EPUB

History

EPUB 2.0 was approved in October 2007, with a maintenance update (2.0.1) approved in September 2010. The EPUB 3.0 specification became effective in October 2011, superseded by a minor maintenance update (3.0.1) in June 2014. New major features include support for precise layout or specialized formatting (Fixed Layout Documents), such as for comic books, and MathML support. The current version of EPUB is 3.1, effective January 5, 2017. The (text of) format specification underwent reorganization and clean-up; format supports remotely-hosted resources and new font formats (WOFF 2.0 and SFNT) and uses more pure HTML and CSS.

In May 2016 IDPF Members approved World Wide Web Consortium (W3C) merger, "to fully align the publishing industry and core Web technology".

Features

The format and many readers support the following:

Reflowable document: optimize text for a particular display

Fixed-layout content: pre-paginated content can be useful for certain kinds of highly designed content, such as illustrated books intended only for larger screens, such as tablets.

Like an HTML web site, the format supports inline raster and vector images, metadata, and CSS styling.

Page bookmarking

Passage highlighting and notes

A library that stores books and can be searched

Re-sizable fonts, and changeable text and background colors

Support for a subset of MathML

Digital rights management—can contain digital rights management (DRM) as an optional layer

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.

Adoption and market share

It is also widely used on many software readers such as iBooks on iOS and Google Books on Android, but notably not by the Amazon Kindle e-readers. iBooks also supports the proprietary iBook format, which is based on the EPUB format but depends upon code from the iBooks app to function.

As final product

As in the eBook market PDF format is not generally sold at sites that sell eBooks, the EPUB participation in the eBook market is very low. The main EPUB producers are suppliers of public domain and open licensed content, as Project Gutenberg, PubMed Central, SciELO and many others. In that context is growing and replacing the PDF format.

Data interchange

EPUB is a popular way to feed ebook creation process, because is an open format and is based in HTML, while Amazon's format is proprietary. EPUB is the "first step" content format in many production process and supply chains.

Implementation

An EPUB file is a ZIP archive that contains, in effect, a website—including HTML files, images, CSS style sheets, and other assets. It also contains metadata. EPUB 3 is the latest version. By using HTML5, publications can contain video, audio, and interactivity, just like websites in web browsers.

Container

An ePub publication is delivered as a single file. This file is an unencrypted zipped archive containing a set of interrelated resources.

An OCF Abstract Container defines a file system model for the contents of the container. The file system model uses a single common root directory for all contents in the container. All (non-remote) resources for publications are in the directory tree headed by the container's root directory, though EPUB mandates no specific file system structure for this. The file system model includes a mandatory directory named META-INF that is a direct child of the container's root directory. META-INF stores container.xml.

The first file in the archive must be the mimetype file. It must be uncompressed so that non-ZIP utilities can read the mimetype. The mimetype file must be an ASCII file that contains the string application/epub+zip. It must be unencrypted, and the first file in the ZIP archive. This file provides a more reliable way for applications to identify the mimetype of the file than just the .epub extension.

An example file structure:

--ZIP Container--mimetypeMETA-INF/ container.xmlOEBPS/ content.opf chapter1.xhtml ch1-pic.png css/ style.css myfont.otf toc.ncx

There must be a META-INF directory containing container.xml. This file points to the file defining the contents of the book, the OPF file, though additional alternative rootfile elements are allowed. Apart 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 container.xml:

Publication

The ePUB container must contain:

At least one content document.

One navigation document.

One package document listing all publication resources. This file should use the file extension .opf. It contains metadata, a manifest, fallback chains, bindings, and a spine. This is an ordered sequence of ID references defining the default reading order.

The ePUB container may contain:

style sheets.

PLS Documents.

media overlay documents.

Contents

Content documents include: HTML 5 content, navigation documents, SVG documents, scripted content documents, and fixed layout documents. Contents also include CSS and PLS documents. Navigation documents supersedes the NCX grammar used in EPUB 2.

Media overlays

Books with synchronized audio narration are created in EPUB 3 by using media overlay documents to describe the timing for the pre-recorded audio narration and how it relates to the EPUB Content Document markup. The file format for Media Overlays is defined as a subset of SMIL.

Software

Many editors exist including calibre, Sigil, LaTeX, and Genebook. An open source tool, called epubcheck, can be used for validating and detecting errors in the structural markup (OCF, OPF, OPS), image, and XHTML files; the tool can either be run from the command line or used in applications or webapps as a library; it is also available on EPUB Validator. Readers exist for all major hardware platforms with the exception of Amazon Kindle, including Google Play Books (Android and iOS) and Apple iBooks (MacOS and iOS).