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V4Ann: Representation and Interlinking of Atom-based Annotations of Digital Content


Abstract and Figures

There is a great potential in creative industries, such as architecture and video game design, for re-using and re-purposing of digital content. Paintings, archival footage, documentaries, movies, reviews or catalogues, and various other forms of artwork can serve as sources of inspiration and design direction towards innovative designs and new concepts. In this paper, we present V4Ann, an ontology-based framework for semantically representing, aggregating and combining annotations (atoms) coming from visual and textual analysis of digital content. The aim is to structure and link data in such a way so as to facilitate the systematic process, integration and organisation of information and establish innovative value chains and end-user applications. The framework is part of the V4Design platform that aims to re-use and re-purpose existing heterogeneous multimedia content by semantically enriching and transforming assets into a 3D representation, so as to inspire and support the design, architecture, as well as 3D and VR game industries.
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V4Ann: Representation and Interlinking of
Atom-based Annotations of Digital Content
Georgios Meditskos, Stefanos Vrochidis, and Ioannis Kompatsiaris
Information Technologies Insititute
Centre for Research and Technology Hellas, Thessaloniki, Greece
{gmeditsk, stefanos, ikom}
Abstract. There is a great potential in creative industries, such as ar-
chitecture and video game design, for re-using and re-purposing of digital
content. Paintings, archival footage, documentaries, movies, reviews or
catalogues, and various other forms of artwork can serve as sources of
inspiration and design direction towards innovative designs and new con-
cepts. In this paper, we present V4Ann, an ontology-based framework
for semantically representing, aggregating and combining annotations
(atoms) coming from visual and textual analysis of digital content. The
aim is to structure and link data in such a way so as to facilitate the
systematic process, integration and organisation of information and es-
tablish innovative value chains and end-user applications. The framework
is part of the V4Design platform that aims to re-use and re-purpose ex-
isting heterogeneous multimedia content by semantically enriching and
transforming assets into a 3D representation, so as to inspire and support
the design, architecture, as well as 3D and VR game industries.
Keywords: Annotations ·Ontologies ·Reasoning ·Semantic enrich-
ment ·Multimodal data
1 Introduction
Vast amounts of multimedia content is being produced, archived and digitised,
resulting in great troves of data of interest. Examples include user-generated
content, such as images, videos, text and audio posted by users on social media
and wikis, or content provided through official publishers and distributors, such
as digital libraries, organisations and online museums. This digital content can
serve as a valuable source of inspiration to the cultural and creative industries
to produce new assets or to enhance and (re-)use the already existing ones.
However, the re-use and re-purposing of digital content is mainly realised
based on individual designers skills and a variety of non-interlinked heteroge-
neous tools. To this end, the content remains largely under-exploited, despite its
great potential for re-use and re-purpose, due to the lack of appropriate solutions
for its retrieval and integration into the design process. For example, existing
heterogeneous multimedia content, such as video and images of buildings and
2 G. Meditskos et al.
objects, can be collected and transformed (e.g. into 3D models1), so as to inspire
and support the creation of new content in creative industries. One of the main
challenges in this area is to maximise the potential for re-purposing of digital
content through the development of innovative technologies to systematically
analyse, combine, link and foster searchability and reusability of heterogeneous
multimedia content in different contexts.
In this paper we describe V4Ann, an ontology-based framework for capturing
and interlinking digital assets and duly annotations at two levels: a) content
analysis level, during which visual and textual content is analysed to extract
labels, called atoms; and b) retrieval and repurposing level, where the assets (e.g.
3D models and images) are interlinked and contextually enriched to facilitate
their discovery. At the content analysis level, V4Ann provides the conceptual
structures to capture and interlink multimedia analysis results on digital content,
such as video, image and text. During retrieval and repurpose, V4Ann provides
practical retrieval capabilities, allowing users, e.g. game designers, to search for
assets relevant to their needs. V4Ann is part of the V4Design platform2, enriching
multimedia processing with a semantic annotation layer.
The contribution of our research can be summarised in the following:
We describe a resource annotation model that implements the W3C standard
for defining annotations (Web Annotation Data Model [17]).
We define a core set of rules that perform valid inferences for annotation
propagation and interlinking, as well as for validity checking.
We propose an atom similarity metric along with a searching algorithm for
keyword-based digital asset retrieval.
The rest of the paper is structured as follows: Section 2 presents related work.
Section 3 gives an overview of the framework and presents our motivation. In
Section 4 we describe the basic concepts of the V4Ann annotation model, while
in Section 5 we elaborate on the inference and validation capabilities. Section 6
describes the atom similarity metric and the searching functionality. In Section
7 we present evaluation results and, finally, in Section 8 we conclude our work.
2 Related Work
Annotations are typically used to convey information about a resource or as-
sociations between resources. Simple examples include a comment or tag on a
single web page or image, video or a blog post about a news article. In 2017, the
Web Annotation Data Model (WADM) [17] became the W3C recommendation
for defining annotations. It provides an extensible, interoperable framework for
expressing annotations, such that they can easily be shared between platforms3.
In the domain of digital libraries, the Europeana Data Model (EDM) [4]
adopts an open and scalable approach that can accommodate the range and level
V4Ann: Representation and Interlinking of Atom-based Annotations 3
of details of particular standards, such as LIDO for museums, EAD for archives
or METS for digital libraries. EDM is not built on any particular standard,
however it is conceptually in line with WADM and the ORE4initiative.
The Open Provenance Model (OPM) [11] enables to specify what caused
“things” to be, i.e., how “things” depended on others and resulted in specific
states. In essence, it allows provenance information to be exchanged between
systems, by means of a compatibility layer based on a shared provenance model.
OPM predates PROV-O [9], and has a very similar approach to modelling prove-
nance by relating agents, artifacts and processes and the concepts of OPM are
covered by equivalent PROV-O concepts. PAV [3] extends PROV-O and specifies
Provenance, Authoring and Versioning information.
The Dublin Core metadata (DCMI) standard5is a simple yet effective general-
purpose set of 15 elements for describing a wide range of networked resources.
Although DCMI favors document-like objects, it can be applied to other re-
sources as well. The SKOS Core Vocabulary [10] is a model for expressing the
basic structure and content of concept schemes. Specifically for multimedia, the
Ontology for Media Resources6was developed by the W3C Media Annotations
Working Group to identify a minimum set of core properties to describe and
retrieve information about media resources. VidOnt [18] provides a formally
grounded core reference ontology for video representation. Several attempts have
been made to map the XML Schema of MPEG-7 to RDFS and OWL [19] and
X3D to OWL (OntologyX3D [6]) and the 3D Modeling Ontology (3DMO7).
V4DAnn aims to serve as the semantic annotation layer of multimedia pro-
cessing results for fostering data exchange among analysis services and for hu-
man consumption. In order to promote interoperability and extensibility, it im-
plements the WADM pattern, introducing the concept of atoms and providing
several annotation entities and properties. In contrast to existing models that
mostly focus on metadata defined by data providers and curators, V4Ann aims
to capture content analysis results (e.g. visual and textual analysis), serving as a
semantic middleware for metadata exchange. For example, EDM views refer to
digital representations, whereas in V4Ann a view represents an atom-based in-
terpretation of a content analysis procedure, e.g. aesthetics extraction. However,
V4Ann provides alignments to conceptual structures of existing models, such as
the EDM, ORE and SKOS (see Section 4 for more details).
As far as semantic enrichment and retrieval are concerned, recent advances
in machine learning and especially deep learning have provided us with tools
like word representations (e.g. word2vec [20] and Glove [14]), which led to the
development of more recent and powerful analysis models [15]. In addition, sev-
eral approaches have been proposed for question answering over Semantic Web
knowledge bases and Linked Data. Most of them generate one or more queries,
while others opt for graph-based approaches to mitigate the rigidness often en-
4 G. Meditskos et al.
digital libraries
online museums
drone footage
news, articles
& reviews
objects, buildings
text analysis
building, object
3D design
Content Analysis User Search
3D model
Fig. 1. The position of V4Ann in the integrated V4Design platform.
tailed in formulating appropriate SPARQL queries. Examples include EARL [5]
and VoxEL [16]. V4Ann aims to provide a practical context enrichment frame-
work to facilitate basic asset discovery, rather than proposing a fully fledged
question answering framework. To this end, it introduces the notions of atom
similarity and local contexts.
3 Key Concepts and Motivation
In a world where visual and textual data are in abundance, creative industries
need to re-use and re-purpose them so as to remain competitive to other indus-
tries and provide to society and creativity a novel financial prism. V4Design is
an H2020 project that aims at exploiting state-of-the-art digital content analy-
sis techniques to generate 3D models, extract aesthetic and stylistic information
from paintings and videos, localise buildings and objects of interest within visual
content, and integrate it with textual information so as to inspire and support
the design, architecture, as well as 3D and VR game industries.
V4Ann aims to enrich V4Design with a semantic annotation layer. From
one hand, V4Ann acts as the semantic middleware, capturing, interlinking and
serving analysis results to multimedia analysis services. On the other hand, it
provides the semantic atom-based query infrastructure to retrieve generated as-
sets. The conceptual architecture of V4Design, along with the position of V4Ann,
is depicted in Fig. 1. All in all, V4Ann aims to address the following challenges:
V4Ann: Representation and Interlinking of Atom-based Annotations 5
Annotation propagation and linking: In a multimodal content analysis set-
ting, like in V4Design, a single media type can be analysed by multiple
technologies. For example, an image can be used for extracting building
masks, as well as for aesthetics (style) extraction. Also, in many cases, there
are interdependencies among the components, e.g. 3D model reconstruction
needs as input video frame masks extracted by building localisation. It is
important to have an efficient and interoperable way to represent, exchange
and further link metadata, both structurally and semantically.
Context-aware retrieval: V4Design aims to create new multimedia content
that can be integrated in existing architecture and video game design plat-
forms, such as Unity8and Rhino9. Therefore, there is a need for practical
and efficient retrieval mechanisms on top of the multimodal annotations.
For example, to allow users to search for assets with certain styles or with
advanced contextual filters, such as “castles near lakes”.
In order to address the aforementioned challenges, V4Ann capitalises on and
combines existing Semantic Web standards for resource annotation and inter-
linking, inference and validation. More precisely, the WADM model is used as
the core resource annotation pattern, combined with existing structured ontolo-
gies and schemata (Section 4). SPIN rules [7] and SHACL shapes [8] are used to
derive additional relations among the annotated resources and for validating the
generated knowledge graphs (Section 5). Finally, keyword-based context-aware
retrieval is facilitated to retrieve assets (Section 6).
4 V4Ann Annotation Model
Fig. 2 illustrates the upper-level concepts of the V4Ann annotation model. The
conceptual model revolves around the notions of annotations,media types,views
and atoms. Annotations serve as resource containers, implementing the annota-
tion pattern of WADM. Each annotation associates a media type (image, video,
text, 3D model) with a view, which encapsulates a set of atoms. Each view de-
fines one or more atoms, e.g. entities, tags, styles, etc. that are derived from
multimedia content analysis. These atoms describe: a) Aesthetics, i.e. architec-
tural styles and creators that are extracted from images and videos; b) Object
and building types that are recognised in images and videos; c) Named entities
and concepts that are extracted from textual descriptions, e.g. image captions;
d) images and video frames used to reconstruct a 3D model. All atoms derived by
aesthetics, localisation and text analysis are disambiguated, i.e. they are already
mapped to WordNet, BabelNet or DBpedia resources by the content analysis
services. Fig. 2 also presents SKOS mappings to the ORE specification, as well
as subclass and subproperty relations to WADM and EDM. In the following we
describe in details each key concept.
6 G. Meditskos et al.
Fig. 2. The core concepts of the V4Ann annotation model defined as specialisation
of WADM (oa namespace). Mappings to other models are also depicted, such as to
Europeana Data Model (EDM) and Object Reuse and Exchange (ORE) initiative.
4.1 Annotation resources
Four domain-specific annotation classes are defined for attaching atom views to
media types10:LocalisationAnnotation,TextualAnnotation,Aesthetics-
Annotation and 3DModelAnnotation. According to the WADM specification,
an annotation has 0 or more bodies (oa:hasBody), which encapsulate descriptive
information, and a 1 or more targets (oa:hasTarget) that the bodies describe.
V4Ann defines two subproperties to restrict the values of these properties, asso-
ciating the targets (i.e. the media types) with view atoms. Intuitively, a V4Ann
annotation has a context that describes amedia type using views. In terms of
OWL 2 semantics, the hasContext (voa:hasBody) property takes as values
only instances of the View class and the describes (voa:hasTarget) property
at least one MediaType value. The Annotation class is defined as:11
Annotation voa:Annotation u
describes.MediaType u ∀hasContext.View (1)
4.2 Media types
In order to define the targets of annotations (describes property assertions),
V4Ann provides the MediaType upper-level class. There are four media types for
annotations: Video,Text,Image,Mask vImage,Texture vImage and 3DModel.
Each media type can be associated with additional descriptive information, such
10 In the rest of the paper, we omit the v4d namespace.
11 We use Description Logics [2] to represent the semantics.
V4Ann: Representation and Interlinking of Atom-based Annotations 7
as the source of the asset (e.g. the URL), license information, date of retrieval,
etc. Intuitively, each media type resource represents a single multimedia asset
for which a set of annotation atoms needs to be captured.
4.3 Views and atoms
Views are container classes that encapsulate the annotation metadata (atoms)
and they are used in hasContext property assertions. Each media type has a dif-
ferent view. For example, the atoms of spatio-temporal building (BuildingView
vView) and object localisation (ObjectView vView) in images and videos
specify their type, i.e. whether the image or video contains a building, object
or a painting. The semantics of OWL 2 allows us to define useful complex class
descriptions to specify further dependencies, as described below. It should be
noted that content analysis is not part of the V4Ann framework. As described
in Section 3, V4Ann aims to semantically capture the results of content analysis,
which is part of the overall V4Design platform [1].
Aesthetics Aesthetics extraction refers to the categorisation of the aesthetics
of paintings and images that contain architecture objects and buildings based on
their style (e.g. impressionism, cubism and expressionism), the creator (mainly
for paintings) and emotion that they evoke to the viewer. Two properties are
defined for creators (v4d:creator schema:creator) and styles (v4d:style),
whose domain is the v4d:AestheticsView class.
AestheticsAnnotation voa:Annotation u
describes.{Image tVideo}u∀hasContext.AestheticsView (2)
AestheticsView v ∀creator.Creator u ∀style.Style (3)
The Creator and Style classes serve as container classes, allowing the cap-
turing of data-specific properties, such as the classification confidence, as well as
contain links to DBpedia and BabelNet. Fig. 3 presents an aesthetics annotation
example (left part). The image depicts the Tholos of Delphi that has been given
the atom (style) “Greek Architecture”.
Object and Building Localisation Building and interior objects localisation
on art and architecture-related movies, documentaries and multiple art-images,
aims to extract content that can be re-purposed and re-used in a meaningful and
innovative way. Examples include buses, trains, as well as statues, buildings, etc.
The extracted atoms (labels) are mapped to the V4Ann annotation model
in terms of generated masks and tags. In videos, the results are also associated
with frame(s) to capture the temporal aspects of localisation.
LocalisationAnnotation voa:Annotation u
describes.{Image tVideo}u∀hasContext.LocalisationView (4)
LocalisationView v ∃hasTag.Tag u ∀hasFrame.integer (5)
8 G. Meditskos et al.
Text Analysis Text analysis provides the atoms that are derived from tex-
tual content. For example, in addition to annotating images with building and
objects, the assets are further enriched with named entities and concepts ex-
tracted from captions, titles and descriptions. V4Ann captures these atoms and
associate them with the media type (video or image) that the textual content
is relevant to through instantiations of the TextAnalysisView class. Example
atoms include name,title,date,creator,designer,artist,location, etc.,
defined as subproperties of Tag.
TextAnnotation voa:Annotation u
describes.{Image tVideo}u∀hasContext.TextView (6)
TextView v ∃hasTag.Tag (7)
3D Reconstruction 3D reconstruction converts input video and images into
3D point clouds and meshes. Apart from the actual 3D object, this process
generates a variety of metadata, such as the number of point clouds, number of
faces, textures, etc. The most important atom is the source of reconstruction,
i.e. the video or the images the 3D model has been extracted from.
3DModelAnnotation voa:Annotation u
describes.3DModel u ∀hasContext.3DModelView (8)
3DModelView v ∃hasSource.{Images tVideo}(9)
A 3D annotation example is depicted in Fig. 3 (right part). The annotation
of the 3D model of Tholos is associated (image vhasSource) with the images
that have been used for the reconstruction. It is assumed that the example image
for aesthetics extraction is part of the set, demonstrating the way multimodal
analysis results are interlinked. As we describe in the next section, these links
are used to materialise additional relationships in the form of inference rules.
5 Inference and Validation
5.1 Implicit Relationships
Additional inferences are derived by combining native OWL 2 RL reasoning and
custom rules. The former is based on the OWL 2 RL profile semantics (OWL 2
RL/RDF rules [12]), which is implemented by state-of-the-art triple stores, such
as GraphDB. However, the semantics OWL 2 is limited. For example, only in-
stances connected in a tree-like manner can be modelled [13]. V4Ann implements
domain rules on top of the graphs to express richer relations. SPARQL-based
CONSTRUCT graph patterns are used that identify the valid inferences that can be
made on the annotation graphs. It is beyond the scope of the paper to include an
extensive coverage of relevant reasoning capabilities. In the following we present
the concept of atom propagation that illustrates the principle idea.
V4Ann: Representation and Interlinking of Atom-based Annotations 9
Fig. 3. Example of atom propagation. The dashed arrow illustrates the enrichment of
the 3D annotation resource with the aesthetics style derived from visual analysis.
Since V4Ann follows a standard-based annotation pattern, additional rela-
tions can be further derived. For example, the aesthetics atoms extracted from
video frames can be used to annotate the 3D models that have been recon-
structed using those frames. The principle idea is that atoms can be propagated
among one or more views, provided that their annotations are associated.
Fig. 3 illustrates atom propagation between an aesthetics and 3D model
annotations. The two annotations are connected at the view level, since the
aesthetics annotation describes an image (img 1) that has been used to generate
the 3D model of Tholos (id 3d 1). In this case, the view that describes the
3D model inherits the atom (style) of the image (Greek Architecture). The
corresponding SPARQL graph pattern is given bellow.
?view :style ?atom .
?a1 a :AestheticsAnnotation;
:describes ?img; :hasContext [:style ?atom] .
?a2 a :3DModelAnnotation; :hasContext ?view .
?view :image ?img .
5.2 Validation and Consistency Checking
The validation process checks the consistency, structural and syntactic quality
of the metadata. We use both native ontology consistency checking (e.g. OWL
10 G. Meditskos et al.
2 DL reasoning) and custom SHACL validation rules, following the closed-world
paradigm. The former handles validation taking into account the semantics at the
terminological level (TBox), e.g. checking class disjointness. The latter detects
constraint violations, e.g. missing values and cardinality violations. An example
SHACL shape is given below that represents a constraint that all 3D model views
should include references to the atoms (images) used to the 3D reconstruction.
rdf:type sh:NodeShape ;
sh:property [
rdf:type sh:PropertyShape ; sh:path v4d:image ;
sh:class v4d:MediaType ; sh:minCount 1 ;
sh:name "one or more images" ; sh:nodeKind sh:IRI ;
] .
6 Context-based Asset Retrieval
In Section 4 we described the process of creating the V4Ann annotation graphs,
which involves the representation and further interlinking (e.g. through annota-
tion propagation) of media type atoms. In this section we describe the approach
of V4Ann towards enabling keyword-based context-aware retrieval of assets, cap-
italising on the concept of local context.
Definition 1. The local content ltof an atom tis defined as the tuple ht, rt, het, hoti,
where rtis the set of conceptually relevant terms, hetis the set of hypernyms
and hotis the set of hyponyms of t.
Intuitively, a local context of an atom constitutes an enriched, pre-constructed
semantic signature of this atom, taking into account conceptual and lexical rela-
tions from existing semantic networks and datasets, such as WordNet, BabelNet
and ConceptNet (Fig. 4). In the case of hypernyms and hyponyms, we use the
threshold hto specify the maximum level of relevant atoms. All in all, the re-
trieval mechanism of V4Ann aims to match incoming local contexts of query
atoms (keywords) against local contexts of annotation atoms.
6.1 The AH Metric
The AH metric represents the similarity of two atoms taking into account their
local context. It depends on a term similarity function S, and on a set Fof local
context filters. In the following, we assume that S(A, B) denotes the similarity
of two atoms Aand B, with respect to the function S, and that S(A, B)[0..1],
with 1 denoting absolute match. We use the notation Af
Bto denote that A
matches to B, with respect to one of the following filters f:
1. exact (e). The two atoms should have either the same URI, or they should
be equivalent concepts, that is, Ae
V4Ann: Representation and Interlinking of Atom-based Annotations 11
Fig. 4. a) Generic local context of atom: relevant atoms are extracted from ConceptNet
and BabelNet properties, hypernyms stem from WordNet and IS-A BebelNet relation-
ships, hyponyms stem from WordNet; b) example local context for “Gendarmenmarkt”.
2. plugin (p). The atom Bshould belong to the set of hypernyms of A(heA) or
to the set of relevant concepts of A(rA), that is, Ap
3. subsume (su). The atom Bshould belong to the set of the hyponyms of A,
that is, Asu
We generalize the Af
Brelation to a set of filters Fand we define that the
atom Amatches the atom B, with respect to a filter set F, if and only if there
is at least one filter fin F, such that Af
B, that is:
B⇔ ∃fF:Af
Definition 2. The AH similarity of two atoms Xand Yis the normalized value
to [0..1] that is defined, with respect to a function Sand a filter set F, as
AH(X, Y, F ) = (S(X, Y )if XF
We generalize (10) on two sets SA,SBof atoms as
AHset(SA, SB, F ) = X
Intuitively, for each atom BSBthere should be at least one atom A
SArelevant to B, with respect to the filter set F. Otherwise, AHset returns
0 (absolute mismatch). The overall AHset similarity is computed as the mean
value of the sum of the maximum AHs for each atom B, since each Bmay have
more than one relevant atoms in SA. In V4Ann, SArepresents the atoms that
are associated with an asset, while SBis the set of keywords.
12 G. Meditskos et al.
6.2 Atom Similarity S
As a similarity function S(A, B), V4Ann uses a heuristic function that takes into
account the information captured in local contexts of Aand B, i.e. in the sets
r, he and ho (see Definition 1). The implementation of Sis summarised in the
following priority rules ri, where r1> r2> r3> r4.
r1:if A=BAB, then S(A, B) = 1.
r2:if BhyABrA, then S(A, B) = a.
r3:if BhoA, then S(A, B) = b.
r4:S(A, B) = 0.
Currently, aand b(a > b) are defined manually based on domain knowledge
regarding the quality of multimedia analysis that produces the atoms (e.g. aes-
thetics extraction). The empirical definition of these values (currently a= 0.7
and b= 0.3) aims to promote plugin matches (r2) over subsumed (r3).
7 Evaluation and Discussion
7.1 Digital Content
Deutsche Welle (DW) and Europeana are two key content providers in V4Design.
DW provides selected parts of their documentary and movie archives so as to
localise building structures and objects. Europeana provides their large archive
of paintings, pictures of contemporary artwork and related critics, for stylistic
and aesthetics extraction and textual analysis. The generated V4Ann annota-
tion graphs contain the atoms that have been extracted from the analysis com-
ponents, along with interconnections among the annotation resources. Table 1
provides some statistics for the annotation graphs.
7.2 Evaluation
User-centred A user-centred evaluation has been performed with a twofold
purpose. First, to collect qualitative feedback on the results, as well as on non-
functional aspects, such as query response time. Second, and most important,
to generate an annotation dataset and assess the performance of V4Ann.
Participants were invited to evaluate the current implementation by perform-
ing keyword-based queries. A list of relevant resources has been provided, such as
square names, monuments, building types, etc., in order to help them conduct
relevant queries. Users filled in a five-point scale questionnaire (1-completely
agree, 5-completely disagree). Sample questions are depicted in Table 2. The
feedback can be summarised as it follows:
Table 1. The number of annotations and atoms in the V4Ann annotation graphs,
along with the average size of local context for each atom (r+hy +ho).
#annotations #atoms avg. local context size
17245 154610 17 per atom
V4Ann: Representation and Interlinking of Atom-based Annotations 13
Table 2. Example questions answered by users.
#Question Mean (SD)
Q1 Atoms that are derived from visual analysis are most of
the time correct
1.7 ±0.83
Q2 Atoms that are derived from text analysis are most of the
time correct
2.34 ±0.79
Q3 Many times irrelevant results are top-ranked 3.97 ±1.29
Q4 There are many irrelevant results 2.43 ±1.41
Q5 It takes too long for the system to provide a response 4.04 ±0.99
Q6 There are too many “No results” responses 4.08 ±0.45
Quality of atoms: The quality and relevance of local contexts depends on
the performance of content analysis, e.g. visual and textual analysis. Table
2 shows that visual analysis provides, in principle, better results than text
analysis (Q1, Q2).
Retrieval results: According to Q3, the system achieves good top-ranked
accuracy, however the complete set of the results contain quite a lot irrelevant
entries (Q4). As we explain in the next section, this is mainly relevant to the
context provided in the query (i.e. number of keywords). Due to the local
context, the system was able to provide a response in most cases (Q6), even
partially correct (Q4).
Response time: The response time of the system was positively assessed
(Q5). The average response time was 4.1 seconds, which includes query anal-
ysis, building of local context and searching algorithm execution.
System evaluation We manually annotated the relevance sets of the performed
queries, so as to quantitatively assess performance. Table 3 depicts the average
precision and recall achieved for h= 1 and h= 3 and using different searching
filters (Section 6.1). As expected, the stricter the filter is, the more accurate
results we obtain (high precision) with low, however, recall. On the other hand,
the more relax is the filter, the higher recall is achieved with a negative impact
on the precision. This is due to the fact that with a strict filter (i.e. exact),
the probability of finding the correct annotation is higher compared to a relaxed
filter (i.e. subsume), since in the second case, impartial matches are also allowed.
It should be noted that the overall performance of V4Ann strongly depends
on the quality of the atoms, which in turn depends on the quality of the results
provided to V4Ann. For example, if the wrong style for a painting is provided by
aesthetics, this will affect precision, since V4Ann does not aim at improving the
classification of incoming atoms. However, we plan to integrate multimodal data
aggregation and fusion techniques to derive the most plausible classification of
atoms and help improve the contextual information captured in local contexts.
Another interesting finding involves the threshold h. We observed that for
h= 1 the framework provides better results than using h= 3, i.e. by enriching
14 G. Meditskos et al.
Table 3. Average precision and recall (top-20 results).
h= 1 h= 3
Recall Precision Recall Precision
exact 0.59 0.77 0.44 0.51
plugin 0.67 0.69 0.52 0.48
subsume 0.73 0.61 0.59 0.42
the local context with additional atoms, up to the third level. Intuitively, h
allows to control the amount of contextual information taken into account during
the definition of local contexts. A higher hvalue leads to more generic local
contexts that affect precision. For example, the third-level WordNet hypernym
of “tower” is “unit”, which is too generic, obfuscating the semantics of the atom.
The optimal value of hdepends on the concreteness of the atoms extracted
from content analysis: the more specific is the label/atom, the more room for
additional context exists. In our experiments, the labels we get tend to be generic,
therefore the best performance is achieved with h= 1.
8 Conclusion
In this paper we presented V4Ann, an ontology-based framework for represent-
ing, linking and enriching results of multimedia analysis on digital content.
V4Ann generates annotation graphs of image, video, textual analysis and 3D
model reconstruction, so as to facilitate the systematic process, integration and
organisation of information and establish practical repurposing mechanisms.
The annotation model of V4Ann reuses existing standards and schemata,
building the atom-based annotations graphs on top of standard ontologies, con-
trolled vocabularies and patterns. The vocabularies are defined in OWL 2 and
atoms are associated with assets using the WADM pattern. As such, it promotes
interoperability, as well as fosters the use of declarative languages to identify fur-
ther inferences and ensure the semantic consistency of the knowledge graphs. We
also elaborated on the concept of local contexts, as well as on the AH metric
for asset retrieval. We evaluated the framework using actual multimedia content
and atoms provided by the V4Design modules and discussed the findings.
V4Ann is accessible through Rhinoceros 3D (Rhino)12 and Unity plugins
developed in the V4Design project through which users (architects and video
games designers) can search for assets and import them in the scene. For future
work we plan to implement context-aware algorithms to improve the classifica-
tion accuracy of incoming atoms, as well as to extend the context-aware retrieval
algorithm with more sophisticated similarity metrics and functions.
V4Ann: Representation and Interlinking of Atom-based Annotations 15
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