EPUB

EPUB (short for electronic publication; alternatively capitalized as ePub, ePUB, EPub, or epub, with "EPUB" preferred by the vendor) is a free and open e-book standard by the International Digital Publishing Forum (IDPF). Files have the extension .epub.

EPUB is designed for reflowable content, meaning that the text display can be optimized for the particular display device used by the reader of the EPUB-formatted book, although EPUB now also supports fixed-layout content. The format is meant to function as a single format that publishers and conversion houses can use in-house, as well as for distribution and sale. It supersedes the Open eBook standard.^[3]

History

EPUB became an official standard of the International Digital Publishing Forum (IDPF) in September 2007, superseding the older Open eBook standard.^[4]

In August 2009, the IDPF announced that they would begin work on maintenance tasks of the EPUB standard.^[5] Two broad objectives were defined by this working group: "One set of activities governs maintenance of the current EPUB Standards (i.e. OCF, OPF, and OPS), while another set of activities addresses the need to keep the Standards current and up-to-date." The working group was expected to be active through 2010, publishing updated standards throughout its lifetime.^[6] On April 6, 2010, it was announced that this working group would complete their update in April 2010. The result was to be a minor revision to EPUB 2.0.1 which "corrects errors and inconsistencies and does not change functionality".^[7] On July 2, 2010, drafts of the version 2.0.1 standards appeared on the IDPF website.^[2]

On April 6, 2010, it was announced that a working group would be formed to revise the EPUB specification.^[7] In the working group's charter draft, 14 main problems with EPUB are identified which the group will address. The group was chartered through May 2011, and was scheduled to submit a final draft on May 15, 2011.^[8] An initial Editors Draft for EPUB3 was published on November 12, 2010,^[9] and the first public draft was published on February 15, 2011.^[10] On May 23, 2011, the IDPF released its proposed specification for final review.^[11] On October 10, 2011, the IDPF announced that its membership had approved EPUB 3 as a final Recommended Specification.

Features

• Free and open
• Re-flowable (word wrap) and re-sizable text
• Inline raster and vector images
• Embedded metadata
• DRM support
• CSS styling
• Support for alternative renditions in the same file
• Use of out-of-line and inline XML islands to extend the functionality of EPUB

File format

Version 3.0 (current version)

EPUB 3 consists of a set of four specifications:^[12]

• EPUB Publications 3.0, which defines publication-level semantics and overarching conformance requirements for EPUB Publications.
• EPUB Content Documents 3.0, which defines profiles of XHTML, SVG and CSS for use in the context of EPUB Publications.
• EPUB Open Container Format (OCF) 3.0, which defines a file format and processing model for encapsulating a set of related resources into a single-file (ZIP) EPUB Container.
• EPUB Media Overlays 3.0, which defines a format and a processing model for synchronization of text and audio.

Detailed descriptions of the differences between 3.0 and 2.0.1 can be found on the IDPF website.

Version 2.0.1

EPUB version 2.0.1 consists of three specifications:

• Open Publication Structure (OPS) 2.0.1, contains the formatting of its content.^[13]
• Open Packaging Format (OPF) 2.0.1, describes the structure of the .epub file in XML.^[14]
• Open Container Format (OCF) 2.0.1, collects all files as a ZIP archive.^[15]

EPUB internally uses XHTML or DTBook (an XML standard provided by the DAISY Consortium) to represent the text and structure of the content document, and a subset of CSS to provide layout and formatting. XML is used to create the document manifest, table of contents, and EPUB metadata. Finally, the files are bundled in a zip file as a packaging format.

Open Publication Structure 2.0.1

An EPUB file uses XHTML 1.1 (or DTBook) to construct the content of a book as of version 2.0.1. This is different from previous versions (OEBPS 1.2 and earlier) which used a subset drawn from XHTML. There are, however, a few restrictions on certain elements. The mimetype for XHTML documents in EPUB is application/xhtml+xml.^[13] For a table of the required XHTML modules and a description of the restrictions, please see Section 2.2 of the specification.

Styling and layout are performed using a subset of CSS 2.0, referred to as OPS Style Sheets. This specialized syntax requires only a portion of CSS properties to be supported by reading systems and adds a few custom ones. Custom properties include oeb-page-head, oeb-page-foot, and oeb-column-number. Font-embedding can be accomplished using the @font-face property, as well as including the font file in the OPF's manifest (see below). The mimetype for CSS documents in EPUB is text/css.^[13] For a table of supported properties and detailed information, please see Section 3.0 of the specification.

EPUB also requires that PNG, JPEG, GIF, and SVG images be supported using the mimetypes image/png, image/jpeg, image/gif, image/svg+xml. Other media types are allowed, but creators must include alternative renditions using supported types.^[13] For a table of all required mimetypes, see Section 1.3.7 of the specification.

Unicode is required, and content producers must use either UTF-8 or UTF-16 encoding.^[13] This is to support international and multilingual books. However, reading systems are not required to provide the fonts necessary to display every unicode character, though they are required to display at least a placeholder for characters that cannot be displayed fully.^[13]

An example skeleton of an XHTML file for EPUB looks like this:

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
  <head>
    <meta http-equiv="Content-Type" content="application/xhtml+xml; charset=utf-8" />
    <title>Pride and Prejudice</title>
    <link rel="stylesheet" href="css/main.css" type="text/css" />
  </head>
  <body>
    ...
  </body>
</html>

Open Packaging Format 2.0.1

The OPF specification's purpose is to "[define] the mechanism by which the various components of an OPS publication are tied together and provides additional structure and semantics to the electronic publication."^[14] This is accomplished by two XML files with the extensions .opf and .ncx.

.opf file

The OPF file, traditionally named content.opf houses the EPUB book's metadata, file manifest, and linear reading order. This file has a root element package and four child elements: metadata, manifest, spine, and guide. All of these except guide are required. Furthermore, the package node must have the unique-identifier attribute. The .opf file's mimetype is application/oebps-package+xml.^[14]

The metadata element contains all the metadata information for a particular EPUB file. Three metadata tags are required (though many more are available): title, language, and identifier. title contains the title of the book, language contains the language of the book's contents in RFC 3066 format or its successors, such as the newer RFC 4646 and identifier contains a unique identifier for the book, such as its ISBN or a URL. The identifier's id attribute should equal the unique-identifier attribute from the package element.^[14] For a full listing of EPUB metadata, please see Section 2.2 of the specification.

The manifest element lists all the files contained in the package. Each file is represented by an item element, and has the attributes id, href, media-type. All XHTML (content documents), stylesheets, images or other media, embedded fonts, and the NCX file should be listed here. Only the .opf file itself, the container.xml, and the mimetype files should not be included.^[14] Note that in the example below, an arbitrary media-type is given to the included font file, even though no mimetype exists for fonts.

The spine element lists all the XHTML content documents in their linear reading order. Also, any content document that can be reached through linking or the table of contents must be listed as well. The toc attribute of spine must contain the id of the NCX file listed in the manifest. Each itemref element's idref is set to the id of its respective content document.^[14]

The guide element is an optional element for the purpose of identifying fundamental structural components of the book. Each reference element has the attributes type, title, href. Files referenced in href must be listed in the manifest, and are allowed to have an element identifier (e.g. #figures in the example).^[14] A list of possible values for type can be found in Section 2.6 of the specification.

An example OPF file:

<?xml version="1.0"?>
<package version="2.0" xmlns="http://www.idpf.org/2007/opf" unique-identifier="BookId">
 
  <metadata xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:opf="http://www.idpf.org/2007/opf">
    <dc:title>Pride and Prejudice</dc:title>
    <dc:language>en</dc:language>
    <dc:identifier id="BookId" opf:scheme="ISBN">123456789X</dc:identifier>
    <dc:creator opf:file-as="Austen, Jane" opf:role="aut">Jane Austen</dc:creator>
  </metadata>
 
  <manifest>
    <item id="chapter1" href="chapter1.xhtml" media-type="application/xhtml+xml"/>
    <item id="stylesheet" href="style.css" media-type="text/css"/>
    <item id="ch1-pic" href="ch1-pic.png" media-type="image/png"/>
    <item id="myfont" href="css/myfont.otf" media-type="application/x-font-opentype"/>
    <item id="ncx" href="toc.ncx" media-type="application/x-dtbncx+xml"/>
  </manifest>
 
  <spine toc="ncx">
    <itemref idref="chapter1" />
  </spine>
 
  <guide>
    <reference type="loi" title="List Of Illustrations" href="appendix.html#figures" />
  </guide>
 
</package>

.ncx file

The NCX file (Navigation Control file for XML), traditionally named toc.ncx, contains the hierarchical table of contents for the EPUB file. The specification for NCX was developed for Digital Talking Book (DTB), is maintained by the DAISY Consortium, and is not a part of the EPUB specification. The NCX file has a mimetype of application/x-dtbncx+xml.

Of note here is that the values for the docTitle, docAuthor, and meta name="dtb:uid" elements should match their analogs in the OPF file. Also, the meta name="dtb:depth" element is set equal to the depth of the navMap element. navPoint elements can be nested to create a hierarchical table of contents. navLabel's content is the text that will appear in the table of contents generated by reading systems that use the .ncx. navPoint's content element points to a content document listed in the manifest and can also include an element identifier (e.g. #section1).^[14][16]

A description of certain exceptions to the NCX specification as used in EPUB can be found in Section 2.4.1 of the specification. The complete specification for NCX can be found in Section 8 of the Specifications for the Digital Talking Book.^[16]

An example .ncx file:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE ncx PUBLIC "-//NISO//DTD ncx 2005-1//EN"
"http://www.daisy.org/z3986/2005/ncx-2005-1.dtd">
 
<ncx version="2005-1" xml:lang="en" xmlns="http://www.daisy.org/z3986/2005/ncx/">
 
  <head>
<!-- The following four metadata items are required for all NCX documents,
including those conforming to the relaxed constraints of OPS 2.0 -->
 
    <meta name="dtb:uid" content="123456789X"/> <!-- same as in .opf -->
    <meta name="dtb:depth" content="1"/> <!-- 1 or higher -->
    <meta name="dtb:totalPageCount" content="0"/> <!-- must be 0 -->
    <meta name="dtb:maxPageNumber" content="0"/> <!-- must be 0 -->
  </head>
 
  <docTitle>
    <text>Pride and Prejudice</text>
  </docTitle>
 
  <docAuthor>
    <text>Austen, Jane</text>
  </docAuthor>
 
  <navMap>
    <navPoint class="chapter" id="chapter1" playOrder="1">
      <navLabel><text>Chapter 1</text></navLabel>
      <content src="chapter1.xhtml"/>
    </navPoint>
  </navMap>
 
</ncx>

Open Container Format 2.0.1

An EPUB file is a group of files conforming to the OPS/OPF standards that is wrapped in a ZIP file.^[3] The OCF specifies how these files should be organized in the ZIP, and defines two additional files that must be included.

The mimetype file must be a text document in ASCII and must contain the string application/epub+zip. It must also be uncompressed, unencrypted, and the first file in the ZIP archive. The purpose of this file is to provide a more reliable way for applications to identify the mimetype of the file than just the .epub extension.^[15]

Also, there must be a folder named META-INF which contains the required file container.xml. This XML file points to the file defining the contents of the book. This will be the OPF file, though additional alternative rootfile elements are allowed.^[15]

Appart from mimetype and META-INF/container.xml, the other files (OPF, NCX, XHTML, CSS and images files) are traditionally put in a directory named OEBPS.

An example file structure:

--ZIP Container--
mimetype
META-INF/
  container.xml
OEBPS/
  book.opf
  chapter1.xhtml
  ch1-pic.png
  css/
    style.css
    myfont.otf

An example container.xml, given the above file structure:

<?xml version="1.0" encoding="UTF-8" ?>
<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>

Digital rights management

An EPUB file can optionally contain DRM as an additional layer, but it is not required by the specifications.^[17] In addition, the specification does not name any particular DRM system to use, so publishers can choose a DRM scheme to their liking. However, future versions of EPUB (specifically OCF) may specify a format for DRM.^[15]

When present, DRMed EPUB files must contain a file called rights.xml within the META-INF directory at the root level of the ZIP container.^[15]

Validation

An open source tool called epubcheck exists for validating and detecting errors in the structural markup (OPS, OPF, OCF) as well as the XHTML and image files. The tool can be run from the command line, or used in webapps and applications as a library. A large part of the original work on the tool was done at Adobe Systems.^[18]

Criticism

The EPUB specification does not enforce or suggest a particular DRM scheme. This could affect the level of support for various DRM systems on devices and the portability of purchased e-books. Consequently, such DRM incompatibility may segment the EPUB format along the lines of DRM systems, undermining the advantages of a single standard format and confusing the consumer.^{[19][20][21][22][23][24]}

The EPUB 3.0 format is intended to address the following criticisms:

• While good for text-centric books, EPUB may be unsuitable for publications which require precise layout or specialized formatting, such as a comic book.^[25]

• A major issue hindering the use of EPUB for most technical publications is the lack of support for equations formatted as MathML. They are currently included as bitmap or SVG images, precluding proper handling by screen readers and interaction with computer algebra systems. Support for MathML is included in the EPUB 3.0 specification.

• Another criticism of EPUB revolves around the specification's lack of detail on linking into, between, or within an EPUB book, as well as its lack of a specification for annotation. Such linking is hindered by the use of a ZIP file as the container for EPUB. Furthermore, it is unclear if it would be better to link by using EPUB's internal structural markup (the OPF specification mentioned above) or directly to files through the ZIP's file structure.^[26] The lack of a standardized way to annotate EPUB books could lead to difficulty sharing and transferring annotations and therefore limit the use scenarios of EPUB, particularly in educational settings, because it cannot provide a level of interactivity comparable to the web.^[27]

Software

Software reading systems

Software that reads, and presumably displays, EPUB files is called a reading system. An EPUB reading system is defined as:

“A combination of hardware and/or software that accepts OPS Publications and makes them available to consumers of content. Great variety is possible in the architecture of Reading Systems. A Reading System may be implemented entirely on one device, or it may be split among several computers....”^[3]

Editing systems

Hardware reading systems

The boundary between hardware and software is not clear cut. Some of these devices are dedicated to e-book tasks while others are platforms that include e-book readers or can have them added. See Comparison of e-book readers for details of dedicated devices (not all support EPUB).

See also

• Comparison of e-book formats
• Open Packaging Convention
• XML Paper Specification

References

External links

• EPUB Wiki/Home Page includes issue and bug tracking.
• EPUB Format Construction Guide (also available in EPUB)
• Video: Format overview of the .epub file
• Publisher Tim O'Reilly explains the importance of EPUB
• Build a digital book with EPUB from IBM developerWorks