigned an identifying number (Object<br />

Descriptor ID), which is unique within the current session. This identifier is used to associate media<br />

objects in the Scene Description with a particular object descriptor, and thus the elementary<br />

streams related to that particular object.<br />

Elementary Stream descriptors include information about the source of the stream data, in<br />

form of a unique numeric identifier (the Elementary Stream ID) or a URL pointing to a remote<br />

source for the stream. ES IDs are resolved to particular delivery channels at the transport layer. ES<br />

Descriptors also include information about the encoding format, configuration information for the<br />

decoding process and the Sync Layer packetization, as well as QoS (quality of service) requirements<br />

for the transmission of the stream and intellectual property identification. Dependencies between<br />

streams can also be signalled, for example to indicate dependence of an enhancement stream to its<br />

base stream in scalable AVO representations, or the availability of the same speech content in<br />

various languages.<br />

A.3.2 Initial Object Descriptor<br />

Initial object descriptors (IODs) serve to access MPEG-4 content. The IOD is an object<br />

descriptor that does not only describe a set of elementary streams, but it also conveys the set of<br />

profile and level information that is needed by a receiver to assess the processing resources needed<br />

for that content.<br />

172

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