=Paper=
{{Paper
|id=Vol-1704/paper13
|storemode=property
|title=Advanced UML Style Visualization of OWL Ontologies
|pdfUrl=https://ceur-ws.org/Vol-1704/paper13.pdf
|volume=Vol-1704
|authors=Jūlija Ovčiņņikova,Kārlis Čerāns
|dblpUrl=https://dblp.org/rec/conf/semweb/OvcinnikovaC16
}}
==Advanced UML Style Visualization of OWL Ontologies==
<pdf width="1500px">https://ceur-ws.org/Vol-1704/paper13.pdf</pdf>
<pre>
    Advanced UML Style Visualization of OWL Ontologies

                            Jūlija Ovčiņņikova*, Kārlis Čerāns

                       julija.ovcinnikova@lumii.lv, karlis.cerans@lumii.lv
              Institute of Mathematics and Computer Science, University of Latvia
                               Raina blvd. 29, Riga, LV-1459, Latvia



        Abstract. The OWLGrEd ontology editor allows graphical visualization and au-
        thoring of OWL 2.0 ontologies using a compact yet intuitive presentation that
        combines UML class diagram notation with textual OWL Manchester syntax for
        expressions. We describe here the approaches available for ontology presentation
        fine tuning within the OWLGrEd editor, namely the ontology visualization op-
        tion framework and the editor plug-in mechanism together with concrete plug-
        ins aimed to enhance the ontology presenting and editing experience.

        Keywords: OWL, OWLGrEd, UML-style ontology visualization, ontology vis-
        ualization options


1       Introduction

Presenting OWL ontologies [1] in a comprehensible form is important for ontology
designers and their users alike. The graphical form in general and UML class diagram
notation in particular offers an option of basic visual ontology construct presentation
that allows linking together constructs that are related in the ontology (e.g. an object
property can be depicted as a line connecting its domain and range class boxes). UML
class diagrams need to be extended to cope with full OWL 2.0 construct modeling. This
is solved in different graphical notations in different ways. So, ODM [2], defines a
UML profile for ontology presentation and OWLGrEd [3] and TopBraid Composer [4]
integrate OWL Manchester Syntax [5] for presenting advanced OWL constructs in tex-
tual form. The uniqueness of OWLGrEd lies in its combined ability to lay out an ontol-
ogy that is imported or created in the editor in a compact graphical UML-style form,
and make further manual ontology editing/adjustment. VOWL [6] visualizes ontologies
by another approach using graphical primitives both for object and data property
presentation, so obtaining a more uniform ontology presentation in a dynamic, yet read-
only, graph-like form. The VOWL presentation of the same ontology will also typically
require more graphical elements, than OWLGrEd.
   The real strength of ontology presentation in an editor like OWLGrEd comes from
the user’s ability to fine-tune the ontology diagram after its automated rendering to
obtain documentation-ready ontology presentations. Such a fine tuning may involve the

* Supported, in part, by Latvian State Research program NexIT project No.1 “Technologies of

    ontologies, semantic web and security”.



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Advanced UML Style Visualization of OWL Ontologies

 diagram object repositioning, as well as its re-structuring up to full ontology editing
 facilities available in the tool (including e.g. manual deletion of irrelevant information).
    In order to achieve a high quality ontology presentation in the tool, even in the pres-
 ence of manual fine tuning options available, the quality of first ontology diagram cre-
 ated upon the import of the ontology into the tool still remains very important. Since
 the UML diagrams, as well as the OWLGrEd notation allow for different presentations
 of the same semantic elements (e.g. a graphical and textual one), and different ontolo-
 gies may correspond to different desirable ontology presentation options, we describe
 here a re-factored ontology visualization option framework offering a number of
 choices that the user can make already before importing the ontology into the tool.
    We describe here also a number of OWLGrEd plugins that can be used for diagram
 refactoring services, as well as its structural and semantics extensions. This part of work
 extends the earlier authors’ work on domain specific ontology visualizations [7,8] (this
 paper describes new re-factoring services plug-in, as well as reviews the plug-in mech-
 anism and concrete plugin architecture from the ontology visual presentation perspec-
 tive). The editor plugins described here are included in the OWLGrEd tool download
 and can be activated on-demand for concrete projects. The OWLGrEd editor with pre-
 installed configuration, as described here, is available at http://owlgred.lumii.lv/pp.


 2      OWLGrEd Notation and Editor

 OWLGrEd (http://owlgred.lumii.lv/) provides a graphical notation for OWL 2 [1],
 based on UML class diagrams. OWL classes are typically visualized as UML classes,
 data properties as class attributes, object properties as association roles, individuals as
 objects, cardinality restrictions on association domain class as UML cardinalities, etc.
 The UML class diagrams are enriched with new extension notations, e.g. (cf. [3,9]):

  fields in classes for equivalent class, superclass and disjoint class expressions writ-
   ten in Manchester OWL syntax [5];
  fields in association roles and attributes for equivalent, disjoint and super properties
   and fields for property characteristics, e.g., functional, transitive, etc.;
  anonymous classes containing equivalent class expression but no name;
  connectors (as lines) for visualizing binary disjoint, equivalent, etc. axioms;
  boxes with connectors for n-ary disjoint, equivalent, etc. axioms;
  connectors (lines) for visualizing object property restrictions some, only, exactly, as
   well as cardinality restrictions.

 Fig. 1 contains a simple demonstration fragment of Latvian Medicine Registries ontol-
 ogy [10] in OWLGrEd notation, illustrating the class, data and object property, as well
 as subclass, sub-property and object property restriction notation, different ways of dis-
 joint classes notations, class-level inline comments and ontology level annotations.
    The OWLGrEd tool allows both for ontology authoring (with option to save the on-
 tology in a standard textual format) and ontology visualization that includes automated
 ontology diagram formation and layouting step, followed by optional manual diagram
 fine tuning to obtain the highest quality rendering of the ontology.



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Advanced UML Style Visualization of OWL Ontologies

                                                                                   treatedIllnessCase              IllnessCase                 <<comment>>
       IllnessTreatment                                                                                                                        "Illness case
                                                                   Person                                                                      and patient
                                                                                            patient
                                                       personID:string                                                                         ontology
                {disjoint}                             dateOfBirth:dateTime                 diabPatient {<patient}                             fragment"
                                                   traumaPatient cancerPatient
                TraumaTreatment
                                                       {<patient}   {<patient}
                                                                                                               Cancer
                                                                          Trauma                      "Cancer illness case"                      Diabetes
            CancerTreatment                                       "Trauma illness case"                                                      "Diabetes illness
                                                                                                                                             case"
                                                                                                      treatedIllnessCase
                                                      treatedIllnessCase                                             only             treatedIllnessCase
                                                                     only                       <<disjoint>>                                         only
       DiabetesTreatment



                           Fig. 1. Demo fragment of Latvian Medicine Registries Ontology


 3          Ontology Visualization Parameters

 The UML notation provides several options for presenting its semantic elements in dif-
 ferent visual ways. This principle is kept also in OWLGrEd by including both graphical
 and textual notations for such semantic elements as subclass relations, object property
 relations, annotations and object properties as association roles or as attributes.
    The automatic ontology visualization in OWLGrEd by default shall use the graphical
 notation, if there is no clear reason for switching to textual one. Fig. 2 shows a larger
 fragment of Medicine Registries Ontology in a graphical notation for all object proper-
 ties and object property restrictions, and with separate disjoint classes axiom rendering.
                                                                                                                   treatedIl lnessCase        IllnessCase
                                                                      Person
                                                           personID:string                     patient
       treatingD octor                                     dateOfBirth:dateTime                diabPatient {<patien t}
            <<EnumClass>>                                   traumaPatient cancerPatient
                               illnessR egDoctor
            Doctor                                              {<patien t}   {<patien t}
     doctorN ame:string                                                                                                                            Diabetes
     doctorC ode:string        diabReg Doctor {<illness RegDoctor}
                                                                                                            Cancer
                               cancerR egDoctor {<illness RegDoctor}
                               traumaR egDoctor {<illness RegDoctor}                                                                     treatedIl lnessCase
                                                                                                                                                         only
                         workpla ce                                                           treatedIl lnessCase
                     <<EnumClass>>                                                                            only
                                             treatedIl lnessCase only             Trauma
       MedicalEstablishement                                                                                                  <<disjoi nt>>
       estNam e:string
       estType :string
       treatme ntPlace
     IllnessTreatment                                                                         traumaD iagnosis     cancerD iagnosis
                                                                        treatedD iagnosis     {<illness Diagnosis} {<illness Diagnosis}
                                                                                                         <<EnumClass>>
                                                                                                                         diabDia gnosis {<illness Diagnosis}
                                                                                     Diagnosis
                                                                    diagnCo de:string                                    illnessD iagnosis
      TraumaTreatment                      <<disjoi nt>>            diagnDe scr:string                                   traumaTrDiagnosis {<treate dDiagnosis}

                                      CancerTreatment               cancerTrDiagnosis           diabTrD iagnosis
                                                                   {<treate dDiagnosis}     {<treate dDiagnosis}

                DiabetesTreatment




            Fig. 2. Medicine Registries Ontology: a larger fragment in a graphical notation

   For different ontologies and their parts, the appropriate element visualization meth-
 ods may differ depending on the ontology content, e.g. if the ontology contains a small




                                                                              138
Advanced UML Style Visualization of OWL Ontologies

 number of object property restrictions, a better way to display object property re-
 strictions would be graphical. However, it would not carry much information to display
 graphically object property restrictions that are based on owl:Thing as the target class.
 A textual form object property restriction visualization may be favorable also in the
 presence of a large number of these restrictions in the ontology.
     The ontology visualization parameter framework offers parameters for inclusion /
 not inclusion of different object types in ontology visualization (e.g. one can choose to
 visualize ontology without data property annotations or without disjoint class axioms).
 Most of the framework parameters, however, refer to different ways (e.g. a graphical,
 or a textual form) of different type entity and axiom information visualization.
     Figure 3 illustrates parameter setting interface including the option to represent sub-
 class relations as text or graphically. In graphical mode there is a possibility to draw
 subclass relations as lines or combine them through forks, if there are more than one
 subclass for a given class.




                   Fig. 3. Ontology Visualization Parameter form illustration

    There are also parameters for disjoint class axiom rendering in Fig. 3. Along with
 disjoint classes representation as text or graphically (in a separate graphical node), there
 is a possibility to group disjointness relations, into boxes if there are more than two
 disjoint classes, and it is possible to hide information if it is not presented graphically.
    An important option in UML is a possibility to mark the generalization sets as dis-
 joint. This notation is brought also over to OWLGrEd since it can greatly simplify the
 graphical diagram appearance, if the disjointness axiom matches a level within the class
 hierarchy. If the ‘Use Disjoint mark at forks’ box is checked in the import parameter
 form, the ontology importer is able to identify the cases when all class subclasses are
 disjoint and add specific {disjoint} mark to the generalization element (cf. the general-
 ization set notation for the subclasses of the IllnessTreatment class in Fig. 1).
    Figure 3 also shows possible extra choices that can be made for the subclass rela-
 tions, such as “Mark top-level named classes as subclasses of ‘Thing’”, “Draw subclass
 relations to named class A from expression ‘A and ...’”, “Draw subclass relations from
 named classes to their union”, “Hide subclass information, if not presented graphically”
 and an option to choose whether to create anonymous subclasses as boxes, or not. The




                                             139
Advanced UML Style Visualization of OWL Ontologies

 extra parameters are left unchecked by default to obtain less loaded graphical presenta-
 tion, however they can be turned on to show more clear class hierarchy, if desired.
           Person                         IllnessCase                            IllnessTreatment                                 <<EnumClass>>
  personID:string                  illnessR egDoctor:Docto r           treatingD octor:Doctor                                 Diagnosis
  dateOfBirth:dateTime             illnessD iagnosis:Diagn osis        treatme ntPlace:Medica lEstablishemen t      diagnCo de:string
                                   patient:Person                      treatedD iagnosis:Diagn osis                 diagnDe scr:string
                                                                       treatedIl lnessCase:Illne ssCase
                     Trauma                              {disjoint}                                              {disjoint}
  traumaR egDoctor:Doctor{<illnessRegD octor}
  traumaD iagnosis:Diag nosis{<illnessD iagnosis}                      CancerTreatment
  traumaPatient:Person{<patient}                        <treated IllnessCase on ly Cancer                                        <<EnumClass>>
                                                        cancerTrDiagnosis:Dia gnosis{<treated Diagnosis}
                                                                                                                   MedicalEstablishement
                     Cancer                                                                                         estNam e:string
  cancerR egDoctor:Docto r{<illnessRegD octor}                          DiabetesTreatment                           estType :string
  cancerD iagnosis:Diagn osis{<illnessD iagnosis}          <treated IllnessCase on ly Diabetes
  cancerPatient:Person{<patient}                           diabTrD iagnosis:Diagn osis{<treatedD iagnosis}                       <<EnumClass>>
                                                                                                                                Doctor
                   Diabetes                                                                                            doctorN ame:string
  diabReg Doctor:Doctor{<illnessRegDo ctor}                                 TraumaTreatment                            doctorC ode:string
  diabDia gnosis:Diagnos is{<illnessDiag nosis}           <treated IllnessCase on ly Trauma                            workpla ce:
  diabPatient:Person{<pa tient}                           traumaTrDiagnosis:Dia gnosis{<treate dDiagnosis}             Medical Establishemen t




          Fig. 4. Demo fragment of Latvian Medicine Registries Ontology: textual notation

    Figure 4 illustrates the same Medicine Registries Ontology of Fig.2 with subclass
 relations presented graphically and disjoint marks at forks, while presenting textually
 the object properties and object property restrictions. The obtained ontology visualiza-
 tion has less elements that can be a benefit, however, the textual representation of object
 properties and object property restrictions somewhat hides the ontology structure.


 4         Ontology Editor Plugins

 To enhance the ontology visualization fine tuning experience after the ontology initial
 loading into the tool, as well as to support different custom ontology visualization
 means and the ontology editing work in general, the OWLGrEd ontology editor can be
 extended by means of plugins. The ontology editor plugins offer editor notation and
 environment extension means, in a similar manner, as profiles do for UML class dia-
 grams [11,12]. A plugin to the ontology editor, may include structural editor symbol
 extensions with fields and visual effects, as well as editor behavior extensions.
    The OLWGrEd plugin for ontology re-structuring includes transformations for ad-
 justing the ontology visual presentation after its automatic visual rendering. It allows
 switching between the textual and graphical presentation form for concrete object prop-
 erty and object property restriction presentations within the ontology diagram, as well
 as re-factoring individual disjoint classes axiom presentation (e.g. from a separate dis-
 joint-box to the ‘disjoint’ mark at the sub-class form symbol). The plugin can be used
 to transform e.g. the ontology diagram of Fig. 2 or Fig. 4 into the one of Fig. 5, where
 certain object properties (in the concrete example – the ones not having super-proper-
 ties) are displayed in a graphical form to balance the structure presentation and com-
 pactness requirements. For an experienced OWLGrEd user it should take about 5
 minutes to change manually the ontology diagram of e.g. Fig. 2 into the one of Fig. 5.
    The ontology presentation in Fig. 5, as well as the presentation in Fig. 2 and Fig. 4
 uses also a custom “enumerated class” boolean user field presentation.



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Advanced UML Style Visualization of OWL Ontologies

            Person                                IllnessCase                                            IllnessTreatment
  personID:string                        illnessR egDoctor:Docto r                          treatingD octor:Doctor
  dateOfBirth:dateTime         patient   illnessD iagnosis:Diagn osis   treatedIl lnessCase treatme ntPlace:Medica lEstablishemen t
                                                                                            treatedD iagnosis:Diagn osis

                     Trauma                              {disjoint}                                                              <<EnumClass>>
                                                                                            {disjoint}
  traumaR egDoctor:Doctor{<illnessRegD octor}
  traumaD iagnosis:Diag nosis{<illnessD iagnosis}
                                                                                                                            Diagnosis
                                                                        CancerTreatment                            diagnCo de:string
  traumaPatient:Person{<patient}
                                                         <treated IllnessCase on ly Cancer                         diagnDe scr:string
                                                         cancerTrDiagnosis:Dia gnosis{<treated Diagnosis}
                     Cancer                                                                                                      <<EnumClass>>
  cancerR egDoctor:Docto r{<illnessRegD octor}                                                                     MedicalEstablishement
  cancerD iagnosis:Diagn osis{<illnessD iagnosis}                         DiabetesTreatment                        estNam e:string
  cancerPatient:Person{<patient}                            <treated IllnessCase on ly Diabetes                    estType :string
                                                            diabTrD iagnosis:Diagn osis{<treatedD iagnosis}
                                                                                                                    workpla ce
                   Diabetes
                                                                                                                                 <<EnumClass>>
  diabReg Doctor:Doctor{<illnessRegDo ctor}                                 TraumaTreatment
  diabDia gnosis:Diagnos is{<illnessDiag nosis}                                                                                  Doctor
                                                            <treated IllnessCase on ly Trauma                            doctorN ame:string
  diabPatient:Person{<pa tient}                             traumaTrDiagnosis:Dia gnosis{<treate dDiagnosis}             doctorC ode:string



 Fig. 5. Demo fragment of Latvian Medicine Registries Ontology: custom notation with plugins

    In general, the editor extensions with symbol fields and graphical effects enhance
 the ontology presentation options by introducing domain-specific notations into the on-
 tology presentation (the extensions can be configured to take into account the domain
 specific notations also during the ontology import); they are handled by a generic User
 Fields plugin [8] that is currently part of the default OWLGrEd editor configuration.
    The available editor enhancements, supported by the User Fields plugin [8] include:

      custom fields, together with their semantics mappings (e.g. “enumerated class”);
      custom visual effects for text and choice fields and symbols dependent on con-
       crete text or choice field values (e.g. a brown/darker enumerated class color);
      views applying certain visual effects to the entire diagram (e.g. hiding certain in-
       formation from the presentation). The default OWLGrEd configuration includes
       views for horizontal and vertical alignment, and hiding all annotations.

    An example of ontology editor plugin that involves both the user fields and custom
 functionality definition is the data ontology support plugin [13]; it contains the above
 considered “enumerated class” field definition. It also adds extra maximum cardinality
 1 axioms to all data and object properties that are rendered in the editor in the textual
 form and do not have explicit cardinality specification. The plugin also records the class
 attribute ordering into an annotation so that this ordering could be further used in auto-
 mated user interface generation for ontology-conformant data browsing (cf. [13]).
    The other currently maintained OWLGrEd editor plugins are: (i) Controlled Natural
 Language (CNL) interface support plugin (used to augment the ontology with lexical
 information in [14]) and (ii) database to ontology mapping support plugin DBExpr [13].


 5         Conclusions

 The UML style visualization and authoring of OWL ontologies has been proven to be
 a successful alternative to other ontology visualization and authoring means. The
 OWLGrEd editor is able to offer a compact ontology notation by using OWL Manches-
 ter syntax [5] for textual presentation of advanced ontology constructs.



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Advanced UML Style Visualization of OWL Ontologies

    We have described three mechanisms that can be applied to enhance the ontology
 visual presentation and editing experience: (i) ontology visualization parameters; (ii)
 ontology profiles that may enhance the user experience in creating comprehensible and
 visually appealing ontology presentations; (iii) ontology visualization transformation
 means. Each of them can be further developed and enhanced to reflect new ontology
 rendering patterns, as well as to respond to the needs of concreate use cases.
    A future work direction is also to move the OWLGrEd editor to the web environ-
 ment, this depends on suitable tool definition framework availability providing the nec-
 essary editor definition infrastructure; this is work in progress in itself, as well.


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