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fread.ui is work-in-progress ebook reader, web browser and primary UI for the GNU/Linux distro for e-paper devices.

fread.ui aims to run well on a system with 256 MB of total memory while providing all the advanced text rendering options expected of a modern ebook reader (using webkit). To compare, all Kindle models starting with the 3rd generation have 256 MB of ram or more.

So far fread.ui supports no ebook formats with EPUB support in progress.

This project is based on WebKit2GTK and JavaScriptCore.

Installing dependencies

sudo apt install build-essential libwebkit2gtk-4.0-dev libjavascriptcoregtk-4.0-dev libzip-dev libmagic-dev



Building the web app

Ensure you have a recent node.js (only necessary for building), then:

cd app/
npm install
npm run build-css
npm run build

For more info on developing the web app see app/


./fread.ui "ebook:///path/to/book.epub"

Keyboard shortcuts

  • Toggle developer console: F12
  • Reload page: ctrl-r
  • Navigate back: alt-left
  • Navigate forward: alt-right


fread.ui is implemented as a combination of a custom URI scheme, a WebKit Web Process Extension and a minimal web application that uses preact. This is all on top of WebKit2GTK.

The custom URI scheme allows opening of EPUB files using e.g. ebook://path/to/my/book.epub and also allows browsing of the files contained in a zip archive (epub uses zip) using the syntax ebook://path/to/my/book.epub//internal/zip/archive/file.htm.

The Web Process Extension adds a global Fread javascript object which allows calling a set of C functions from javascript. Currently these add filesystem utility functions such as Fread.zip_ls which lists the files in a zip file residing on the filesystem. In the future this will include functions for updating the electronic paper display. The API can be glarked from web_extensions/fread.js.

The web app provides parsing of the various EPUB-specific metadata formats and rendering of that metadata into something presentable to the user. It is located in app/.


Splitting the ebook up into individual pages is handled by the ebook-paginator library.

APIs and manuals


JavasScriptCore (new glib API)

See also the Web Process Extension that ships with the epiphany browser for an example.



Ensure you have libmagic-dev installed, then:

man libmagic


  • Integrate CFI parsing and link following
  • Render table of contents
  • Configuration file for setting keyboard shortcuts
  • Change existing jsc filesystem functions to async
  • Add setting to let user enable: process-swap-on-cross-site-navigation-enabled
  • Unit testing

Top menu

  • Add/remove bookmark
  • Show bookmarks
  • Theme (switch css file)
  • Font size up/down
  • Toggle dark mode (webkit has a built-in dark mode)

Book browser


  • Write code to generate/refresh thumbnails and database of ebooks (IndexedDB?)
  • Thumbnail view
  • List view
  • Order by: Title, Author, Filename, Date changed, Date published
  • Search

Settings page


In epub mode we should disable enable-javascript-markup but not disable javascript since that would prevent our own javascript for epub parsing from running.

Other stuff to disable in epub mode:

  • enable-xss-auditor
  • enable-webgl
  • enable-webaudio
  • enable-resizable-text-areas
  • enable-site-specific-quirks
  • enable-html5-database
  • enable-html5-local-storage
  • enable-offline-web-application-cache
  • enable-java
  • enable-media
  • enable-media-stream
  • enable-mediasource
  • enable-mock-capture-devices
  • enable-page-cache (don't cache visited pages in memory)
  • enable-accelerated-2d-canvas
  • hardware-acceleration-policy: WEBKIT_HARDWARE_ACCELERATION_POLICY_NEVER

Stuff to expose to the user:

  • auto-load-images

Other interesting settings:

  • enable-spatial-navigation (keyboard up-down-left-right navigation)

Memory usage

Memory usage was tested on the latest Debian 10.2 x86 netinstall in a virtualbox with 256 MB of ram, with disabled swap and apt install xinit awesome to get a basic X environment. The main process and two WebKit processes together use around 94 to 144 MB depending on how you measure after starting and loading up the mobile front page of english wikipedia:

  PID User     Command                         Swap      USS      PSS      RSS 
 3604 juul     /usr/lib/i386-linux-gnu/web        0    12636    18524    31420 
 3603 juul     /usr/lib/i386-linux-gnu/web        0    47500    58284    77720 
 3596 juul     ./fread.ui https://en.m.wik        0     8332    19186    37932 

Unshared:   66.86 MB
PSS:        93.74 MB
Resident:  143.62 MB

The script used to generate the above output is memory_usage/show_memory_usage.rb. To run this script you need apt install ruby smem. Make sure you're not running any other WebKit-based programs when running that script. The PSS field is a measure made by smem that takes into account a proportion of shared library memory usage. Some pie charts for PSS and RSS are provided in memory_usage/ to further illuminate how memory is used by the system. Keep in mind that you need to add up the WebKitWebProcess, WebKitNetworkProcess and fread.ui in those pie charts.

The pie charts show that 19.2 % of memory (PSS) or 26.4 % (RSS) are used by the app, again with the mobile english front page of wikipedia loaded.

Generating the pie charts require apt install python-matplotlib and where made using:

# for RSS
smem --bar=name -s rss

# for PSS
smem --bar=name -s pss

We can disable some features to make libwebkitgtk smaller, e.g -DENABLE_GEOLOCATION=OFF though this may only result in fewer dependencies and may not affect non-file-backed memory usage. These options can be found in Source/cmake/WebKitFeatures.cmake. We can probably disable the following:


EPUB support


Specified by the three standards:


Specified by:

Only big difference between 3.0.0 and 3.0.1 is the addition of Fixed Layout Documents which is a non-reflowable type of EPUB document.


Here's a list of changes from 3.0.1.

The biggest things added are:

  • WOFF 2.0 and SFNT font support
  • Allows remotely hosted fonts, audio, video and any resource loaded by a script

Script support is optional now though so we can still disable script tag support and manually load fonts, audio and video, but we should probably have a user setting for it with the default being "Ask the user".


Only finalized in May of 2019.

The list of changes from 3.0.1 is odd. From a quick read, apart from some reformatting and rewording, it looks identical to the list of changes from 3.0.1 to 3.1 and why isn't it a list of changes from 3.1 to 3.2?


A file called mimetype contains the string "application/epub+zip".


Can have multiple root elements for multiple versions of the same book but just using the first one is allowed. Used to find the .opf file (the Package Document) for the book.

Rootfile element can also contain multiple <link rel="foo" href="bar media-type="" /> Documentation for rel: Probably the only ones relevant: stylesheet, author, tag, license


Worth checking for so we know we can't read the file.


Looks like these proprietary extensions are used to specify stuff like fonts and we'll need to parse them. Readium already does.

Looks like nothing else in the META-INF/ is useful as of 3.0.1 since the formats of other files such as metadata.xml are not specified.


Open Publication Structure.

There's a parser in readium.


OPF is the Open Packaging Format which is an XML format that contains metadata about the epub. There will be a content.opf file inside the .epub.

OPF metadata spec. The <metadata> tag can contain <dc:foo> elements, <meta> elements or arbitrary elements. The dc: elements may be inside a <dc-metadata> sub-element. There can be multiple <dc:creator> tags and they can have a role= and file-as= (which is used when sorting authors alphabetically). All EPUB 2.0 must include the metadata tags: dc:title, dc:identifier and dc:language. EPUB 3 adds <meta property="dcterms:modified">2011-01-01T12:00:00Z</meta> as a required field. In EPUB 3.0 the role=, file-as= etc. properties on creator are specified as:

    <dc:creator id="creator">Haruki Murakami</dc:creator>
    <meta refines="#creator" property="role" scheme="marc:relators" id="role">aut</meta>
    <meta refines="#creator" property="alternate-script" xml:lang="ja">村上 春樹</meta>
    <meta refines="#creator" property="file-as">Murakami, Haruki</meta>
    <meta refines="#creator01" property="display-seq">1</meta>

The display-seq property specifies author name display sequence and is not present in a <dc:creator display-seq=?> version.

<dc:date>2000-01-01T00:00:00Z</dc:date> is publication date and not required.

ISBN is stored as <dc:identifier opf:scheme="ISBN"> so that might be the standard way.

The <manifest> element contains a list of all files in the book. They need id=, href= and media-type= (mimetype) + optional fallback=<id> and fallback-style=<id>. For 3.0 exactly one manifest <item> must be declared as the EPUB Navigation Document using the properties="navn" property. Though remember properties= can be a space-separated list of properties.

The <spine> element contains references to the ids in the <manifest> section and simply specifies the reading order. The attribute toc=, if it exists, points to the id of a <manifest> element which is the table of contents NCX file. All EPUB 2.0 files must include an NCX file but reading systems only should support it. The NCX item in manifest cannot have a fallback. Another attribute in 3.0 is page-progression-direction= which can be "rtl", "ltr" and "default".

The <guide> element may or may not be present and references mayor sections like "cover", "toc" and "bibliography". There is only a short list of possible section types so it should be easy to support.

Here's some javascript for parsing OPF:


The NCX file is not required for epub 3.0+ files but must be present in 2.0 files. It provides the table of contents. The <docTitle> are obvious <docAuthor> and then <navMap> is the main table of contents with <navList>

EPUB Navigation Document

This is the EPUB 3.0 replacement for NCX (though both can be present). They are basically an xhtml version of the NCX file.

In the <body> there are a set of <nav epub:type="toc" id="toc"> elements where epub:type= is either "toc", "page-list" or "landmarks". The <nav> element can contain a single heading tag (h1, h2, h3, h4, h5, h6 or hgroup) and a single <ol> tag. If <nav> elements contain the hidden= property then they must not be displayed (doesn't matter what the attribute value is).

EPUB-specific CSS



These are specified here. They are included in EPUB 3.2 but discouraged from use.

Some of these are actually in normal CSS without the -epub- prefix, e.g. ruby-position.

If we want to support -epub-* CSS properties we can use postcss-epub but a simpler solution would be to remove the "-epub-" prefix from all of these before handing them to the browser. We should check if PostCSS can do this in javascript without being a memory hog and if not then we should find a CSS parsing library in C and implement it during loading of the CSS file from the .epub file.

The MyCSS CSS parser (written in C) could be used. It looks like the current codebase is only maintained as part of Modest. There is also a python binding.

EPUB Adaptive Layout

It seems like this was never really adopted and few readers support it. It does not seem worth the trouble to implement. A javascript implementation exists and another one in typescript in Vivliostyle. Vivliostyle has a notice that they may drop support in the future. It seems like this is a dead standard.


If the user wants to turn on javascript (we should keep it default off) we need to provide this special object.


See koreader/koreader#4440


WebKit only has a bit of MathML support.

We could use MathJax to render MathML in javascript but the ram usage jumps to 170 MB (PSS) or 280 MB (RSS) when loading the MathJax sample page, which is only rendering a few formulas.

We could use Lasem but it's unclear how complete it is. It can render to SVG or PNG so we could just pull MathML html nodes using javascript, pass them to Lasem and take back the SVG or image data and replace in the DOM. It might also be possible to do all this without javascript.

After tring Lasem on the Mozilla MathML Torture tests, it seems to do pretty well. The only problems I noticed was that the horizontal brackets not spanning correctly on tests 19 and 22.

KOReader also does not have MathML support so we can't use the same code.


A CFI or Canonical Fragment Identifier is sorta an extended version of URL anchors (the part after the # in a URL) which look like e.g:


To navigate, we need to be able to parse these.

Here's some javascript for parsing CFIs:

  • readium - Needs jquery but seems like that dependency could easily be removed
  • epub.js - Partial implementation

Other resources:

Other formats

There are only really three classes of ebook formats that are worth concerning ourselves with. Formats meant to be reflowed, e.g. EPUB, formats not meant to be reflowed, e.g. PDF, and sets of images, e.g. CBZ. If we can support one of each of these then tools like Calibre can facilitate fairly high quality conversion between most other formats into one of these formats.

Other than EPUB it would be nice to support:

  • PDF
  • CBZ

Lower priority:

  • DjVu
  • CBR
  • HTML in dir, zip, tar or tar.gz
  • TXT
  • RTF
  • Markdown
  • FictionBook2
  • CHM

Even lower priority:

  • Mobipocket
  • AZW - Kindle format v7 and lower (slightly modified Mobipocket)
  • AZW3 - Kindle format v8
  • LIT (similar to CHM)
  • Open eBook
  • Kindle Print Replica - Just PDF + some stuff

Looks like we can get XPS support for free from MuPDF (a Microsoft PDF-like format) so why not.

Fairly comprehensive list here.

PDF support

Probably using MuPDF is a good idea. We could potentially also enable the WebKit PDF plugin with the ENABLE_PDFKIT_PLUGIN.

We should also add PDF reflowing support.

DjVu support

Use DjVuLibre and look at DjView4 for an example implementation.

CBZ support

We already have unzip support for reading EPUB so we just need a decent


Unfortunately since RAR is a proprietary standard there is no libre software to uncompress newer versions of the RAR format. The UniquE RAR library is GPLv2 and supports up to RAR 2.0 but it looks like nothing that's actually open source supports above RAR 2.0 (we're currently at RAR 5.0).

The Debian package unrar-free has a few security patches that are not in the official UniquE RAR source code.

Other epub readers


MuPDF actually has EPUB support and can do justification! It allows user-supplied CSS but has very few other options in the app though we should check out its API. One bad thing is that the app loads the entire EPUB file into memory. It doesn't keep its place in the text when font size is changed. It doesn't render pages so the bottom line of text is often cut off vertically.

MuPDF only has a partially documented API.

We should probably use this only for its PDF and XPS support.


Readium.js is an in-browser EPUB reader that can also be installed as a Chromium app.

Possibly a good alternative to Epub.js as a source of epub parsing code.

Opening a < 100 kB epub using the Chromium app resulted in the following total memory usage

Virtual       22338.96 MB
Resident        834.85 MB
PSS (smem)      406.01 MB
Dirty           275.17 MB


Epub.js is an in-browser EPUB reader.


Foliate is an interesting ebook reader which is entirely (or almost) written in javascript using Epub.js and the GJS javascript bindings for the GNOME platform libraries. It also uses WebKit2GTK.

When opening a < 100 kB epub in Foliate the memory usage after startup is > 240 MB (RSS). When opening a larger epub it's obvious that the entire epub is loaded into memory in a way that isn't just the normal linux aggressive caching. This probably happens because the way foliate opens epub files (which are zip files) is to use Epub.js which runs inside the browser javascript context and unzips the entire file inside of browser memory using the JSZip library.

Foliate uses the python tool KindleUnpack to read Kindle format ebooks.


Work-in-progress ebook reader, web browser and primary UI for the distro






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