La Posada del Hombre.

2.6.1 Open Annotation Data Model

The Open Annotation Data Model is an initiative of the Open Annotation Com-munity (OAC) Group, a working group of the W3C, whose purpose is to work to-wards a common, RDF-based, specification for annotating digital resources [13].

The OAC came up with the Open Annotation Data Model Specification to provide a standard description mechanism for sharing annotations between sys-tems [52]. It touches on the core ideas on the structure of annotations without demanding structural changes of the platform that wishes to implement them. It is also designed to cover as many annotation use cases as possible.

Their effort started by working towards a reconciliation of two proposals that had emerged from around 2011: the Annotation Ontology¹⁷ and the Open An-notation Model¹⁸. Their final deliverable was a specification, published under an

16The Europeana Collections Ports is an interface to digitised artefacts from Europe such as films, books, paintings etc. Seehttp://europeana.eu/portal/

17https://code.google.com/p/annotation-ontology/

18http://www.openannotation.org/spec/core/20130208/index.html

appropriate open license, informed by the aforementioned proposals and com-munity feedback [13].

Figure 2.1: Illustration of the basic annotation data model by Sanderson, Cic-carese, and Van de Sompel[82].

As shown in Figure 2.1, the data model describes the relationship of two or more resources via an RDF¹⁹graph. The annotation itself is made up of two parts:

the Bodyand theTarget [52, 82]. Both can be of different media types and serve different functions. The Body of an annotation usually has content that provides extra information about the Target while the Target is the resource that is being annotated.

The basic model can be extended with other attributes to: i) embed other resources; ii) include the annotator’s motivations while making the annotation;

iii) include semantic tags; andiv) even link to a specific part of a larger resource using fragment URIs [82].

19Resource Description Framework

2.6.2 Web Annotation Data Model

The Web Annotation Data Model²⁰ is a draft specification²¹ that provides a spe-cific JSON format for the implementation of annotations based on a conceptual model laid out in the Open Annotation Data Model [17]. JSON-LD is used to serialise the Linked Data while maintaining JSON compatibility, preserving hu-man readability and allowing flexibility to express almost any kind of real world object [12,17].

The JSON object contains varying attributes depending on the intended pur-pose. For example, Listing 2.1 shows the JSON representation of an annotation that is linked to an image and contains HTML content in the body.

1 {

2 " @ c o n t e x t ": " http :// www . w3 . org / ns / anno . j s o n l d " ,

3 " id ": " http :// e x a m p l e . org / a n n o 5 " ,

4 " type ":" A n n o t a t i o n " ,

5 " body ": {

6 " type " : " T e x t u a l B o d y " ,

7 " text " : " < p > C o m m e n t text </ p >" ,

8 " f o r m a t " : " text / html " ,

9 " l a n g u a g e " : " en "

10 } ,

11 " t a r g e t ": " http :// e x a m p l e . org / p h o t o 1 "

12 }

Listing 2.1: Example JSON representation of an annotation with image Target and an embedded textual body [17]

The model provides a standard that is adaptable to several use cases ranging from the simple to complex, while following Linked Data principles. Some of the considerations of the model include the ability to [17]:

• Reference an external Web resource— The Body and Target can reference a resource available at a URI and also include other information about the resource, such as its language and format.

• Have different classes for Web resources— In some cases it may be useful to know the type of context that a Body or Target references. This allows clients to intelligently determine the best way to download and render the

20http://www.w3.org/TR/2016/WD-annotation-model-20160331/

21The latest draft of the specification was published on 31^stMarch 2016 and is still considered a work in progress.

content. Examples of resources where this may be immediately applicable are image, audio and video files.

• Target segments of Web resources— URIs referencing Bodies or Targets do not have to reference the resource as a whole but may reference a section of the resource. The URI must include a fragment component that describes the section of the resource that is being referenced. However, it is important to note that not all media types support fragments and also that fragments vary depending on media type.

• Embedded textual bodies— Alternatively, one may need to add an anno-tation that contains information added into the Body and not reference an external Web resource. It is possible to include plain text into the Body as well as a representation of what would be in an external Web resource as long as the type and format are specified.

• Assign multiple Bodies and Targets — There is no fixed number for the number of Bodies or Targets that an annotation may have. It is possible to have a annotation without a Body or even have more than one Body. For example, when a user selects a block of text and highlights it, the visual result of that action is the annotation.

• Intelligent choice of Bodies and Targets — In some cases, one may have multiple bodies that have the same content but are served to the client de-pending on predefined criteria. For example, an annotation may include a comment in various languages and the Body is selected based on the lan-guage setting on the client.

• Attach extra contextual information about the annotation — Other than the details of the Body and Target, it is also possible to include other infor-mation about the annotation, such as:

– The author of the annotation.

– The client that was used when creating the annotation.

– The date-time that the annotation was created or had been last up-dated.

– The intended audience of the annotation.

– The motivation and purpose of the annotation.

– Any rights information to state under which conditions the resource may be used.