RVL Demo Videos

Have a look at the first two RVL demo videos to learn about the basic principles of the RDFS/OWL Visualization Language (RVL):

RVL – Demo #1 – PropertyMappings and ValueMappings

RVL – Demo #2 – IdentityMappings and Mapping of Value Intervals

Scheduled next videos:

  • RVL – Demo #3 – Using Submappings to Map Connector Attributes
    • Composing mappings based on the role of graphic objects
    • Setting connector width, color, shape
  • RVL – Demo #4 – Labeling and Label Positioning
    • Labeling nodes
    • Labeling connectors
    • Labeling labels
  • RVL – Demo #5 – Mapping to other Graphic Relations
    • While we started with node-link diagrams for simplicity, RVL is not limited to the  graphic relations “Linking”, “Labeling” and “Containment”. On the contrary: a broad palette of alternative graphic relations such as  “Relative Position” (Clustering), “Separation by a Separator” and “Alignment” and  “Adjacency” that are described in the Visualization Ontology (VISO) should follow.
  • RVL – Demo #6 – OWL- and RDFS-specific Mappings
    • Mapping Domain-Range relations between classes
    • Mapping further class level relations  such as universal and existential restrictions
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Describing a Schema for RVL

This post is about the schema of RVL and discusses alternatives for defining this schema. It is not about the features of RVL itself. For place limitations, this discussion could not be published in the RVL paper. As there are now efforts from the Dublin Core Metadata Inititative (RDF Application-Profiles Group) as well as from the W3C (Data Shapes Working Group) towards a standard for defining prescriptive constraints for the RDF technical space, this blog post serves as a place to describe RVL as a case study. A summary of requirements derivable from the RVL use cases is given at the end of this post.

In order to clarify the distinction between mappings defined with RVL, the RVL schema, and languages used to define this schema, the following table gives an overview on the different “language levels” in the context of RVL. We will look at the first row, i.e., languages that can be used to define schemata:

Language level Name Example (natural language)
Schema languages RDF(S) / OWL / SPIN An rvl:PropertyMapping is an owl:Class .
A rvl:PropertyMapping
has exactly 1
rvl:targetGraphicRelation .
Schema RVL rvl:PropertyMapping
Model Concrete RVL mappings ex:Cost2LightnessMapping

For defining the schema of RVL, multiple options existed, which we discuss in the following, based on three additional requirements that we put with respect to the schema. Finally, we shortly describe our approach for defining the RVL schema using a combination of OWL axioms and SPIN constraints.

  • LR-15 The schema of the language must be restrictive and expressive enough to derive tooling from it.
  • LR-16 The schema language must be aware of ontology semantics, not only URIs.
  • LR-17 Constraints in the mapping language’s schema and constraints defined in VISO/facts should be handled consistently

First, in order to allow for the generation of mapping editors from the language descrip- tion, the RVL schema should define, in a tool-usable way, what is a valid mapping in RVL (LR-15). Allowing for the derivation of an editor from a languages’ schema contributes to the extensibility of a language, since it reduces redundancy and allows for changing the editor automatically along with the language definition. By using constraints directly from the RVL schema, we avoid that this knowledge has to be hard-coded again in the source code when implementing a guided editor for the mapping language.

Second, since we want to easily define constraints on our mapping types, the schema language should be aware of ontology semantics (LR-16), and not only aware of URIs. It will frequently occur that constraints of the mapping language have to reference VISO/graphic terms such as in the following constraint (here defined in natural language):

“An rvl:PropertyMapping
always maps an rdf:Property
to a viso-graphic:GraphicRelation.”

Third, an additional requirement exists because we want to use RVL in a semi-automatic visualization system: External rules on graphic syntax and human perception that are based on facts from the VISO/facts module will have to be accessed for constructing editors. Therefore, we require that both the constraints from the mapping language’s schema and the constraints defined in VISO/facts can be handled consistently (LR-17).

One option was to stay completely within the RDF-based ontology technical space. This is suggested by the fact that both the source data and the graphic elements we are mapping onto are RDF-based. Defining also our mapping language RVL with RDF-based technolo- gies, therefore, could help avoiding technological breaks. Defining restrictions that use terms from the source ontologies and VISO could easily be done, e.g., via OWL class restrictions and also the Domain and Range of properties could easily be stated with OWL. An additional benefit is that mapping definitions, instantiating an RDF-based vocabulary, could conveniently be shipped along with the data they are visualizing. Furthermore, since each mapping was an RDF resource, globally uniquely identified via a URI, linked data principles would apply to it. This would contribute to the requirement of shareable mappings, since other users could dereferentiate mappings and reuse them in their own visualisations. The authors of Fresnel chose this approach and defined the vocabulary using OWL (cf. right column of Table 2). The problem with this approach is that class restrictions and domain–range settings defined in OWL are not meant to prescribe valid user input, but to derive new knowledge under the open world assumption. For this reasons, we do not consider OWL (alone) appropriate to define the RVL language, since the regular OWL semantics and the corresponding tools are not applicable. While OWL may also be interpreted with different (closed world) semantics and specific tooling could be built, OWL also lacks constructs such as defaults and attributes for conveniently defining a rich prescriptive schema.

Another option was to write the RVL schema in a different technical space, such as the meta-modeling technical space or the grammar technical space, and only reference the ontology resources via their URIs (left column of Table 2). If the mapping language was defined by grammar rules or meta model constraints, under a closed world assumption, tooling for constrained-based guidance (editors, warning messages, auto-suggest functions) could conveniently be generated based on these constraints with existing technologies. Around ECore, as as popular base for meta-modeling, the Eclipse Modeling Framework and many frameworks on top of it support building textual or graphical editors for ECore-based languages. On the downside, when this means that ontologies need to be transformed, (e.g., to ECore), it will be difficult with this approach to dynamically adapt to extensions of the ontological models. Re-modeling ontologies in ECore is a drawback, when we want to access changing external knowledge bases and consider facts stored in these knowledge bases in our constraints.

We chose the first option and decided to stay within the ontology technical space. However, we use RDFS/OWL only for modeling the abstract syntax of RVL, and use SPIN for defining the constraints of the RVL schema (center column of Table 2). With TopBraid Composer a modeling environment is available that can be used to build an editor that supports syntactic guidance for creating valid RVL mappings and at the same time allows for conveniently accessing VISO/graphic resources as well as visualization rules from the VISO/facts knowledge base. Table 2 summarises our comparison of the three options described above under the aspects of expressiveness, use of standards, support of shareability of mappings, the availability of tooling and the support of guidance based on both schema knowledge and external facts from existing knowledge bases. In the left column we use the concrete solution of OWLtext as an example for the second approach.

Table 2 - Comparison of three options foor specifying the RVL schema

Table 2 – Comparison of three techniques to define schemata and derive tooling from these schemata: The right column represents the approach of staying completely within the Ontology technical space, exemplified by the solution chosen to define Fresnel. (FSL stands for the Fresnel Selector Language) The left column represents the approach of bridging the Ontology and Metamodelling technical space, exemplified by OWLtext. The center column shows our choice of using OWL in combination with SPIN for the definition of constraints for combined Open and Closed world reasoning.

Concrete Examples of the RVL Schema defined with

In the following, we provide a set of concrete examples to illustrate which parts of RVL are defined with RDFS/OWL and which parts with SPIN and how a SPIN constraint can be defined. Types and relations of RVL are defined with RDFS and OWL:

rvl:PropertyMapping rdfs:subClassOf rvl:Mapping .

rvl:sourceProperty a rdf:Property .

rvl:Clamp a rvl:OutOfBoundHandlingType ;
   rdfs:label "clamp"^^xsd:string ;
   dct:description "Values outside the defined interval are set to
      the boundaries of the interval."^^xsd:string .

In order to prescripe how a mapping type must be used, SPIN is used as a constraint language. SPIN first of all allows for storing SPARQL queries as RDF. However, additional properties, such as spin:constraint and spl:Attribute enable the definition of prescriptions such as attributes, which constrain the usage of certain properties in the context of a specific class. In the following listing we show how SPIN can be used to express the example constraints we already (partially) introduced above as natural language. It states that a rvl:PropertyMapping always maps exactly one rdf:Property to exactly one viso-graphic:GraphicRelation:

      [ a spl:Attribute ;
      rdfs:comment "There has to be exactly one target graphic relation." ;
      spl:maxCount 1 ;
      spl:minCount 1 ;
      spl:predicate rvl:targetGraphicRelation ;
      spl:valueType viso-graphic:GraphicRelation
# ... analog constraint for rvl:sourceProperty

Attributes encapsulate a SPARQL query which can be evaluated to decide whether some property is used as required. The most simple kind of query is a SPARQL ASK query – when it returns “yes” the constraint is violated, when it returns “no” the RVL model meets the constraint. The listing below shows such a constraint that is simply stored as a SPARQL ASK query. For better readability we show the SPARQL query in the usual syntax, not as SPIN, i.e., stored as RDF triples.

   [ a sp:Ask ;
     rdfs:comment "Expressiveness - The chosen visual means cannot express the given source property (based on it’s defined scale of measurement)";
     sp:where (
       ASK WHERE {
       ?this rvl:sourceProperty ?sp .
       ?this rvl:targetGraphicRelation ?tvm .
       ?sp viso-data:has_scale_of_measurement/(rdfs:subClassOf)* ?spSoM .
       ?spSoM (rdfs:subClassOf)+ viso-data:Scale_of_Measurement .
       ?data_kind rdfs:subClassOf ?restriction .
       ?restriction owl:onProperty viso-data:has_scale_of_measurement .
       ?restriction owl:allValuesFrom ?spSoM .
       ?tvm viso-facts:not_expresses ?data_kind .
       FILTER (false) .

An important thing to note is that the last example connects to an external knowledge base, using the viso-facts ontology. This goes beyond the constraints defined above, which only reference RVL concepts.

Summary of Requirements for a Standard Schema Language Deriveable from RVL Use Cases

In order to replace the current OWL+SPIN schema of RVL by a (future) standard “schema language”, this language needs to fullfill the following requirements (in brackets   requirements and use-cases  from the rdf-validation requirements database are listed that match exactly or seem related (~) to the given items (WIP):

  • Class-based restriction of value type and cardinality for properties [R-76, R-75, (opt: R-74), R-17]
  • “Convention over Configuration” –> Definition of default values [R-31, R-38]
  • Constraints may be based on knowledge from various graphs
    • optional: Import of graphs only for the purpose of being used for constraints (complementing owl:imports) [UC-Editor-2]
  • Differentiate between constraint violation levels (e.g. , error / warning / .. ? ) [~UC-3, R-205, ~UC-7,]
  • Support of Guidance / UI Construction [R-195, R-125]
    • could be an additional (low) constraint violation level:  “recommended”. Only values matching on the “recommended” constraint level are suggested to users via UIs. [UC-3, ~R-205, R-72]
    • opt: extensible constraint levels (for example “not effective”,  “not expressive” in the visualization context)
    • opt: Define quick fixes along with the constraints (to avoid hardcoding them in a platform dependent way) [?, R-192]
  • Combinable with open world reasoning –> interpreting OWL (completely) as closed world is not an option. For example, we still want to use standard reasoners to conclude … [R-173]
    • Every rvl:PropertyMapping is an rvl:Mapping
    • Everything having an rvl:sourceValue is an rvl:ValueMapping
  • Consise, compact definition of frequently used constraints [R-184]
    (in the example above we use spin:attribute)
    • –> extensibility and reusability, e.g. by a templating mechanism
  • Document the constraints in natural language [R-192]
  • SPARQL queries can be used to formulate complex constraints [R-188, R-186]
    • optional: Path selector language for convenient, compact selector expressions in simpler constraints [~R-103]
  • Recommend properties from third-party vocabularies to be used, even when not constraining their usage [~UC-Editor-4]
  • optional (may also be an extra language like Forms , or Fresnel could be reused): Suggest visibility and order of widgets for each class [~UC-OER-5]

The above mentioned requirements  can be connected to two main use cases – most of them relate to both:

  1. Support of Guidance / UI Construction
  2. Validation
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More RVL Examples – Visualizing Ontologies from the Life Sciences

Some more examples of RVL mappings have been added to the blog. Explore the d3.js graphics generated with an initial prototyp of an RVL interpreter and compare the corresponding RVL mapping code!

Have fun and – feedback (including critical) is appreciated!

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Example of a visual mapping described in the RDFS/OWL Visualisation Language (RVL)

RVL is a declarative RDF-based language for specifying visual mappings from RDFS/OWL data to graphic means. In the following we use the language to visualise example data specified using vocabulary from the Citation Ontology:

The above graphic was created by processing the RVL mappings below. A first mapping simply says map cito:cites to directed links
(e.g. connector lines with arrowheads).

@prefix :          <http://purl.org/rvl/example/mapping/> .
@prefix cito:      <http://purl.org/spar/cito/> .
@prefix rvl:       <http://purl.org/rvl/> .
@prefix vg:        <http://purl.org/viso/graphic/> .
@prefix rdf:       <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .

      a rvl:PropertyMapping ;
      rvl:sourceProperty cito:cites ;
      rvl:targetObjToObjRelation vg:Linking_Directed_Relation ;

If you move the pointer over the edges, you can observe that also sub-properties of cito:cites have been used in some cases.

Let’s add another mapping to distinguish the different properties by color. We realize this with a sub-mapping attached to the first mapping:

      a rvl:PropertyMapping ;
      rvl:sourceProperty cito:cites ;
      rvl:targetObjToObjRelation vg:Linking_Directed_Relation ;
      rvl:subMapping [
        rvl:subMapping-onRole vg:linking_connector;
        rvl:subMapping-onTriplePart rdf:predicate;
        rvl:subMapping-mapping :PredicateID2Color;

      a rvl:PropertyMapping ;
      rvl:sourceProperty rdf:ID ;
      rvl:targetAttribute vg:color_named ;
      rvl:valueMapping [
        rvl:sourceValueOrderedSet (
            cito:confirms cito:cites cito:critiques
        rvl:targetValueList (
            vg:Green vg:Yellow vg:Red

More examples will follow :)

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VISO now at GitHub

Since there were several requests for reusing the VISO ontology, we decided to move it to GitHub. A lean core version of VISO is currently build from the existing version and a few modules are already available. The existing version will remain for documentation purposes and serve as a base for the new version, which will start with a small set of stable concepts. Concepts that are required by VISO users are migrated incrementally.

If you decide to fork VISO, please use a sorted Turtle notation as it is offered, for example, by TopBraid Composer (Settings… -> Input/Output) in order to ease later merging of your extensions.

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RDFS/OWL Visualisation Language (RVL) published

An initial version of the RDFS/OWL Visualisation Language (RVL), which was developed as part of my PhD thesis has been published at the HSWI13 (Workshop on Human-Semantic Web Interaction). The slides can be found within an internal report and on slideshare (see below).

Download RVL paper (authors version)




Abstract: Information on how to visualize RDF data is stored differently by each visualization tool to date. We propose the RDFS/OWL Visualization Language (RVL), a declarative language for sharing visualization settings as simple as CSS styles. The mapping definitions can be given a URI and shared along with the data to be visualized, they can be composed, extended and reused. The declarative approach has the benefit that this can be done independently of specific platforms. Unlike styling or presentation languages for RDF or pure visualization languages, RVL combines rich visual mapping capabilities with the direct awareness of RDFS/OWL language constructs.

The documentation of RVL can be accessed at http://purl.org/rvl/ (work in progress). Prototypical tooling for language is currently built and will be published on this blog.

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VISO (Visualization Ontology) has new documentation

The VISO (Visualization Ontology) has a new documentation and can now be accessed more easily, simply via http://purl.org/viso/. VISO is used as a basis also for the RDF visualisation framework I’m currently building for the eScience – network.

VISO modules overview

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Presentation of the eScience Project at OUTPUT-DD 2012

The eScience project on publication visualisation was presented at OUTPUT-DD 2012. Thank you for all discussions around the topic of literature search and visualisations! The feedback served as input to improve the final survey which was opened for participation in late 2012. The result of this survey will soon be published as a report on this blog.

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Weighted Faceted Browsing

Screenshot of the Weighted Faceted Browser instatiated for the domain of visualisation in the context of VizBoardUsually a selection in a Faceted Browser means all or nothing − between this there are no other options.

Artur Werstler, a student supervised by Martin Voigt and me, analysed how Faceted Browsing and weights could be combined. This was done with respect to both: weights in the data and weighted query expressions. A prototype he developed in the context of the visualisation workbench VizBoard was now presented by means of a short paper on the EICS 2012. It shows how weighting parts of a query and faceted browsing can be combined consistently in a single user interface. Besides using weights for sorting multiple results, also filtering may be influenced by weights given that a certain threshold has been previously defined. By using weights, it is possible to avoid filtering out good results (with respect to most criteria) too early − only because some item did not match the very first restricted facet.

Please note that the items which can be filtered in the prototype are from the visualisation domain as well. So, this specific instance of a WFB is a visualisation of visualisations.


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Visualization Ontology (VISO) online

In cooperation with Martin Voigt from the chair of Multimedia Technology we recently published an (alpha) version of the Visualisation Ontology (VISO). Look at the VISO blog for details.


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