Why Real-World Multimedia Assets Fail to Enter the
Semantic Web
Tobias Bürger ∗
Digital Enterprise Research Institute (DERI)
Technikerstrasse 21a
6020 Innsbruck, Austria
tobias.buerger@deri.at
ABSTRACT
Making multimedia assets on the one hand first-class objects
on the Semantic Web, while keeping them on the other hand
conforming to existing multimedia standards is a non-trivial
task. Most proprietary media asset formats are binary, optimized
for streaming or storage. However, the semantics
carried by the media assets are not accessible directly. In
addition, multimedia description standards lack the expressiveness
to gain a semantic understanding of the media assets.
There exists an array of requirements regarding media
assets and the Semantic Web, already. Based on a critical
review of these requirements we investigate how ontology
languages fit into the picture. We finally analyse the usefulness
of formal accounts to describe spatio-temporal aspects
of multimedia assets in a practical context.
Categories and Subject Descriptors
H.5.1 [Information Systems]: Multimedia Information
Systems; I7.4 [Document and Text Processing]: Electronic
Publishing
General Terms
Multimedia Semantics, Semantic Web
Keywords
Multimedia Semantics, Semantic Web, Multimedia Model,
Requirements Analysis
1. INTRODUCTION
Today a huge explosion of content can be experienced on
the Web generated by, and for the home users [27]: An increasing
number of people produce media assets (as photos,
video clips, etc.), and share them on popular sites as Flickr 1 ,
and YouTube 2 .
∗
Tobias Bürger is also affiliated with Salzburg Research
1
http://www.flickr.org
2
http://www.youtube.com
Michael Hausenblas
Joanneum Research
Steyrergasse 17
8010 Graz, Austria
michael.hausenblas@joanneum.at
More recently, the popular attraction was guided away from
image sharing to richer content sharing of videos. This can
be seen by the launch of video portals like iFilm.com, Ziddio.com
or the dozen of other portals that appeared recently
to compete with YouTube 3 .
Unsurprisingly there is already a portal called VideoRonk 4
trying to combine other portals by providing a MetaSearch
interface, which is quite of an help as one does not want
to search on ten or more different sites. However, what is
missing is the link between the contents of all these sites,
enabling distributed recommendations, cross-linking, etc.
Still, for example a cross-site search on the semantic level
is close to impossible. The most obvious reason is due to
a lack of metadata coming along with all the content. The
power of providing metadata along with content on the Web
can be seen at prospering mashups that not just combine
APIs—provided by parties as Google 5 — but also trying to
mashup things on a semantic level. This can be observed for
example at Joost [34]. Having metadata about everything,
as video content, blog posts, news feeds and the users of the
system makes this new experience of watching TV through
the Internet possible. To take this even one step further:
Would every stream or video available on the Internet be
described more detailed even content on the Internet could
be matched with user profiles from applications like Joost
and could be offered to watch.
As pointed out in [39, 29], high-quality metadata is essential
for multimedia applications. Our recent work within
initiatives [40] and research projects 6 has shown, there is a
need for going beyond current metadata standards to annotate
media assets. Current XML-based standards [21] are
diverse, often proprietary and not ad hoc interoperable; cf.
also [38]. In SALERO, for example, we are facing the problem
to offer a semantic search facility over a diverse set of
multimedia assets, e.g., image, videos, 3D objects or character
animations. The same is true for the Austrian project
GRISINO 7 where we aim to realize a semantic search facility
for cultural heritage collections. Automating the handling
of metadata for these collections and automating linkage between
parts of these collections is hard as the vocabularies to
3
http://www.youtube.com
4
http://www.videoronk.com
5
http://code.google.com/apis/
6
as, e.g., EU project SALERO, http://www.salero.info
7
http://www.grisino.atdescribe them are mostly diverse and do not offer facilities
to attach formal descriptions.
A Motivating Scenario. Imagine a person that wants to
watch the recent clips similar to the ones of his favourite experimental
artist. Tons of clips are potentially distributed
on the Web, which makes searching for them sometimes time
consuming and laborious. Thus a central facility to search
for and negotiate content is needed. This facility should allow
to formulate a search goal, including the characteristics,
the subject matter, a maximum price, and the preferred encoding
and file format of the clip. In a next step, all portal
offerings will be scanned in order to retrieve and negotiate
content that matches the users’ intention. Note that also
parts of a video may match his intention which means that
videos need to be fine granular and sufficiently well enough
described.
In order for this scenario to work, the descriptions of (1) the
goal formulation, (2) the description of the media content by
all content owners and (3) the negotiation semantics have
to be compatible. Three important focal points of these
semantic descriptions are:
• Expressivity for high level semantic descriptions of content
as typical users are not thinking in terms of colour
histograms and spatial / temporal constructs. The
characteristics of the media should be described detailed
enough.
• The need for rules: To effectively identify the part of
the content that matches the users’ intention, rules are
needed to map high level semantic concepts to spatial
and temporal segments of the video (eg., because ratings
and classifications could only apply to parts of the
content, ie., a scene including crime is only suitable for
adults)
• Fine grain semantic descriptions as of bandwidth, user
effort, or cost reason to transfer the whole content is
not possible. Thus parts of the content should be described
detailed enough.
To reach out, we want to provide answers to the question:
Why do we need rich semantic descriptions of media assets
on the Web, and (why) is there a need to bundle these descriptions
together with the multimedia assets? Simultaneous,
we want to provide answers to the questions: How can
descriptions be provided? Why are the metadata features
of multimedia standards not enough?
2. REQUIREMENTS FOR THE DESCRIP-
TION OF MULTIMEDIA ASSETS
Requirements for multimedia content descriptions have been
researched in a number of papers [15, 39, 29, 5] before and
investigations of the combination of multimedia descriptions
with features of the Semantic Web are yet numerous [22,
3, 36, 37, 2]. In the following, we give a summarisation
of the proposed requirements and add two additional ones
(Authoring & Consumption and Performance & Scalability).
Representational Issues. A basic prerequisite is the formal
grounding and neutral representation of the format used
to describe multimedia assets.
• Neutral Representation: The ideal multimedia metadata
format has a platform and application independent
representation, and is both human and machine
processable;
• Formal Grounding: Knowledge about media assets must
be represented in formal languages, as it must be interpretable
by machines to allow for automation.
Extensibility & Reusability. It is requested that the format
at hand is extensible, e.g., via an extension mechanism
as found in MPEG-7. It should be possible to integrate or
reference existing vocabularies [21].
Multimedia Characteristics and Linking. The format should
reflect the characteristics of media assets, hence allow linking
between data and annotations:
• Description Structures. The format should support description
structures at various levels of detail, including
a rich set of structural, cardinality, and multimedia
data-typing constraints;
• Granularity. The language has to support the definition
of the various spatial, temporal, and conceptual
relationships between media assets in a commonly
agreed-upon format;
• Linking. It has to facilitate a diverse set of linking
mechanisms between the annotations and the data being
described, including a way to segment temporal
media.
Authoring & Consumption. A major drawback of existing
metadata approaches is its lacking support for authors
in creating annotations along with the lacking benefits of
generated annotations.
• Engineering support. Appropriate tools are a prerequisite
for uptake of new vocabularies. There is the need
for at least authoring and consumption environments
making use of the vocabularies to demonstrate their
usefulness.
• Deployment. Multimedia Assets need to be exchangeable,
and there must be ways to deploy descriptions
along with the assets.
Performance & Scalability. The language should yield
descriptions that can be stored, processed, exchanged and
queried effectively and efficiently.3. ENVIRONMENT ANALYSIS:
THE SEMANTIC WEB
A good starting point for the analysis of our targeted hosting
environment—the Semantic Web—is the Architecture of
the World Wide Web [23], in which its three main building
blocks are discussed: identification, interaction, and data
formats. The Semantic Web, as an extension of the wellknown
Web roughly has the following characteristics:
• It is a highly distributed system. Identification of resources
is based on URIs—for both data and services;
• There is no single, central “registry”, viz. authorities
are decentralised; data and metadata are under control
of a lot of distinct individuals (companies, standardisation
bodies, private, etc.)
• Alike in the Web a fundamental building block are
relations between data, whereas the relations in the
Semantic Web are named, may be of any granularity
and allow the automatic interchange of data;
• Contribuser 8 inhabit it; each participant may play different
roles at once: consuming content and contributing
via comments, links, etc.
• Finally, there exists a number of standards. As, RDF
allowing formal definitions of the intended meaning,
SPARQL for querying, RDF(S), OWL or SKOS to
classify content and OWL, WSML, or RIF for describing
logical relationships.
Any multimedia metadata format that is after the successful
application on the Semantic Web has to be in-line with the
above listed characteristics. While some requirements, as
formats (e.g. XML) are rather easy to meet, other can pose
serious problems regarding the integration into the Semantic
Web.
4. MULTIMEDIA ASSETS ON THE
SEMANTIC WEB
Firstly, addressing the environmental requirements together
with an efficient layering of the semantic descriptions on top
of the existing metadata (sub-symbolic level - symbolic level
- semantic level) is a necessary prerequisite for multimedia
assets to enter the Semantic Web successfully. Secondly,
from the requirements gathered in section 2 and the environmental
analysis done in section 3 we deduce the following
characteristics for multimedia assets on the Semantic Web:
Formality of Descriptions. Formal descriptions are the
basic building blocks of the Semantic Web. To enable automatic
handling like retrieval, and negotiation of multimedia
assets formality of descriptions is a pre-requisite.
Three different (semantic) levels of multimedia metadata
can be identified [15]: (1) At the subsymbolic layer covering
the raw multimedia information typically binary formats are
used which are optimized for storage or streaming and which
mostly do not provide metadata. (2) The symbolical layer
8
a portmanteau word; contributor and user
provides an additional structural layer for the binary essence
stream. For this level standards like MPEG-7, Dublin Core
or MPEG-21 can be used. The semantics of the information
encoded with these standards are only specified within each
standards framework. (3) Therefore the semantic and logical
layer is needed to provide the semantics for the symbolical
layer. This layer should be formally described.
Efficient layering and referencing of descriptions. It is
necessary to support different levels of meaning attached
to multimedia assets, i.e., meaning at the bit-level, traditional
metadata and semantic (high-level) information. As
there are already widely adopted standards available for the
description of multimedia assets, the semantic layer must
be efficiently put upon those traditional description layers
and should not aim to replace it. Furthermore semantic
descriptions from these traditional layers shall be re-used.
As content, parts of content, and traditional and semantic
descriptions may be distributed efficient referencing mechanisms
for multimedia content must be present.
Interoperability among descriptions. Many formats used
in various communities cause interoperability problems when
dealing with multimedia content. To overcome this, an RDF
based semantic layer should be added on top of these numerous
formats to ease their semantic and syntactic integration.
However, there are some open problems resp. the
integration of existing annotation standards and semantic
approaches [39, 29]: The stack of Semantic Web languages
and technologies provided by the W3C is well suited to the
formal, semantic descriptions of the terms in a multimedia
document’s annotation. But, as also pointed out in [35], the
Semantic Web based languages lack the structural advantages
of the XML-based approaches. Additionally, there is
a huge amount of work already done on multimedia document
annotation within the framework of other standards.
This is why a combination of the existing standards is the
most promising path for multimedia document description
in the near future.
Subjectivity and granularity of descriptions. Opinions
and views of content differ among users because of their
personal background, culture or previous experiences. As
many users are potential contributors to descriptions of assets,
opinions may differ. Many of these opinions sometimes
do not serve to a unique whole opinion. This is why it should
be possible to separately attach these opinions to multimedia
assets and keep them separate.
Trust and IPR issues. The Web consists of decentralized
authorities and a huge number of contribusers. As descriptions
of content—especially in the new changing Web 2.0
environment—are subject to vandalism, there need to be
ways to guarantee the validity of the descriptions and to
secure descriptions that are just read-only for a user group.
Popular portals like Flickr or YouTube show that there is no
need to own content in order to annotate it. Furthermore
copyright is critical when dealing with multimedia content.Functional Descriptions. Sometimes the fact that metadata
is created to support some specific function is forgotten
when summarizing the requirements for a metadata schema.
For the metadata creator it should be clear beforehand for
what purpose the metadata will be used and what benefits
he gains from it [28], ie., using this part of the metadata
scheme enhances retrieval, raises social attention or helps
you protect your assets.
This in turn also applies to the consumer of the metadata,
functional descriptions of what type of information can be
inferred from the attached metadata or what type of actions
can be performed on the content are essential: this is
especially true for information that is obfuscated prior to a
possible negotiation phase of the content.
Engineering Support. The presence of metadata is a prerequisite
to make multimedia assets accessible, and deployable
on the Semantic Web, hence to enable their automated
processing. From a developers perspective, there must be
tools and standards enabling an integrated authoring, testing,
and deployment of multimedia assets along with their
associated metadata. In the following the most important
areas of engineering support are listed:
• Edit & Visualise. To aid the engineer in handling the
annotations, editor tools, and IDEs 9 are needed. These
may include validator services 10 , converter or mapper,
and visualisation modules.
• Libraries & Applications. When developing applications,
the availability of APIs is a core requirement.
In special for Semantic Web applications, interface and
mapping issues are of importance [17].
• Deployment Multimedia containers as HTML, SMIL,
etc. require the metadata either being referenced from
within the media assets, or being embedded into it.
As the data model needs to be RDF—in contrast to
existing, flat (tags, etc.) technologies—upcoming approaches
as RDFa [1] need to be utilised thoroughly.
5. FORMAL DESCRIPTIONS OF MULTI-
MEDIA ASSETS
In this part ontology languages which are thought to be used
for the advanced requirements which were identified in the
sections before. In its core it comprises a a comparison of
two families of ontology languages against the requirements
postulated in section 4.
The reader is invited to note that not all of the existing
languages have the same expressiveness and not all have
the same inferential capabilities. Further, the underlying
knowledge representation paradigms can differ (eg., Description
Logics, Logic Programming, etc.). Corcho and Gomez-
Perez [18] present a framework that allows for analysing and
comparing the expressiveness and reasoning capabilities of
ontology languages, which can be used in the decision process.
The process of choosing and selecting the appropriate
ontology language includes questions like:
9
as for example http://www.topbraidcomposer.com/
10
http://phoebus.cs.man.ac.uk:9999/OWL/Validator
• What expressiveness does an ontology language have?
• What are the inference mechanisms (reasoning capabilities)
of it?
• Are there any supporting tools for that language?
• Is the language appropriate for exchanging ontologies
between applications?
• Are there translators that transform the ontology implemented
in a source into a target language (to enhance
reusability, exchangeability or interoperability)?
We are going to take these questions into consideration and
simultaneously verify if the languages meet the requirements
discussed in section 4.
5.1 Ontology Languages
A number of logical languages have been used for the description
of different kinds of knowledge (i.e. ontologies and
rules) on the Semantic Web: First Order Logic, Description
Logics, Logic Programming and Frame-based Logics. Each
of which allow the description of different statements and
each imply different complexity results for certain reasoning
tasks with these languages.
In this section we want to introduce two of the most promising
ontology language families, ie., the OWL- and the WSMLfamily
of languages. The OWL family of languages is a
standardisation effort of the W3C and the WSML family of
languages is an effort of the WSMO working group, whereas
WSML is a formal language for the description of ontologies
and Semantic Web Services.
5.1.1 Web Ontology Language (OWL) Family
The Web Ontology Language (OWL) family was designed
in a W3C standardisation process because of the need for
an ontology language that can be used to formally describe
the meaning of terminology used in Web documents, thus,
making it easier for machines to automatically process and
integrate information available on the Web. This language
should be layered on top of XML and RDF (W3C’s Resource
Description Framework 11 ) in order to build on XML’s ability
to define customized tagging schemes and RDF’s approach
to representing data.
Currently OWL 1.1 12 is under development; it extends OWL
DL in several ways: the underlying DL now is is SROIQ,
which provides increased expressive power with respect to
properties and cardinality restrictions. Further, OWL 1.1
has user-defined datatypes and restrictions involving datatype
predicates, and a weak form of meta-modelling known as
punning.
The usage of rules in combination with DL has been investigated
for some time [12, 19]—in the Semantic Web stack,
it is expected that a rule language will complement the ontology
layer.
11
http://www.w3.org/TR/rdfprimer/
12
http://owl1_1.cs.manchester.ac.uk/owl_
specification.html5.1.2 The WSML family of languages
The activities of the WSMO Working group 13 have yielded
proposals of new ontology languages, namely WSML (WSML-
Core, WSML-DL, WSML-Flight, WSML-Rule, WSML-Full),
OWL- (”OWL minus”) [6] and OWL Flight [8]. In [14]
unique key features of WSML in comparison of other language
proposals are presented. Compared to OWL key features
include (1) WSML offers one syntactic framework for
a set of layered languages, and (2) it separates between conceptual
and logical modelling. The different variants of the
WSML framework, all having different expressiveness [26],
are:
• WSML-Core corresponds with the intersection of DL
and Horn Logic (without function symbols and without
equality), extended with datatype support in order
to be useful in practical applications. WSML-Core is
fully compliant with a subset of OWL.
• WSML-DL extends WSML-Core to an expressive Description
Logic, namely, SHIQ, thereby covering that
part of OWL which is efficiently implementable.
• WSML-Flight extends WSML-Core in the direction
of Logic Programming. WSML-Flight has a rich set of
modelling primitives for different aspects of attributes,
such as value and integrity constraints. Furthermore,
WSML-Flight incorporates a rule language, while still
allowing efficient decidable reasoning.
• WSML-Rule extends WSML-Flight to a fully-fledged
Logic Programming language, by allowing function symbols
and unsafe rules.
• WSML-Full unifies all WSML variants under a common
First-Order umbrella with non-monotonic extensions
which allow to capture non-monotonic negation
of WSML-Rule.
5.1.3 The Relation of WSML to OWL
The relation of WSML to OWL is presented in [7]: WSML-
Core is a semantic subset of OWL Lite and WSML-DL is
semantically equivalent to OWL DL. A major difference between
ontology modelling in WSML and ontology modelling
in OWL is that WSML separates between conceptual modelling
for the non-expert users and logical modelling for the
expert user as it—unlike OWL—uses an epistemology which
abstracts from the underlying logical language. WSML-
Flight and WSML-Rule are based on the Logic Programming
paradigm, rather than the Description Logic paradigm.
Thus, their expressiveness is quite different from OWL. On
the one hand, WSML-Flight/Rule allow chaining over predicates
and non-monotonic negation, but do not allow classical
negation and full disjunction and existential quantification.
With WSML Logic Programming and the Description Logics
paradigm are captured in one coherent framework whereas
interaction between the paradigms is achieved through a
common subset, WSML-Core.
5.2 Rules
Due to the manifold availability of rule systems, harmonisation
efforts have not been successful so far. A relatively
13
http://www.wsmo.org
new W3C initiative, the Rule Interchange Format Working
Group, is now after defining a core rules language for exchanging
rules. This Rule Interchange Format Core 14 (RIF
Core) language aims at achieving maximum interoperability
while preserving rule semantics; from a theoretical perspective,
RIF Core corresponds to the language of definite Horn
rules. As standardisation is still in its infancy, we will not
go further into detail regarding rules, but one has to note
that the careful integration of ontology languages is an issue
to be addressed; for example the usage of DL concepts in a
rule has to be well-defined.
5.3 Comparing Formal Descriptions Regarding
the Requirements
In the following a high-level comparison of formal description
paradigms for multimedia assets is performed. We chose
OWL+RIF on the one side, and WSML/OWL-Flight on the
other to achieve a somehow realistic scenario; the result can
be found in Table 1 15 .
Requirement OWL 1.1 +
RIF
WSML/OWL-
Flight
Formal Description ++ ++
Layering of Descriptions + +
Interoperability ++ +
Granularity + +
Trust & IPR issues - -
Functional Descriptions - +
Engineering Support ++ +
Datatype Support + ++
Table 1: Comparison of Formal Descriptions for Media
Assets.
In the following, we elaborate in detail on each of the items
in Table 1, and argue therefore our findings regarding the
comparison of OWL 1.1 + RIF vs. WSML/OWL-Flight.
5.3.1 Formal Description
Both OWL and WSML provide a framework for the formal
(machine-processable) descriptions of ontologies. An ontology
in WSML consists of the elements concept, relation, instance,
relationInstance and axiom. The primary elements
of an OWL ontology concern classes and their instances,
properties, and relationships between these instances. The
formality of the descriptions is based on logics that allow
machines to reason on the information. Whereas OWL is
based on Description Logics, the WSML family members
are based on different logic languages (ie. Description Logics,
Logic Programming or First Order Logic).
Despite the fact, that OWL is more widely adopted and
used we believe that WSML with its layered framework is
conceptually superior to OWL. A major difference between
ontology modelling in WSML and ontology modelling in
OWL is that WSML separates conceptual modelling for the
non-expert users, and logical modelling for the expert user
as it—unlike OWL—uses an epistemology, which abstracts
14
http://www.w3.org/TR/rif-core/
15
++ ...good support, + ...available , - ...not supportedfrom the underlying logical language making the surface syntax
nicer. Even if an application later requires OWL, one
is able to use WSML tools to convert ontologies that reside
in popular logic/language fragments automatically into
equivalent OWL ontologies. Furthermore the WSML family
framework enables one to choose exactly which language
with the needed expressiveness is intend to be used, and
later allows an easy switch to another family member because
of its common grounding. WSML Rule and WSML
Flight also include rule-support. Thus, unlike with OWL,
no additional rule language is needed.
5.3.2 Layering of Descriptions
An array of existing multimedia metadata formats have been
used for years in diverse application areas. However, when
one aims at using these formats (as MPEG-7, ID3, etc.) in
the context of the Semantic Web, the options are limited.
Hence, to enable an efficient layering of RDF-based vocabularies
on top of existing multimedia metadata, one may use
hybrid techniques.
As a result of our works in the media semantics area, we recently
proposed the RDFa-deployed Multimedia Metadata
(ramm.x) specification [20]. ramm.x is a light-weight framework
allowing existing multimedia metadata to hook into
the Semantic Web using RDFa [1].
A different but as well Web compatible approach is described
in [25]. There, the authors propose the concept of semantic
documents; semantic documents include any information
regarding the document and its relationships to other
documents. The concept is realised by including XMP descriptions
in PDF documents which can be rendered in any
browser with available plugins. XMP is a format for embedding
metadata in documents using RDF.
5.3.3 Interoperability
To adhere to the architecture of the WWW, OWL uses (1)
URIs for naming and (2) RDF to provide extensible descriptions.
(3) OWL builds on RDF and RDF Schema and adds
additional vocabulary for describing properties and classes.
(4) The datatype support for OWL is grounded on XML
Schema.
WSML has a number of features which allow to integrate it
seamlessly in the Web: (1) WSML uses IRIs 16 [13] for the
identification of resources. (2) WSML adopts the namespace
mechanism of XML, and WSML and XML Schema
datatypes are compatible. (3), WSML has an XML- and
RDF based syntax for exchange over the Web.
To reach compatiability between WSML and OWL, WSML
has a set of defined translators between OWL and WSML
[9, 10].
5.3.4 Granularity
As stated above, when referring to granularity, we understand
the support of the definition of various spatial, temporal,
and conceptual relationships regarding annotations.
In this sense, OWL and WSML meet the minimal requirements,
but do not explicitly address this issue. Depending
16
IRIs are the successors of URIs
on the granularity, obviously scalability and performance issues
come along. In this respect, again, OWL and WSML
can be perceived comparable.
5.3.5 Trust and IPR
In an interdependent, interconnected environment as the Semantic
Web, two important aspects immediately arise: data
provenance and trust [4]. Requirements regarding trust issues
gathered from [30, 16] contain costs and benefits w.r.t.
implementation, technology-driven vs. social networking,
etc.
Both WSML and OWL do not have explicit provisions for
handling trust and IPR issues, respectively.
However, as WSML also is a language capable of describing
different aspects of a Web service, it is capable of describing
so called ’non-functional properties’. Non-functional properties
typically are used to constrain functional—i.e. the formal
specification of what a service can do—and behavioural
aspects, namely how the functionality can be achieved in
terms of interaction of a Web services; they may also be
utilised to specify trust and IPR properties [31].
5.3.6 Functional Descriptions
WSML is a language for the specification of ontologies and
different aspects of Web services. As such it not only provides
means for modeling and description of ontologies but
also functional (service) descriptions, i.e. the description of
a service capability by means of precondition, assumptions,
postconditions and effects [24].
OWL does not have support for such kind of descriptions.
5.3.7 Engineering Support
Tool Support for WSML and especially OWL is constantly
growing. However, the amount of tools available for OWL
[41] and WSML [11] can drastically not be compared. As
OWL is a W3C Recommendation, the support for it is huge.
5.3.8 Data Type Support
The reader is invited to note that both OWL and WSML
ground their datatype support on XML Schema. In WSML,
XML Schema primitive datatypes, simple types and XML
Schema derived datatypes are supported [33]; OWL adopts
the RDF(S) specification of datatypes [32], though some
XML Schema built-ins are problematic.
6. CONCLUSIONS
The first question we kept open is ”What are real-world multimedia
assets”? Real-world multimedia assets are multimedia
objects which can be currently found embedded in
HTML pages on the Web, as images, videos, etc. We see
three main reasons why media assets fail to enter the Semantic
Web:
1. There is a lack of the critical mass of annotated content
which is mainly due to the large scale automation of
(semantic) visual analysis has not gone that far. This
is why the user is the central person in the process in
order to provide manual annotations. Motivating user
to attach complex annotations to content is not easy
to achieve.2. Current traditional and Web 2.0 based approaches to
multimedia annotation are not useful to achieve the
goals of the Semantic Web: The most important aspects
that the Semantic Web intends to solve are (i)
Annotation, (ie., how to associate metadata to a resource),
(ii) Information Integration (ie., how to integrate
information about resources), and (iii) Inference
(ie., reasoning over known facts to unleash hidden
facts).
Existing multimedia metadata standards as MPEG-7
can be used to annotate but keep a certain amount
of ambiguity amongst these annotations. As it is a
standard it allows easy integration based on it (a requirement
for that is that everyone adheres to this
standard!) but inference is not possible with the information
attachable to a MPEG-7 file. The problem
with tagging is manifold: There are big open problems
amongst tagging: (a) How can you guarantee consistency
among tags of different users? (b) How do you
reconcile tags? (c) How do you associate tags with
parts of the tagged content? This huge amount of uncertainty
will not allow reliable information integration,
nor allow to reason on it.
3. As we argued in this paper, more requirements have
to be fulfilled, which can not be solely solved by traditional
or Web 2.0 based approaches and which make
more formalized descriptions of content necessary. However,
before not being able to attach these directly to
the media being described, multimedia assets will not
be able to enter the Semantic Web.
Acknowledgements
The research leading to this paper was partially supported
by the European Commission under contract FP6-027026,
“Knowledge Space of semantic inference for automatic annotation
and retrieval of multimedia content - K-Space”and
SALERO (contract number FP6-027122).
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