EPub

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Definition

ePub is a popular open e-book standard.

“".epub" is the file extension of an XML format for reflowable digital books and publications. ".epub" is composed of three open standards, the Open Publication Structure (OPS), Open Packaging Format (OPF) and Open Container Format (OCF), produced by the IDPF.” (dipf, retrieved 22:38, 26 February 2009 (UTC))

Software and Formats

ePub can be authored and read with an increasing set of software. Since it is an open standard, it does have support from various vendors and publishers (see e.g. Tim O'Reilly Unplugged: The Kindle 2 And Transforming Industries).

Formats

ePub formats are defined with Relax NG but rely on other standards too.

Overview

The ePub Specification comes in three parts:

(1) The open publication structure (OPS) - 09/11/07
  • The Open Publication Structure 2.0 (OPS) is an XML-based standard for authoring digital publications. Contents can be marked up either with a subset of XHTML or Daisy DTBook.
  • The Open Packaging Format (OPF) 2.0 describes the structure of an .epub in XML
(2) The open container format - 10/27/06
  • The OPS Container Format 1.0 (OCF) is a zip-based standard used to encapsulate publication components for transport and delivery.

ePub contents may be DRM controlled, but must not ...

The container and packaging

The *.epub zip file by example:

If we create an e-pub version of this page we get a file called xxx.epub. This *.epub file is an OCF zip file. Here is the structure:

edutechwiki_epub.epub (the zip file)
META-INF folder
container.xml
OEBPS folder
fonts folder (includes ttf fonts used)
content1.xhtml
content.opf
style.css
image_name1
image_name2
....
Mimetype

That kind of packaging structure follows quite a similar philosophy as the IMS Content Packaging standard. I.e. a zip file includes a central xml file (content.opf) that includes the definition of organization (the "spine") and the metadata. It then includes all the assets needed for rendering.

The mime-type, i.e. contents of the Mimetype file is application/epub+zip.

Let us now describe some of its files:

The Open Packaging Format (OPF)

File content.opf describes the various content elements of the epub package. E.g. an example made with an automatic online converter for this page looks like this: 

<?xml version='1.0' encoding='UTF-8'?>
<package xmlns:dc="http://purl.org/dc/elements/1.1/"
         xmlns="http://www.idpf.org/2007/opf" version="2.0" unique-identifier="bookid">
  <metadata>
    <dc:title>EPub</dc:title>
    <dc:identifier id="bookid">web2fb2_200904221954_3347363837</dc:identifier>
    <dc:language>En</dc:language>
    <dc:creator>Daniel K. Schneider</dc:creator>
    <dc:type>reference</dc:type>
  </metadata>
  <manifest>
    <item id="css" href="style.css" media-type="text/css"/>
    <item id="content1" href="content1.xhtml" media-type="application/xhtml+xml"/>
    <item id="i0ced13f269" href="i0ced13f269" media-type="image/png"/>
    <item id="ib166f0f69c" href="ib166f0f69c" media-type="image/png"/>
    <item id="i7fa52f212a" href="i7fa52f212a" media-type="image/png"/>
    <item id="i33954a4ae2" href="i33954a4ae2" media-type="image/png"/>
    <item id="i8be224f209" href="i8be224f209" media-type="image/png"/>
  </manifest>
  <spine>
    <itemref idref="content1"/>
  </spine>
</package>
The spine is mandatory

“Following manifest, there must be one and only one spine element, which contains one or more itemref elements. Each itemref references an OPS Content Document designated in the manifest. The order of the itemref elements organizes the associated OPS Content Documents into the linear reading order of the publication.” (Open Packaging Format (OPF) 2.0 specification). This spine (one could translate this to "parts") can include three different kinds of files:

  • XHTML
  • XML (yours, i.e. what they call out-of-line XML
  • DTBook

Spine elements refer to resources defined in the manifest and may include a table of contents. E.g. a simple example would look like this: 

<manifest>
     <item id="intro"
           href="intro.html"
           media-type="application/xhtml+xml" />
     <item id="chap1"
           href="chap1.html"
           media-type="application/xhtml+xml" />
     <item id="chap2"
           href="chap2.dtb"
           media-type="application/x-dtbook+xml" />
     <item id="chap3"
           href="chap3.html"
           media-type="application/xhtml+xml" />
     <item id="f1"
           href="fig1.jpg"
           media-type="image/jpeg" />

     <!--  ...... other multimedia assets here .... -->

     <item id="toc_item"
           href="toc.ncx"
           media-type="application/x-dtbncx+xml" />
</manifest>

<spine toc="toc_item">
     <itemref idref="intro" />
     <itemref idref="chap1" />
     <itemref idref="chap2" />
     <itemref idref="chap3" />
</spine>
The metadata

The metadata are defined using Dublin Core plus possible user-defined tags. Some of these metadata are mandatory, i.e. title, identifier and language.

The manifest

This manifest (like in IMS content packaging) must include all files that are part of the publication in any order. It must have a structure like this according to the specification. I.e. each item must have an id, an href to a resource and a media-type. In addition, one can define fall-back elements. 

 <manifest>
        <item id="intro" href="introduction.html"
                media-type="application/xhtml+xml" />
        <item id="c1" href="chapter-1.html"
                media-type="application/xhtml+xml" />
        <item id="c2" href="chapter-2.html"
                media-type=application/xhtml+xml" />
        <item id="toc" href="contents.xml"
                media-type="application/xhtml+xml"
                fallback="fall1"  />
        <item id="oview" href="arch.png"
                media-type="image/png" />
        <item id="fall1" fallback="fall2"
                href="SomeDoc.pdf"
                media-type="application/pdf" />
 </manifest>

The XHTML files can include various formats, e.g. binary pictures, SVG and in-line XML. All these formats can be style with a subset of CSS2.

The Open Packaging Format (OPF) 2.0 v0.9871.0 relax schema

META-INF files in the epub package

All valid OCF Containers must include a directory called META-INF at the root level of the container file system. This directory contains the files specified below that describe the contents, metadata, signatures, encryption, rights and other information about the contained publication. (OCF 1.0 specification, retrieved 17:59, 22 April 2009 (UTC)).

The container.xml file describes in a simple case where to find the content.opf file. In our simple example it looks like this: 

<?xml version="1.0"?>
<container version="1.0" xmlns="urn:oasis:names:tc:opendocument:xmlns:container">
  <rootfiles>
    <rootfile full-path="OEBPS/content.opf" media-type="application/oebps-package+xml"/>
  </rootfiles>
</container>

An other example taken from the OCF 1.0 specification show that one could include an alternative PDF file for example: 

<?xml version="1.0"?>
<container version="1.0" xmlns="urn:oasis:names:tc:opendocument:xmlns:container">
  <rootfiles>
    <rootfile full-path="OEBPS/My Crazy Life.opf"
     media-type="application/oebps-package+xml" />
    <rootfile full-path="PDF/My Crazy Life.pdf"
     media-type="application/pdf" />
  </rootfiles>
</container>

In addition to container.xml, there can be five other files:

  • manifest.xml
  • metadata.xml
  • signatures.xml
  • encryption.xml
  • rights.xml

The formal specification of these META-INF files is done with a little Relax NG schema:

  • There is one rule for container (see the examples above).
  • One rule for signatures and that includes the xmldsig-core-schema.rng Schema
  • One rule for encryption that also refers to extenal *.rng Schemas.

The XHTML modules used in OPS

OPS uses a set of XHTML modules with some additional restrictions. E.g. OPS is always XHTML compatible, but no the other way round.


XHTML 1.1 Module Name Elements (non-normative)
Structure body, head, html, title
Text abbr, acronym, address, blockquote, br, cite, code, dfn, div, em, h1, h2, h3, h4, h5, h6, kbd, p, pre, q, samp, span, strong, var
Hypertext a
List dl, dt, dd, ol, ul, li
Object object, param
Presentation b, big, hr, i, small, sub, sup, tt
Edit del, ins
Bidirectional Text bdo
Table caption, col, colgroup, table, tbody, td, tfoot, th, thead, tr
Image img
Client-Side Image Map area, map
Meta-Information meta
Style Sheet style
Style Attribute (deprecated) style attribute
Link link
Base base

Remark: EPub also can use the DTBook (DAISY/NISO standard) for markup.

SVG

Readers must support SVG 1.1. SVG animation and scripting features are not supported and must not be used by publication authors; a Reading System should not render such content. CSS styling of SVG must be fully supported.

SVG content can be used from XHML img and object elements but also within XHTML (probably in the standard way with namespaces).

XML

You also may use your own XML both inline within XHTML and out-of-line as documents. Both can have fallback options (to be used when the contents can't be rendered by a client).

OPS style Sheets

OPS Style sheets are CSS2 styles in the XML tradition, i.e. selectors and attribute names are case sensitive. Again, like for XHTML, there are some restrictions.

Authoring software

“Web-standard formats such as XHTML make up the core of OPS. OPS files can be created using a wide variety of Web and eBook publishing tools. They can also be created using XML editors such as Altova’s XML Spy. Adobe InDesign CS3 supports the direct generation of OCF-packaged OPS content as an export function. OCF files can also easily be created using standard ZIP applications like WINZIP” (IDPF FAQ, retrieved 22:38, 26 February 2009 (UTC).

Conversion software

There exist several tools that can convert from one format to another. Mileage varies according to input. E.g. PDF is more difficult to convert than XHTML for example,

  • PDF input leads to really ugly output
  • Web pages like this mediawiki page translate badly, e.g. bullets go away.
Calibre
Calibre is a one stop solution to all your e-book needs. It is free, open source and cross-platform in design and works well on Linux, OS X and Windows.
List of features: Library Management - Format conversion (all major ebook formats can be converted from) - Syncing to ebook reader devices - Fetching news from the web and converting it into ebook form - Viewing many different ebook formats - Giving you access to your book collection over the internet using just a browser
Source formats: LIT, MOBI, EPUB, HTML, PRC, RTF, TXT, PDF. Some convert better than others, e.g. PDF pictures don't translate.
Output formats: EPUB, LRF, MOBI
Easy to install under Ubuntu (tested with 8.04 Hardy Heron)
On-line conversion tools
  • Web2FB2 is a webpage to F3B and EPUB converter. (tested April 2009). tip: make use of the advanced options to set both title and author. The file name created will be named authorname_first_name_title.epub.

Reader Software

Most readers support several e-book formats and several support ePub. See also: EPUB at mobileread.com and Wikipedia's Comparison of e-book formats.

There exist several readers. Here are just some of these:

FBReader
FBReader e-book reader for Unix/Windows computers. It supports several formats.
Wikipedia entry
Unix/Windows (It's included in Ubuntu's Synaptic)
Features: Several formats (partially or fully) . Several reading options.
Calibre
Multi-purpose tool, see above
Openberg
OpenBerg, was an opensource initiative to write a system (reader, authoring etc.). The reader was a Firefox extension. Now dead ?
Wikipedia entry
Adobe Digital editions
Adobe Digital Editions is a free software with built-in DRM mechanism. If you buy contents and if you register with Adobe you can use the contents on a limited set of other computers - needs some clarification)
Wikipedia entry
Stanza
Lexcycle Stanza (favorite free iPhone/iPod reader). Win/Mac/i*.
Wikipedia Entry

Links

General
  • EPUB (mobileread.com wiki)
Official
Contents
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