Readium Annotations

User annotations convey textual information about a resource segment in a publication.

This document defines a syntax for Annotation Documents, serialised in JSON and ready for embedding in a packaged publication (especially EPUB), shared as a file or referenced in a Readium Web Publication Manifest.

This document defines a profile of the W3C Web Annotation Data Model by specifying a subset of the properties allowed in this model and adding a few properties.

It is complemented by a proposal for new Selectors; these new Selectors should be discussed in the context of a new version of the W3C Annotation Data Model.

Editors:

Laurent Le Meur

Participate:

Use cases

A user decides to export from his reading application an EPUB ebook he has previously annotated. He selects the option to save his annotations in the ebook. The exported EPUB contains his annotations. If he imports the ebook into another reading application that supports this specification, his annotations will reappear.
A user decides to export the annotations he has created in an ebook from his reading application. He chooses a file name and a destination folder, and he validates his choice. An annotation file is created on his computer. If he imports this file into another reading application that supports this specification, and if the reading application already contains this ebook, his annotations will now appear in the ebook.
A teacher reads an ebook and prepares annotations. He exports an annotation file and shares this file with his students. The students import the ebook and the detached annotation file into their reading application: the annotations made by the teacher appear in the ebook. The students can add their annotations to the ebook, and these student annotations will not be mixed with the annotations created by their teacher.
A user annotates an ebook. He shares his annotations with other users via a Cloud mechanism handled by his reading application.
A user annotates an ebook. He adds semantics to his annotations, e.g. differentiating “to be corrected” from “to be discussed” annotations.

1. Annotation

1.1. Annotation Object

This document retains the following annotation properties from the W3C Annotation Data Model:

Name	Description	Format	Required?
`@context`	The context that determines the meaning of the JSON as an Annotation. It MUST be “http://www.w3.org/ns/anno.jsonld”.	string	Yes
`id`	The identity of the annotation. A uuid formatted as a URN is recommended.	URI	Yes
`type`	The RDF structure type. It MUST be "Annotation".	string	Yes
`motivation`	The motivation for the annotation's creation. Only used for tagging bookmarks.	"bookmarking"	No
`created`	The time when the annotation was created.	ISO 8601 datetime	Yes
`modified`	The time the annotation was modified after creation.	ISO 8601 datetime	No
`creator`	The creator of the annotation. This may be either a human or an organization.	Creator	No
`target`	The target content of the annotation.	Target	Yes
`body`	The annotation body.	Body	No

An annotation without Body structure can correspond to a "highlight" or a "bookmark". A bookmark is specified by the inclusion of a motivation property with bookmarking as a value.

Sample 1: Core structure of a Readium annotation

{
  "@context": "http://www.w3.org/ns/anno.jsonld",
  "id": "urn:uuid:123-123-123-123",
  "type": "Annotation",
  "created": "2023-10-14T15:13:28Z",
  "modified": "2024-01-29T09:00:00Z",
  "target": {
   },
  "body": {
  }
}

1.2. Creator

The creator of an annotation is a person or an organisation.

This document defines the following creator properties:

Name	Description	Format	Required?
`id`	The identity of the creator.	URI	Yes
`type`	The RDF structure type. It MUST be "Person" or "Organization".	string	Yes
`name`	The name of the creator.	string	No

1.3. Target

The target of an annotation associates the annotation to a specific segment of a resource in the current publication.

This document defines three target sub-properties:

Name	Description	Format	Required?
`source`	The identity of the target resource.	URI	Yes
`selector`	The segment of the target resource that is annotated.	Array of Selector objects	No
`meta`	Indications that help locate the selector in the resource.	Meta	No

A Target with no Selector indicates that the annotation is targeting the entire target resource.

1.3.1. Source

The target resource MUST be identified by the URL of an existing resource in the publication.

If the annotation is attached to an EPUB, the source value must also be present as one of the item/@href values of the manifest element.

If the annotation is attached to a Web Publication, the source value must also be present as one of the readingOrder/@href or resource/@href values on the Web Publication Manifest.

Sample 2: the source of the annotation is the relative URL identifying an HTML document in an EPUB.

{
  "@context": "http://www.w3.org/ns/anno.jsonld",
  "type": "Annotation",
  "target": {
    "source": "OEBPS/text/chapter1.html",
    "selector": [
    ],
    "meta": {
    }
  }
}

1.3.2. Selector

An annotation refers to a segment of a resource, which is identified by one or more Selectors. The nature of the Selectors and methods to describe segments depend on the resource type. Providing more than one Selector allows an annotation software to choose the most accurate selector from those it can handle and helps accommodate evolutions on the annotated resource.

Annotation selectors are specified in W3C Annotation Data Model, section Selectors. This specification filters selectors deemed useful for annotating publications and details the use of these selectors.

Note: New selectors will undoubtedly be defined in the coming months after discussion with members of the W3C Publishing Maintenance Working Group.

1.3.2.1. Text Quote Selector

This Selector describes a range of text by copying it, and including some of the text immediately before (a prefix) and after (a suffix) it to distinguish between multiple copies of the same sequence of characters.

Whitespaces present in the source document are preserved in the “prefix”, “exact”, and “suffix” segments and are represented as per the JSON Grammar (e.g. \n, \t etc.).

Note: There is no restriction on the preceding and following text that can be included in the selector, but this amount should be left as low as possible.

Important: This selector contains complete annotated text segments and, therefore, must not be used when the publication is protected by a DRM, which limits the number of characters that may be copied.

Implementation: This selector is exported from Thorium Reader 3.1 if the publication is in EPUB or Web Publication format and free from any DRM.

Sample 3: A text segment represented as a TextQuoteSelector.

{
  "selector": [
    {
    "type": "TextQuoteSelector",
    "prefix": "trouver quelqu’un    \n  \t\t    comme vous. ",
    "exact": "Combien de fois \n\n    ne m’avait-il",
    "suffix": " pas \n\n      reproché de travailler ma"
    }
  ]
}

1.3.2.2. CssSelector + TextPositionSelector

References in the W3C Annotation Data Model: CSS Selector, Text Position Selector, Refinement of Selection

A TextPositionSelector describes a range of text by recording the start and end positions of the selection in the stream. Position 0 would be immediately before the first character, position 1 would be immediately before the second character, and so on. The start character is thus included in the list, but the end character is not. The W3C Annotation Data Model specifies that the selection of the text MUST be in terms of unicode code points (the "character number"), not in terms of code units (that number expressed using a selected data type). Selections SHOULD NOT start or end in the middle of a grapheme cluster. The selection MUST be based on the logical order of the text, rather than the visual order, especially for bidirectional text.

In an HTML resource, rebuilding a DOM range from a purely textual range using tree walking is cumbersome and is not an optimal solution. Such a selector can only be efficient if it is used a refinement of a CSS Selector that targets the closest common ancestor of the element nodes containing the start and end characters, and if the segment of text is small enough. The use of the closest common ancestor element also makes the selector more robust against evolutions of the HTML resource.

Implementation: This selector is exported from and processed by Thorium Reader 3.1 if the publication is in EPUB or Web Publication format.

Sample 6: A CSS Selector refined by a Text Position Selector:

{
  "selector": [
    {
      "type": "CssSelector",
      "value": "#intro > p:nth-child(2)",
      "refinedBy": {
        "type": "TextPositionSelector",	 
        "start": 4,
        "end": 19
      }
    }
  ]
}

This selects "q" from "quick" as start position and "x" from "fox" as end position in the following HTML snippet:

<div id="intro">
  <p>Some text.</p>
  <p>The quick <em>brown</em> fox jumps over the lazy dog.</p>
  <p>The lazy <em>white</em> dog sleeps with the crazy fox.</p>
</div>

1.3.2.3. ProgressionSelector

A ProgressionSelector contains a decimal value representing the annotation's position as a percentage of the total size of the resource.

While such positioning is imprecise and does not correctly identify a fragment, it is helpful to order annotations in a list and help position the annotation near the corresponding fragment if other selectors fail.

Sample: This ProgressionSelector indicates that the annotation is positioned just after the middle of the resource:

{
  "selector": [
    {
    "type": "ProgressionSelector",	 
    "value": 0.534234255
    }
  ]
}

1.3.3. Meta

Meta information MAY be added to an annotation as “breadcrumbs”, to ease the display of contextual information relative to the global position of the annotation in the publication.

The meta property contains:

Name	Description	Format	Required?
`headings`	Ancestor headings of the annotation.	Array of Heading objects	No
`page`	Page of the publication containing the annotation. It may be either a synthetic page or a print equivalent. It is essentially a visual indicator.	string	No

1.3.3.1. Headings

The Headings object contains:

Name	Description	Format	Required?
`level`	Heading level.	number	No
`txt`	Heading title.	string	No

Sample 7: Meta information contains ancestor headings and a page number:

{
  "@context": "http://www.w3.org/ns/anno.jsonld",
  "type": "Annotation",
  "target": {
    "source": "OEBPS/text/chapter11.html",
    "selector": [
    ],
    "meta": {
      "headings": [
        {
        "level": 1,
        "txt": "Section 11"
        },
        {
        "level": 2,
        "txt": "Sub Section 1"
        }
      ],
      "page": "XI"       
    }
  }
}

1.4. Body

The body of an annotation contains plain text, style, and an optional keyword.

The body property contains:

Name	Description	Format	Required?
`type`	The body type. It MUST be “TextualBody”.	string	Yes
`value`	The textual content of the annotation.	string	Yes
`format`	The media-type of the annotation value; 'text/plain' by default; 'text/markdown' is recommended.	rfc6838, rfc7763	No
`color`	The colour of the annotation; yellow by default.	"pink" \| "orange" \| "yellow" \| "green" \| "blue" \| "purple"	No
`highlight`	The style of the annotation; solid background by default.	"solid" \| "underline" \| "strikethrough" \| "outline"	No
`language`	The language of the annotation.	BCP47	No
`textDirection`	The direction of the text; left-to-right by default.	"ltr" \| "rtl"	No
`keyword`	Free text categorising the annotation.	string	No

Note: read “Best practices for Reading Systems” about using a keyword in an annotation.

Sample 8: An annotation Body.

{
  "@context": "http://www.w3.org/ns/anno.jsonld",
  "type": "Annotation",
  "body": {
    "type" : "TextualBody",
    "value" : "j'adore !",
    "keyword" : "teacher",   
    "color" : "blue",
    "language" : "fr",
    "textDirection" : "ltr"
  }
}

2. Annotation Set

An Annotation does not contain information about its associated publication. If a set of annotations is shared as a detached file, it is mandatory to export with them information that will help find the associated publication even if the publication is not adequately identified.

Note: the AnnotationCollection defined by the W3C does not provide an adequate structure for sharing annotations either as a detached file or as a file embedded in a Zip package. The AnnotationCollection is intrinsically paginated and provides a way to retrieve annotations via a REST API.

The AnnotationSet object contains:

Name	Description	Format	Required?
`@context`	The context that determines the meaning of the JSON as an annotation set. It MUST be “http://www.w3.org/ns/anno.jsonld”.	string	Yes
`id`	The identity of the annotation set. A uuid formatted as a URN is recommended.	URI	Yes
`type`	The RDF structure type. It MUST be "AnnotationSet".	string	Yes
`generator`	The agent responsible for the generation of the object serialisation.	Generator	No
`about`	Information relative to the publication.	About object	Yes
`generated`	The time when the set was generated.	ISO 8601 datetime	No
`title`	A title helping on the identification of the set.	string	No
`items`	The annotations of the set.	Array of Annotation objects	Yes

2.1. Generator

The Generator object contains information relative to the software from which the serialized annotation has been produced.

Name	Description	Format	Required?
`id`	The identity of the generator software. The recommended value is the GitHub URL of the application source code.	URI	Yes
`type`	The RDF structure type. It MUST be "Software".	string	Yes
`name`	The name of the generator software.	string	Yes
`homepage`	The home page presenting the generator software.	URL	No

2.2. About

The About object contains information relative to the publication. Such metadata in intended to help associating an annotation set with a publication:

Name	Description	Format	Required?
`dc:identifier`	Publication identifiers. An ISBN is preferred.	Array of strings	No
`dc:title`	The title of the publication.	string	No
`dc:format`	The media type of the publication.	string	No
`dc:publisher`	The name of the publisher.	string	No
`dc:creator`	The author(s) of the publication.	array of strings	No
`dc:date`	The release year.	calendar year using four digits	No

Note: all properties defined above are from the Dublin Core vocabulary, referenced in the Web Annotation Data Model.

Sample 9: An AnnotationSet containing one annotation.

{
  "@context": "http://www.w3.org/ns/anno.jsonld",
  "id": "urn:uuid:123-123-123-123",
  "type": "AnnotationSet",
  "generator": "https://github.com/edrlab/thorium-reader/releases/tag/v3.1.0",
  "generated": "2023-09-01T10:00:00Z",
  "title": "Annotations Mme Prof, La Peste, cours 1ere B",
  "about": {
     "dc:identifier": [
        "urn:isbn:1234567890"
     ],
     "dc:format": "application/epub+zip",
     "dc:title": "Alice in Wonderland",
     "dc:publisher": "Example Publisher",
     "dc:creator": ["Anne O'Tater"],
     "dc:date": "1865"
  },
  "items": [
    {
      "@context": "http://www.w3.org/ns/anno.jsonld",
      "id": "urn:uuid:234-234-234-234",
      "type": "Annotation",
      "target": {
      },
      "body": {
      }
    }
  ]
}

2.3. Media type of an Annotation Set

This specification introduces a dedicated media type value to identify an AnnotationSet: application/rd-annotations+json.

HTTP responses associated with annotation files must indicate this media type in their header.

Note: I propose using a “rd-” prefix (for ReaDium). It would allow for easier registration at IANA. Should we extend this to other media types, including “application/webpub+json”?

2.4. File extension of an Annotation Set

This specification introduces a dedicated file extension for serialized AnnotationSets: .ann.

3. Embedding annotations in publications

3.1. In EPUB

The OPTIONAL annotations.ann file in the META-INF directory holds an AnnotationSet.

3.2. In Readium Web Publications

The JSON file holding an AnnotationSet MUST be represented as a link object in the links collection, with an annotations relation.

Sample 10: A Readium Web Publication Manifest containing a link to an annotations file.

{
  "@context": "https://readium.org/webpub-manifest/context.jsonld",
  "metadata": {
  },
  "links": [
    { 
      "rel": "self", 
      "href": "http://example.com/manifest.json", 
      "type": "application/webpub+json"
    },
    {
      "rel": "annotations", 
      "href": "https://example.com/annotations.ann", 
      "type": "application/rd-annotations+json"
    }
  ]
}

When a Web Publication is packaged using the Readium Packaging Format, it is up to the generator to embed the annotation file in the package or keep it remote.

4. Best Practices for Reading Systems

This section is non-normative.

4.1. Displaying filtered annotations

Reading systems should enable filtering by colour, highlight mode, keyword and creator. For instance, a user can display "blue" annotations only or “teacher” annotations only. Filtering on multiple criteria is a plus.

4.2. Using multiple selectors

It is recommended that Reading Systems export CssSelector + TextPositionSelector + ProgressionSelector, as they can be used for sorting annotations when no other sortable selector is present.

When displaying an annotation, a Reading System is free to use the most precise Selector available. It will select an alternative Selector as a fallback in case the preferred one does not return a correct position in the publication: this can happen if the publication has been modified after the annotation has been created.

4.3. Exporting annotations as a detached file

When a user decides to export an annotation set from a reading system, he SHOULD be proposed to filter the annotations by keywords (multiple choice). “Annotations with no keyword” and “All annotations” SHOULD be proposed as options. The advantage of this practice is that, for instance, a user can export personal annotations (usually with no keyword) and leave “teacher” annotations unexported.

He MAY enter a title for the annotation set (empty by default). Such a title SHOULD become the exported filename.

He MUST be able to choose the directory in which the annotation set will be stored.

The file extension MUST be .ann.

The application may propose alternative formats at export time: an HTML or markdown format may be handy as a list of annotations with human-friendly references to the location of each annotation.

4.4. Exporting annotations in a publication

When a user decides to export a publication from the Reading System, he SHOULD be proposed to embed the annotations associated with the publication.

If the user decides to embed annotations in a publication, he SHOULD be proposed to filter the annotations by keywords (multiple choice).

4.5. Importing annotations

To simplify the association of annotations with a publication, a Reading System MUST offer a way to select a publication before selecting an annotation set. The drag and drop of an annotation set into a Reading System MAY also be proposed, but identifying the proper publication from the metadata in the annotation set is more complicated.

When importing an annotation set, a Reading System SHOULD display a message with the title of the annotation set and the number of annotations in the set. The Reading System MUST offer the user the choice to abort the import.

Each annotation is uniquely identified. If during the import of an annotation set, one or more annotations are re-imported, the Reading System MUST offer to the user the choice to override existing annotations or abort the import of the annotation set.

4.6. Dealing with colours

This document specifies a closed set of six colours chosen because of their extensive support in well-known reading systems. However, most existing reading apps offer a smaller set to their users.

If an application imports annotations with a colour it does not support, it should display them with a neutral colour. The recommended neutral colour is grey.

Some applications may support colours not in the set defined by this specification (e.g. brown). In this case, a 1-to-1 substitution at export time is required (e.g. brown to orange).