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Parsing EPUB Metadata

The goal of this document is to provide directions that each implementation of Readium can follow when parsing EPUB 2.x and 3.x publications.

While the default context is very flexible in the way each metadata can be represented, when parsing a publication in the streamer we always use the most complex form for each metadata to harmonize our output.

Related Repository: Readium Web Publication Manifest

Localized Strings

In many cases, the default context supports alternate representations of the same string in different scripts and languages by means of JSON-LD language maps. To fill such a map from an EPUB metadata element, proceed as follows:

  • Determine the language used in the content of the carrying element as defined in the XML specification, i.e. check whether the carrying element has or inherits an xml:lang attribute.
  • In the EPUB 3.x case, check if the element is refined by some meta elements that have or inherit an xml:lang attribute and whose property is alternate-script. For each one, add to the map the corresponding language associated with the content of the meta element.
  • When no language hint is available, use null or und depending on the platform.

Sorting keys

Localized sorting keys are supported in RWPM for publication title, contributor/collection' names and subject' names. While computing the localized string, use the language of the carrying element as defined in the XML specification and fallback to null or und.

Title

The title and sortAs keys of a publication are objects where each key is a BCP 47 language tag and each value of this key is a string.

When parsing an EPUB, we need to establish:

  • which title is the primary one
  • a language map of the representations of the title
  • a language map of strings used to sort the title of the publication
  • which title is the subtitle
  • a language map of the representations of the subtitle

EPUB 2.x

The first <dc:title> element should be considered the primary one.

Parse it as a localized string to compute a language map.

The value of sorting key of the publication is given by the content attribute in a meta whose name is calibre:title_sort.

The subtitle can’t be expressed.

EPUB 3.x

The primary title is defined using the following logic:

  1. it is the <dc:title> element whose title-type (refine) is main;
  2. if there is no such refine, it is the first <dc:title> element.

Parse it as a localized string to compute a language map.

The sorting key of the publication is carried by the main title’s refine whose property is file-as. If there is none, fallback to the EPUB 2.x case.

The subtitle is the value of the <dc:title> element whose title-type (refine) is subtitle. In case there are several, use the one with the lowest display-seq (refine). Parse it as a localized string to compute a language map.

Identifier

The identifier of a publication is a key whose value is a string. It must be a valid URI.

When parsing an EPUB, we need to establish:

  • which identifier is the primary one
  • which scheme is this identifier’s
  • the identifier’s valid URI

EPUB 2.x

The primary identifier is marked as the unique identifier via the package element unique-identifier attribute. It’s id must therefore match the value of this unique-identifier attribute.

To determine the scheme, first look at the opf:scheme attribute e.g.

<dc:identifier id="pub-id" opf:scheme="ISBN">123456789X</dc:identifier>

If there is none, then the identifier must be value parsed in order to guess it.

The valid URI is the result of this second step e.g. urn:isbn:123456789X.

EPUB 3.x

The primary identifier is marked as the unique identifier via the package element unique-identifier attribute. It’s id must therefore match the value of this unique-identifier attribute.

To determine the scheme, first look if there is a refine whose property is identifier-type e.g.

<dc:identifier id="pub-id">urn:isbn:123456789X</dc:identifier>
<meta refines="#pub-id" property="identifier-type" scheme="onix:codelist5">15</meta>

If there is none, then the identifier must be value parsed in order to guess it.

The valid URI is the result of this second step e.g. urn:isbn:123456789X.

Contributors

The contributor’s key depend on the role of the creator or contributor. It is an object that contains a name, a sortAs and an identifier key.

The name and sortAs keys of each contributor are objects where each key is a BCP 47 language tag and each value of the key is a string.

The contributor object may also contain an identifier string that must be a valid URI.

When parsing an EPUB, we need to establish:

  • the key of the contributor;
  • a language map for the name of this contributor;
  • a language map used to sort the name of the contributor.

EPUB 2.x

The following mapping should be used to determine the key of the contributor’s object:

element opf:role key
dc:creator <empty> or <unknown> author
dc:contributor <empty> or <unknown> contributor
dc:creator or dc:contributor aut author
dc:creator or dc:contributor pbl publisher
dc:creator or dc:contributor trl translator
dc:creator or dc:contributor edt editor
dc:creator or dc:contributor ill illustrator
dc:creator or dc:contributor art artist
dc:creator or dc:contributor clr colorist
dc:creator or dc:contributor nrt narrator
dc:publisher N/A publisher

Where opf:role is the value of the attribute of the <dc:element>.

Parse the carrying element as a localized string to compute a language map for the contributor’s name.

Finally, the string used to sort the name of the contributor is provided by the value of the opf:file-as attribute of this element.

EPUB 3.x

The following mapping should be used to determine to key of the contributor’s object:

element role key
dc:creator <empty> or <unknown> author
dc:contributor <empty> or <unknown> contributor
dc:creator or dc:contributor aut author
dc:creator or dc:contributor pbl publisher
dc:creator or dc:contributor trl translator
dc:creator or dc:contributor edt editor
dc:creator or dc:contributor ill illustrator
dc:creator or dc:contributor art artist
dc:creator or dc:contributor clr colorist
dc:creator or dc:contributor nrt narrator
dc:publisher N/A publisher
media:narrator N/A narrator

Where role is the value of the refine whose scheme is a value of marc:relators.

Parse the contributor element as a localized string to compute a language map for the contributor’s name.

Finally, the string used to sort the name of the contributor is carried by the contributor's refine whose property is file-as.

Language

The language of a publication is a key whose value is a string or an array of valid BCP 47 language tags.

When parsing an EPUB, we need to establish:

  • which language is the primary one
  • additional languages

EPUB 2.x

The first <dc:language> element must be considered the primary language of the publication.

If there is more that one <dc:language> elements, then the value of the key is an array whose first entry is the primary language.

EPUB 3.x

The first <dc:language> element must be considered the primary language of the publication.

If there is more that one <dc:language> elements, then the value of the key is an array whose first entry is the primary language.

Description

The description of a publication is a key whose value is a string in plain text.

The string is the value of the <dc:description> element.

Publication Date

The published date of a publication is a key whose value is a string conforming to ISO 8601.

EPUB 2.x

The string is the value of the <dc:date> element whose opf:event attribute has the value publication.

EPUB 3.x

The string is the value of the <dc:date> element.

Modification Date

The modified date of a publication is a key whose value is a string conforming to ISO 8601.

EPUB 2.x

The string is the value of the <dc:date> element whose opf:event attribute has the value modification.

EPUB 3.x

The string is the value of the meta element whose property attribute has the value dcterms:modified.

Subjects

The subject of a publication is a key whose value is, in the most complex form, an array of subject objects.

Although each subject should have its own <dc:subject> element, this is not necessarily the case in practice, authors and authoring tools often separating multiple subjects using commas or semicolons in the same element. So, if there is a single dc:subject that is not refined by any property, split its content at every comma and semicolon and consider you have several dc:subject with shared attributes.

Parse each <dc:subject> element as a localized string to compute a language map for the subject’s name.

EPUB 2.x

sortAs, code and scheme cannot be expressed.

EPUB 3.x

The sortAs string used to sort the subject is the value of the refine whose property has the value of file-as.

The code property has the same value as the refine whose property has the value of term.

The scheme property has the same value as the refine whose property has the value of authority.

Collections and Series

The belongsTo of a publication is an object containing a collection or series object.

This latter object contains name, sortAs and identifier keys, whose values are a string, and a position key, whose value is a number.

When parsing an EPUB, we need to establish:

  • which object to use;
  • the name of the collection/series;
  • the string used to sort the name of the collection/series;
  • the URI for the collection/series;
  • the position of the publication in this collection/series.

EPUB 2.x

The object to use will always be series.

The string for its name is the value of the content attribute in the <meta> element whose name attribute has the value calibre:series.

The position of the publication is the value of the content attribute – converted to a number – in the <meta> element whose name attribute has the value calibre:series_index.

Please be aware that it can be a floating point number with up to two digits of precision e.g. 1.01, and zero and negative numbers are allowed.

EPUB 3.x

The object to use depends on the refine whose property has the value of collection-type:

  1. if it is series, use series;
  2. else use collection.

The string for its name is the value of the <meta> element whose property has the value of belongs-to-collection and which is refined.

The sortAs string used to sort the name is the value of the refine whose property has the value of file-as.

The identifier string is the value of the refine whose property has the value of dcterms:identifier.

The position of the publication is the value of the refine whose property has the value of group-position.

If there is no series, try to parse calibre:series as in the EPUB 2.x case.

Progression Direction

The readingProgression of a publication is a key whose value is a string amongst the following:

  • auto;
  • ltr;
  • rtl.

This string is the value of the page-progression-direction attribute of the <spine> element.

If no value is set, it defaults to auto.

Number of pages

The numberOfPages of a publication is a key whose value is an integer.

EPUB 2.x

Does not apply.

EPUB 3.x

The integer is the value of the meta element whose property attribute has the value schema:numberOfPages.

Rendition / Presentation

The presentation of a publication is an object containing the following keys:

  • continuous;
  • layout;
  • orientation;
  • overflow;
  • spread.

In addition, the following elements may be included in properties in a Link Object from the readingOrder (spine overrides):

  • layout;
  • orientation;
  • overflow;
  • page;
  • spread.

Flow

The rendition:flow metadata is mapped to overflow and continuous.

The overflow of a publication is a key whose value can be the following string:

  • auto;
  • paginated;
  • scrolled.

This overflow can be continuous, which is a key whose value is a boolean: true or false

EPUB 2.x

Does not apply.

EPUB 3.x

Global Property

The string is the value of the <meta> element whose property attribute has the value rendition:flow with the following mapping:

rendition:flow values
auto "overflow": "auto" + "continuous": false
paginated "overflow": "paginated" + "continuous": false
scrolled-doc "overflow": "scrolled" + "continuous": false
scrolled-continuous "overflow": "scrolled" + "continuous": true

If no value is set, it defaults to auto.

Spine Overrides

For each spine item, the value of overflow must be inferred from the properties attribute whose value contains rendition:flow-.

Properties value
rendition:flow-auto auto
rendition:flow-paginated paginated
rendition:flow-scrolled-doc scrolled
rendition:flow-scrolled-continuous scrolled

Layout

The layout of a publication is a key whose value can be the following string:

  • reflowable;
  • fixed.

EPUB 2.x

If the publication either has a com.kobobooks.display-options.xml or com.apple.ibooks.display-options.xml in its META-INF folder, then check whether an <option> element whose name attribute is fixed-layout exists e.g.

<display_options>
  <platform name="*">
    <option name="fixed-layout">true</option>
  </platform>
</display_options>
  • If the value is true, then the value of the layout key is fixed.
  • If the value is false, then the value of the layout key is reflowable.
  • If no such option is set, then the value of the layout key defaults to reflowable.

EPUB 3.x

Global Property

The string is the value of the <meta> element whose property attribute has the value rendition:layout with the following mapping:

rendition:layout value
reflowable reflowable
pre-paginated fixed

If no value is set, it defaults to reflowable.

Spine Overrides

For each spine item, the value of layout must be inferred from the properties attribute whose value contains rendition:layout-.

Properties value
rendition:layout-reflowable reflowable
rendition:layout-pre-paginated fixed

Orientation

The orientation of a publication is a key whose value can be the following string:

  • auto;
  • landscape;
  • portrait.

EPUB 2.x

If the publication has a com.apple.ibooks.display-options.xml in its META-INF folder, then:

  1. prioritize the platform element whose name attribute has a value of *;
  2. if there is none, then prioritize ipad;
  3. if there is still none, then fall back on iphone.

The string is the value of the <option> element whose name attribute is orientation-lock with the following mapping:

option value
none auto
landscape-only landscape
portrait-only portrait

If no such option is set, it defaults to auto.

EPUB 3.x

Global Property

The string is the value of the <meta> element whose property attribute has the value rendition:orientation.

If no value is set, it defaults to auto.

Spine Overrides

For each spine item, the value of orientation must be inferred from the properties attribute whose value contains rendition:orientation-.

Properties value
rendition:orientation-auto auto
rendition:orientation-landscape landscape
rendition:orientation-portrait portrait

Spread

The spread of a publication is a key whose value can be the following string:

  • none;
  • auto;
  • landscape;
  • both.

EPUB 2.x

Does not apply.

EPUB 3.x

Global Property

The string is the value of the <meta> element whose property attribute has the value rendition:spread with the following mapping:

rendition:spread value
none none
auto auto
landscape landscape
portrait both
both both

If no value is set, it defaults to auto.

Spine Overrides

For each spine item, the value of spread must be inferred from the properties attribute whose value contains rendition:spread-.

Properties value
rendition:spread-none none
rendition:spread-auto auto
rendition:spread-landscape landscape
rendition:spread-portrait both
rendition:spread-both both

Page-spread-* properties

The page of a Link Object properties is a key whose value can be the following string:

  • center;
  • left;
  • right.

EPUB 2.x

Does not apply.

EPUB 3.x

For each spine item, the value of page must be inferred from the properties attribute whose value contains rendition:page-spread-.

Properties value
rendition:page-spread-center center
rendition:page-spread-left left
rendition:page-spread-right right