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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 defining 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 when 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 accomodating 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 certainly 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 amount of 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 full 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. EPUB CFI Selector

Reference in the W3C Annotation Data Model: EPUB CFI Selector

This Selector describes a range of text in an EPUB resource by expressing a starting and ending character using the EPUB Canonical Frangment Identifier syntax.

To correspond to a text segment, the EPUB CFI expression MUST correspond to a range in the form epubcfi(P,S,E), where P represents a common Parent path, S and E the start and end subpaths respectively.

Note: Both the left-hand part (resource location in the publication) and the right-hand part (anchoring of the text fragment) of the path must be present.

Important: The reading system community lacks a reference and open-source implementation of EPUB CFI to DOM Range conversion (back and forth). Current implementations are not fully interoperable, essentially because some implementations use unicode code points instead of code units. Our implementation assumes the use of code units.

Implementation: This selector is exported from and processed by Thorium Reader 3.1 when the publication is in EPUB format.

Sample 4: A text segment represented as an EPUB CFI:

{
  "selector": [
    {
    "type": "FragmentSelector",	 
    "conformsTo": "http://www.idpf.org/epub/linking/cfi/epub-cfi.html",
    "value": "epubcfi(/6/4[chap01ref]!/4[body01]/10[para05],/2/1:1,/3:4)"
    }
  ]
}

Note: It would be more efficient to create a simpler EPUBCFISelector, as an equivalent of the FragmentSelector conforming to http://www.idpf.org/epub/linking/cfi/epub-cfi.html defined in the W3C Annotation Model, which would avoid expressing the "epubcfi()" part and the left part of the CFI which relates to the resource already selected by the "source" property of the annotation.

1.3.2.3. CSS Selector

Reference in the W3C Annotation Data Model: CSS Selector

This Selector describes an HTML element in an EPUB resource using the CSS Selectors Level 3 syntax.

It is used to select an image, an audio or video clip or any other element in the DOM that does not correspond to a text fragment. The annotation is therefore contextual to the HTML resource, not directly attached to the media resource.

Note: such construct can be mapped from and to a DOM Range using simple code. See https://www.npmjs.com/package/css-selector-generator for an example of open-source codebase generating CSS Selectors from a Node; For getting a Node from a CSS Selector, developers will use document.querySelector().

Implementation: Thorium Reader 3.1 does not allow the selection of a media resource. This will be added in a future version.

Sample 5: A CSS Selector points at the 5th img in the resource.

{
  "selector": [
    {
    "type": "CSSSelector",
    "value": "img:nth-child(5)"
    }
  ]
}
1.3.2.4. 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>

1.3.3. 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; 'plain/text' by default. rfc6838 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 criterias is a plus.

4.2. Using multiple selectors

It is recommended that Reading Systems export EPUB CFI Selectors, 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 let “teacher” annotations unexported.

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

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

The file extention of the file MUST be .ann.

The application may propose alternative formats at export time: a 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 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&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 large support in well-known reading systems. But 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 these annotations with a neutral colour. The recommended neutral colour is grey.

Some applications may support colours that are 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).

Note: we didn't spot applications which offer more than six annotation colours.

5. JSON Schemas

6. References

Open Annotation in EPUB, 2015: https://idpf.org/epub/oa/

Open Annotation in EPUB, CFI pros and cons, 2014: https://docs.google.com/document/d/1d0IRsb2h9LM-ZWPwjS4dZ4Fffg4u9qA3Dkw0P9_3o5Y/edit

EPUB CFI cannot reference content in non-spine items, 2023: w3c/epub-specs#226

W3C Web Annotation Data Model, 2017: https://www.w3.org/TR/annotation-model/

EPUB CFI, 2017: https://idpf.org/epub/linking/cfi/

EPUB 3.3, 2023: https://www.w3.org/TR/epub-33/

Readium Web Publication Manifest: https://readium.org/webpub-manifest/