MPEG-21 Digital Item Declaration (DID)
MPEG doc#: N7422
Date: July 2005
This document provides a brief overview of part 2 of ISO/IEC 21000, MPEG-21 Digital Item Declaration (DID) 
2 Digital Items
A Digital Item is a structured digital object with a standard representation, identification and metadata within the MPEG-21 framework. This entity is the fundamental unit of distribution and transaction within this framework. 
3 Digital Item Declaration Model
Declaring a Digital Item involves specifying the resources, metadata, and their interrelationships for a Digital Item.
Part 2 of ISO/IEC 21000 defines a set of abstract terms and concepts to form a useful model for declaring Digital Items. The goal of this model is to be as flexible and general as possible, while providing for the “hooks” that enable higher level functionality. This, in turn, allows the model to serve as a key foundation in the building of higher level models in other MPEG-21 elements. This model specifically does not define a language in and of itself. Instead, the model helps to provide a common set of abstract concepts and terms that can be used to define such a scheme, or to perform mappings.
A container is a structure that allows items and/or containers to be grouped. A container itself is not an item; containers are groupings of items and/or containers. Descriptors allow for the "labelling" of containers with information appropriate for the purpose of the grouping.
An item is a grouping of sub-items and/or components that are bound to relevant descriptors. These descriptors contain information about the item. Items may contain choices, which allow them to be customized or configured. Items may be conditional (on predicates asserted by selections defined in the choices).
An item that contains no sub-items can be considered a whole. An item that does contain sub-items can be considered a compilation. Items may also contain annotations to their sub-parts.
A component is the binding of a resource to a set of descriptors. These descriptors are information concerning all or part of the specific resource instance. A component itself is not an item; components are building blocks of items. Components may be conditional.
An anchor binds descriptors to a fragment, which corresponds to a specific location or part of a resource. These descriptors are information concerning all or part of the fragment. Anchors may be conditional.
A descriptor associates information with the enclosing entity. This information may be a component (such as a thumbnail of an image, or a text component), or a textual statement. Descriptors may be conditional.
A condition describes the enclosing entity as being optional, and links it to the selection(s) that affect its inclusion. Multiple predicates within a condition are combined as a conjunction (an AND relationship). Any predicate may be negated within a condition. Multiple conditions associated with a given entity are combined as a disjunction (an OR relationship) when determining whether to include the entity.
A choice describes a set of related selections that can affect the configuration of an item. The selections within a choice are either exclusive (choose exactly one) or inclusive (choose any number, including all or none). Choices themselves may be conditional.
A selection describes a specific decision that will affect one or more conditions somewhere within an item. If the selection is chosen, its predicate becomes true; if it is not chosen, its predicate becomes false; if it is left unresolved, its predicate is undecided. Selections themselves may be conditional.
An annotation describes a set of information about another identified entity of the model without altering or adding to that entity. The information can take the form of assertions, descriptors, and anchors.
An assertion defines a full or partially configured state of a choice by asserting true, false or undecided values for some number of predicates associated with the selections for that choice.
A resource is an individually identifiable Asset such as a video or audio clip, an image, or a textual Asset. A resource may also potentially be a physical object. All resources shall be locatable via an unambiguous address.
A fragment unambiguously designates a specific point or range within a resource. Fragment may be resource type specific.
A statement is a literal textual value that contains information, but not an Asset. Examples of likely statements include descriptive, control, revision tracking or identifying information (such as an identifier as described in ISO/IEC 21000-3).
A predicate is an unambiguously identifiable declaration that can be true, false or undecided.
Figure 1 is an example showing the most important entities within this model, how they are related, and as such, the hierarchical structure of the Digital Item Declaration Model.
4 Digital Item Declaration Language
Part 2 of ISO/IEC 21000 also specifies the XML based Digital Item Declaration Language (DIDL). This provides for the standard representation in XML of a Digital Item in the MPEG-21 Multimedia framework. The entities of the DID Model are represented in XML by the elements and attributes of DIDL. In addition a W3C XML Schema definition of DIDL is provided. This schema is also freely available at the ITTF web site. The schema can be used for schema validation of a DIDL document, however it is not sufficient for full DIDL validation since DIDL includes validation rules that are not able to be expressed in W3C XML Schema. The complete syntax and semantics, including the additional validation rules, are specified in the specification itself.
Figure 1 — Example Digital Item Declaration model
This Annex provides a sample DIDL document that has a structure matching that of Figure 1.
<?xml version="1.0" encoding="UTF-8"?>
<title xmlns="http://purl.org/dc/elements/1.1/">Product Specification</title>
<Resource mimeType="text/plain"><![CDATA[Embedded text resource]]></Resource>
 MPEG MDS subgroup, Text of ISO/IEC 21000-2 2nd Edition FDIS, ISO/IEC JTC 1/ SC 29/WG 11/N6927, January 2005, Hong Kong, CN.
 ISO/IEC, Information Technology – Multimedia framework (MPEG-21) – Part 1: Vision, Technologies and Strategy, ISO/IEC TR 21000-1:2004, November 2004.
Presentation of MPEG-21 Digital Items
Author: tiejun Huang
This document provides a brief overview of Presentation of Digital Item which is an extension of MPEG-21 Digital Item Declaration (DID). A Digital Item defined by DID is a structured digital object with a standard representation, identification and metadata. As one of the elements in the DID model, a Descriptor could be employed to associates information with its parent element.
A Presentation element is a specific Descriptor which contains the presentation information for the Digital Item which contains it. Typically it is a scene representation language that can describe the Structured Information of the parent Digital Item. It can be the well-formed XML or just a pointer which points to a remote fragment containing the presentation Information. The Presentation Information can reference the metadata from a Digital Item.
The Presentation uses the mimeType attribute of the Resource element to specify the data type of the Presentation. For example, if the Presentation is described as LASeR XML, the mimeType should be “application/x-laser+xml”; if the Presentation is described as SVG XML, the mimeType should be “image/svg+xml”. An Item element may contain one or more Presentations which can be described by LASeR XML, BIFS and so on.
Accordingly, the MPEG-21 Intellectual Property Management and Protection Components  is extended to cover the new element.
 ISO/IEC, Information Technology – Multimedia framework (MPEG-21) – Part 2: Digital Item Declaration, ISO/IEC 21000-2:2005, Oct. 2005.
 ISO/IEC, Information Technology –Coding of audio-visual objects (MPEG-4) – Part 20: Lightweight Application Scene Representation (LASeR) and Simple Aggregation Format (SAF), ISO/IEC 14496-20:2008, Dec. 2008.
 W3C, Scalable Vector Graphics (SVG) 1.1, Jan. 2003.
 ISO/IEC, Information Technology –Coding of audio-visual objects (MPEG-4) – Part 1: Systems, ISO/IEC 14496-1:2010, Jun. 2010.
 ISO/IEC, Information Technology – Multimedia framework (MPEG-21) – Part 4: Intellectual Property Management and Protection Components, ISO/IEC 21000-4:2006, Apr. 2006.