Asciidoctor EPUB3 Documentation

Asciidoctor EPUB3 is a set of Asciidoctor extensions for converting AsciiDoc documents directly to the EPUB3 e-book format.

Introduction

Asciidoctor EPUB3 is not merely a converter from AsciiDoc to EPUB3. Rather, it’s a tool for creating highly aesthetic, professional, easy-to-read e-books. Let’s face it, many of the technical e-books out there—especially those produced from software documentation—are hideous. Asciidoctor EPUB3 is on a mission to disrupt the status quo.

Figure 1. An excerpt from an e-book produced by Asciidoctor EPUB3 shown in Day, Night, and Sepia modes.

Project Mission

The Asciidoctor EPUB3 project aims to produce EPUB3 documents that meet the following objectives:

Fully Semantic

Produce deeply semantic XHTML5 documents, including use of the recommended epub:type attribute.

Exceptional Readability

Readers should be drawn into the text so that they read and absorb it. Maximize the readability of the text using carefully crafted styles that are focused on:

Custom, readable fonts with strong UTF-8 character support
Sufficient line spacing and margins
Modular font size scale
Subtle, pleasing colors with good contrast
A responsive design that scales well from small to large screens
Widowed and orphaned content avoided where possible

Complete and Accurate Metadata

Fully populate the EPUB3 package metadata using information in the AsciiDoc source document.

Consistent Rendering

Render consistently across a broad range of EPUB3 (and select EPUB2+) e-readers and respond to any size screen.

Polish, Polish, and More Polish

Add polish to the final product such as font-based icons and callout numbers.

We believe that the e-books produced by Asciidoctor EPUB3 are the very best output you can expect to find in digital publishing today. Of course, there’s always room for improvement, so we’ll continue to work with you to achieve and maintain this goal.

Asciidoctor EPUB3 only produces variable layout (i.e., reflowable) EPUB3 documents since this layout is best suited for the types of documents typically written in AsciiDoc. We may explore the use of fixed layout documents in the future if the need arises.

Notable Features

Direct AsciiDoc to EPUB3 conversion
Highly-aesthetic and readable styles with optimized text legibility
Respects font settings (if supported by the e-reader) without altering headings, code or icons
EPUB3 metadata, manifest and spine (assembled by Gepub)
Document metadata (title, authors, subject, keywords, etc.)
Internal cross-reference links
Syntax highlighting with Rouge, CodeRay or Pygments
Unicode callout numbers
Page breaks avoided in block content (so much as it’s supported by the e-reader)
Orphan section titles avoided (so much as it’s supported by the e-reader)
Table border settings honored
Support for SVG images in the content
Stem blocks via AsciiMath

Structuring your Manuscript

An EPUB3 archive is composed of multiple files. The content of each “chapter” is typically stored in a dedicated XHTML file. Therefore, the Asciidoctor EPUB3 converter “chunks” the AsciiDoc source document into multiple XHTML files to add to the EPUB3 archive. Like other converters, Asciidoctor EPUB3 handles this chunking task by automatically slicing up the XHTML output at predetermined heading levels.

When the doctype attribute is set to book, each top-level section will become a separate e-book "chapter" file. This includes preface, bibliography, appendix, etc. This behavior can be configured via the epub-chapter-level document attribute.

Otherwise, the whole document is converted to a single ebook chapter.

You may specify custom chapter filenames by assigning IDs to sections:

[#custom-chapter-id]
= Chapter

Here’s an example showing the structure of a book:

= Book Title
Author Name
:doctype: book
:imagesdir: images
//...and so on

== Chapter One

Some interesting text here.

== Chapter Two

Even more exciting stuff.

In older Asciidoctor EPUB3 versions, there were strict rules on document organization: a 'spine' master document with chapter includes. This is no longer the case. If you followed the old rules, chances are your document will work with the newer Asciidoctor EPUB3 either as-is or after minor adjustments.

Prerequisites

All that’s needed to use Asciidoctor EPUB3 is Ruby 2.7 or newer and a few Ruby gems, which we’ll explain how to install in the next section.

To check if you have Ruby available, use the ruby command to query the installed version:

$ ruby --version

Getting Started

You can get Asciidoctor EPUB3 by installing the published gem.

Install the Published Gem

Asciidoctor EPUB3 is published on RubyGems.org. You can install the published gem using the following command:

$ NOKOGIRI_USE_SYSTEM_LIBRARIES=1 gem install asciidoctor-epub3

This optional environment variable tells the gem installer to link against the C libraries on the system, if available, instead of compiling the libraries from scratch. This speeds up the installation of Nokogiri considerably.

If you want to syntax highlight source listings, you’ll also want to install Rouge, CodeRay, or Pygments. Choose one (or more) of the following:

Rouge

$ gem install rouge

CodeRay

$ gem install coderay

Pygments

$ gem install pygments.rb

You then activate syntax highlighting for a given document by adding the source-highlighter attribute to the document header (Rouge shown):

:source-highlighter: rouge

If a style is not specified, the black and white theme (i.e., bw) is used. This default is used so that the syntax highlighting is legible regardless of which reading mode the reader selects (white, black, sepia, etc). To override this default, you must set <highlighter>-style document header attribute to a valid highlighter style name (e.g., :rouge-style: pastie).

Assuming all the required gems install properly, verify you can run the asciidoctor-epub3 script:

$ asciidoctor-epub3 -v

If you see the version of Asciidoctor EPUB3 printed, you’re ready to use Asciidoctor EPUB3. Let’s get an AsciiDoc document ready to convert to EPUB3.

The metadata in the generated EPUB3 file is populated from attributes in the AsciiDoc document. The names of the attributes and the metadata elements to which they map are documented in this section.

The term package metadata in Table 1 is in reference to the <metadata> element in the EPUB3 package document (e.g., package.opf). The dc namespace prefix is in reference to the Dublin Core Metadata Element Set.

AsciiDoc attributes that control the EPUB3 metadata (i.e., package.opf)
Name	Description
`uuid`	Populates the required unique identifier (`<dc:identifier>`) in the package metadata. An id will be generated automatically from the doctitle if not specified. The recommended practice is to identify the document by means of a string or number conforming to a formal identification system.
`lang`	Populates the content language / locale (`<dc:language>`) in the package metadata.
`scripts`	Controls the font subsets that are selected based on the specified scripts (e.g., alphabets). (values: latin, latin-ext, latin-cyrillic or multilingual)
`revdate`	Populates the publication date (`<dc:date>`) in the package metadata. The date should be specified in a parsable format, such as `2014-01-01`.
`doctitle`	Populates the title (`<dc:title>`) in the package metadata. The title is added to the metadata in plain text format.
`author`	Populates the contributors (`<dc:contributor>`) in the package metadata. The authors in each chapter document are aggregated together with the authors in the master file.
`username`	Used to resolve an avatar for the author that is displayed in the header of a chapter when the `doctype` is set to a value other than `book`. The avatar image should be located at the path {imagesdir}/avatars/{username}.jpg, where `{username}` is the value of this attribute.
`producer`	Populates the publisher (`<dc:publisher>`) in the package metadata.
`creator`	Populates the creator (`<dc:creator>`) in the package metadata. If the creator is not specified, Asciidoctor is set as the creator with the role "mfr" (an abbreviation for manufacturer).
`description`	Populates the description (`<dc:description>`) in the package metadata.
`keywords`	Populates the subjects (i.e., `<dc:subject>`) in the package metadata. The keywords should be represented as comma-separated values (CSV).
`front-cover-image`	Populates the front cover image and the image on the cover page (EPUB3 only) in the package metadata. The image is also added to the e-book archive. The value may be specified as a path or inline image macro. Using the inline image macro is preferred as it allows the height and width to be specified.
`copyright`	Populates the rights statement (`<dc:rights>`) in the package metadata.
`source`	Populates the source reference (`<dc:source>`) in the package metadata. The recommended practice is to identify the referenced resource by means of a string or number conforming to a formal identification system.
`epub-properties`	An optional override of the properties attribute for this document’s item in the manifest. Only applies to a chapter document.
`epub-chapter-level`	Specify the section level at which to split the EPUB into separate "chapter" files. This attribute only affects documents with `:doctype: book`. The default is to split into chapters at level-1 sections. This attribute only affects the internal composition of the EPUB, not the way chapters and sections are displayed to users. Some readers may be slow if the chapter files are too large, so for large documents with few level-1 headings, one might want to use a chapter level of 2 or 3.
`series-name, series-volume, series-id`	Populates the series statements (`belongs-to-collection`) in the package metadata. Volume is a number, ID probably a UUID that is constant for all volumes in the series.
`epub3-frontmatterdir`	The path to a directory that contains frontmatter files. The file names must match `front-matter.html` and will be included in alphabetic order. The files are expected to be valid EPUB HTML files. If only one front matter page is required, the default 'front-matter.html' file can be used instead.*
`epub3-stylesdir`	The path to a directory that contains alternate epub3.css and epub3-css3-only.css files to customize the look and feel.
`doctype`	Used to control the inclusion of special content in the generated HTML. If set to a value other than book, the byline information (author and avatar) is included below the chapter header and a typographic end mark is added at the end of the last paragraph. Suggested values include: book (default), article.
`toc`	Adds table of contents at the beginning of the book. Depth is controlled by `:toclevels:` attribute.
`outlinelevels`	Sets the depth of table of contents metadata. If not set, defaults to `:toclevels:`

With that out of the way, it’s time to convert the AsciiDoc document directly to EPUB3.

Performing the Conversion

You can convert AsciiDoc documents to EPUB3 from the command line using the asciidoctor-epub3 script provided with the Asciidoctor EPUB3 project.

Convert AsciiDoc to EPUB3

Converting an AsciiDoc document to EPUB3 is as simple as passing your document to the asciidoctor-epub3 command. This command should be available on your PATH if you installed the asciidoctor-epub3 gem. Otherwise, you can find the command in the bin folder of the project. We also recommend specifying an output directory using the -D option flag.

$ asciidoctor-epub3 -D output samples/sample-book.adoc

When the script completes, you’ll see the file sample-book.epub appear in the output directory. Open that file with an EPUB reader (aka e-reader) to view the result.

Below are several screenshots of this sample book as it appears on an Android phone.

Figure 2. An example of a chapter title and abstract shown side-by-side in Day and Night modes

Figure 3. An example of a section title followed by paragraph text separated by a literal block

Figure 4. An example of a figure and an admonition

Figure 5. An example of a sidebar

Figure 6. An example of a table

The asciidoctor-epub3 command is a temporary solution for invoking the Asciidoctor EPUB3 converter. We plan to remove this script once we have completed proper integration with the asciidoctor command.

As another example, point asciidoctor-epub3 at the GitHub Guides that we’ve ported to AsciiDoc, then compare the output to the real GitHub Guides.

Validate the EPUB3 Archive

Next, let’s validate the EPUB3 archive to ensure it built correctly.

EPUB3 with validation

$ asciidoctor-epub3 -D output -a ebook-validate samples/sample-book.adoc

Validation success

Validating using EPUB version 3.0.1 rules.
No errors or warnings detected.
Messages: 0 fatal / 0 errors / 0 warnings / 0 info
EPUBCheck completed

If the EPUB3 archive contains any errors, they will be output in your terminal.

EPUB Standard & Validator

The electronic publication (EPUB) standard is developed by the International Digital Publishing Forum (IDPF). EPUB 3.1, released in January 2017, is the latest version of this standard.

An EPUB3 archive contains:

a package document (metadata, file manifest, spine)
a navigation document (table of contents)
one or more content documents
assets (images, fonts, stylesheets, etc.)

The IDPF also supports EPUBCheck. EPUBCheck parses and validates the file against the EPUB schema.

If you want to browse the contents of the EPUB3 file that is generated, or preview the XHTML files in a regular web browser, add the -a ebook-extract flag to the asciidoctor-epub3 command. The EPUB3 file will be extracted to a directory adjacent to the generated file, but without the file extension.

$ asciidoctor-epub3 -D output -a ebook-extract samples/sample-book.adoc

In this example, the contents of the EPUB3 will be extracted to the output/sample-book directory.

Tuning Listing Captions

Unlike the built-in converters, the EPUB3 converter is configured to add a signifier (e.g., Listing) at the start of the caption for all listing and source blocks that have a title. This behavior is triggered because the listing-caption attribute is set by default.

If you don’t want the signifier to be included at the beginning of the caption on listing and source blocks, simply unset the listing-caption when invoking Asciidoctor EPUB3.

$ asciidoctor-epub3 -a listing-caption! book.adoc

Now the behavior will match that of the built-in converters. For more information about this attribute and other related attributes, see internationalization and numbering.

Command Arguments

-h, --help: Show the usage message
-D, --destination-dir: Writes files to specified directory (defaults to the current directory)
-a ebook-epubcheck-path=<path>: Specifies path to EPUBCheck executable to use with -a ebook-validate. This attribute takes precedence over EPUBCHECK environment variable.
-a ebook-extract: Extracts the EPUB3 to a folder in the destination directory after the file is generated
-a ebook-validate: Runs EPUBCheck to validate output file against the EPUB3 specification
-v, --version: Display the program version

Environment variables

EPUBCHECK: Specifies path to EPUBCheck executable to use with -a ebook-validate. Effect of this variable can be overriden with -a ebook-epubcheck-path attribute.

EPUB3 Archive Structure

Here’s a sample manifest of files found in an EPUB3 document produced by Asciidoctor EPUB3.

META-INF/
  container.xml
EPUB/
  fonts/
    awesome/
      fa-solid-900.ttf
    font-icons.ttf
    mplus-1mn-latin-bold.ttf
    mplus-1mn-latin-light.ttf
    mplus-1mn-latin-medium.ttf
    mplus-1mn-latin-regular.ttf
    mplus-1p-latin-bold.ttf
    mplus-1p-latin-light.ttf
    mplus-1p-latin-regular.ttf
    noto-serif-bold-italic.ttf
    noto-serif-bold.ttf
    noto-serif-italic.ttf
    noto-serif-regular.ttf
  images/
    avatars/
      default.png
    figure-01.png
    figure-02.png
  styles/
    epub3-css3-only.css
    epub3.css
  chapter-01.xhtml
  chapter-02.xhtml
  ...
  cover.xhtml
  nav.xhtml
  package.opf
  toc.ncx
mimetype

Working with Images

Images referenced in your AsciiDoc document must be stored in the images catalog. The images catalog is defined by the imagesdir attribute. If set, the value of this attribute is resolved relative to the document and must be at or below (i.e., within) the directory of that document. (In other words, it cannot point to a location outside the document directory). If this attribute is not set, the images catalog defaults to the directory of the document. Asciidoctor EPUB3 will discover all local image references and insert those images into the EPUB3 archive at the same relative path.

Default Images

The sample book contains a placeholder image for the author avatar.

Adding the Cover Image

E-readers have different image resolution and file size limits regarding a book’s cover. Kindle covers tend to be 1050x1600 (16:9 aspect ratio). To ensure your cover displays correctly, you’ll want to review the documentation or publisher guidelines for the e-reading platform you’re targeting.

We’ve found that if the book cover is more than 1600px on any side, Aldiko will not render it and may even crash.

Feel free to use the SVG of the sample cover in the data/images folder as a template for creating your own cover. Once your image is ready, you can set the cover image by defining the front-cover-image attribute in the header of the master document.

:front-cover-image: image:cover.png[Front Cover,1050,1600]

The image is resolved relative to the directory specified in the imagesdir attribute, which defaults to the directory of the3 document. The image can be in any format, though we recommend using PNG, JPG, or SVG as they are the most portable formats.

You should always specify the dimensions of the cover image. This ensures the viewer will preserve the aspect ratio if it needs to be scaled to fit the screen. If you don’t specify a width and height, then the dimensions are assumed to be 1050x1600.

How to Organize Images by Chapter

You can set the imagesdir attribute by chapter (as long as the attribute is not overridden by the API). To do so, use an attribute entry to set the value of the imagesdir attribute on the line above the include directive for a chapter.

:imagesdir: chapter-one/images
include::chapter-one.adoc[]

:imagesdir: chapter-two/images
include::chapter-two.adoc[]

About the Theme

EPUB3 files are styled using CSS3. However, each e-reading platform honors a reduced set of CSS3 styles, and the styles they allow and how they implement them are rarely documented. All we’ve got to say is thank goodness for CSS hacks, media queries and years of CSS experience!

The theme provided with Asciidoctor EPUB3 has been crafted to display EPUB3 files as consistently as possible across the most common EPUB3 platforms and to degrade gracefully in select EPUB2 platforms. The theme maintains readability regardless of the reading mode (i.e., day, night or sepia) or the display device’s pixel density and screen resolution.

The theme’s CSS files are located in the data/style directory.

Asciidoctor EPUB3 only provides one theme, and, at this time, you can not replace it with a custom theme using the stylesheet attribute. However, you can use your own epub3.css and epub3-css3-only.css files by specifying the directory where they are located using the epub3-stylesdir attribute.

Device-specific Styles

For readers that support JavaScript, Asciidoctor EPUB3 adds a CSS class to the body element of each chapter that corresponds to the name of the reader as reported by the epubReadingSystem JavaScript object. This enhancement allows you to use styles targeted specifically at that reader.

Below you can find the readers that are known to support this feature and the CSS class name that gets added to the body element.

Reader	HTML Element	CSS Class Name
Gitden	body	gitden-reader
Namo PubTreeViewer	body	namo-epub-library
Readium	body	readium-js-viewer
iBooks	body	ibooks
Adobe RMSDK >= 11	body	rmsdk
Google Books	div	gb-reader-container

Reader

HTML Element

CSS Class Name

Gitden

body

gitden-reader

Namo PubTreeViewer

body

namo-epub-library

Readium

body

readium-js-viewer

iBooks

body

ibooks

Adobe RMSDK >= 11

body

rmsdk

Google Books

div

gb-reader-container

Kobo does not support the epubReadingSystem JavaScript object, despite the fact that it does support JavaScript.

Pushing to Android

While it’s certainly possible to view the EPUB3 on your desktop/laptop, you’ll probably want to test it where it’s most likely going to be read—on a reading device such as a smartphone or a tablet. Assuming you have an Android device available, transferring the EPUB3 to the device is easy once you get a bit of setup out of the way.

You transfer files from your computer to an Android phone over a USB connection using a command from the Android SDK Tools called adb. Follow these steps to get it set up:

Download the Android SDK Tools zip from the table labeled SDK Tools Only on the Get the Android SDK page
Extract the archive
Locate the path to the adb command (Hint: Look in the platform-tools folder)
Set the environment variable named ADB to the path of the adb command
```
$ export ADB=~/apps/android-sdk/platform-tools/adb
```

Now you can use the adb push to push the EPUB3 files to your Android device.

Publish EPUB3 file to Android device

$ adb push output/sample-book.epub /sdcard

You’ll have to manually import the EPUB3 into your e-reader of choice.

E-book Reader Recommendations and Quirks

EPUB3 e-readers will provide the best reading experience when viewing a book generated by Asciidoctor EPUB3. Here’s a list of some of the e-readers we know to have good EPUB3 support and the systems on which they run:

Amazon Kindle (most platforms)
Gitden (Android and iOS)
iBooks (iOS, OSX)
Readium (Chrome)
Kobo (Android, iOS, OSX and Windows)
Namo PubTreeViewer (Android, iOS and Windows)
Calibre (ebook-viewer) (Linux, OSX, Windows)

To get the full experience, ensure that the e-reader is configured to use the publisher’s styles. Different e-readers word this setting in different ways. Look for the option screen that allows you to set the fonts and font colors and disable it. With publisher’s styles active, you’ll still be able to adjust the relative size of the fonts and margins and toggle between day, night and sepia mode.

When the book is viewed in EPUB2 e-readers and Kindle apps/devices which have reached their end-of-life (EOL), the e-book relies on the strong semantics of HTML and some fallback styles to render properly. EPUB2 e-readers, such as Aldiko, don’t understand CSS3 styles and therefore miss out on some of the subtleties in the formatting.

As mentioned in the theme section, the stylesheet attempts to provide as consistent a reading experience as possible in the common EPUB3 e-readers, despite the different CSS implementation rules and limitations unique to each e-book application. Most of these obstacles were addressed using media queries or explicit classes. Some we haven’t conquered. Yet.

The Kindle quirks list shows you just a few of the constraints we encountered. To see all the workarounds and why we chose certain style options, check out the code and comments in the epub3.css and epub3-css-only.css files.

Kindle Quirks

overrules margins and line heights like a medieval tyrant
font-family can’t be set on <body>
requires !important on text-decoration
position: relative isn’t permitted
strips (or unwraps) <header> tags
@page isn’t supported
page-break: avoid isn’t supported
page-break-* cannot be applied using a compound or nested CSS selector; must be a simple ID or class
max-width isn’t supported
widows are left in the cold
won’t style footers without an explicit class
-webkit-hyphens: auto causes Kindle for Mac (and perhaps others) to crash
text-rendering: optimizeLegibility causes file to be rejected by KFP (and causes the text to disappear in some previewers)
Kindle Direct Publishing (KDP) strips out select font-related CSS rules (e.g., font-family) under certain conditions (for reasons that have proved nearly impossible to reverse engineer); the known workaround is to add a layer of indirection by using @import to hide the CSS files from the script

Asciidoctor EPUB3 Documentation

Introduction

Project Mission

Notable Features

Structuring your Manuscript

Prerequisites

Getting Started

Install the Published Gem

EPUB-related AsciiDoc Attributes

Performing the Conversion

Convert AsciiDoc to EPUB3

Validate the EPUB3 Archive

Tuning Listing Captions

Command Arguments

Environment variables

EPUB3 Archive Structure

Working with Images

Default Images

Adding the Cover Image

How to Organize Images by Chapter

About the Theme

Device-specific Styles

Pushing to Android

E-book Reader Recommendations and Quirks