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11Feb/12Off

Typesetting Arabic with LuaTeX: Part 2 (documentation, tools and libraries)

Introduction

I've been thinking about the next article in this series and what should it address so I've decided to skip ahead and give a summary of the documentation, tools and libraries which made it possible for me to experiment with typesetting Arabic. I'm listing these because it actually took a long time to assemble the reading materials and tools required, so it may just save somebody, somewhere, the many hours I spent hunting it all down. For sure, there's a ton of stuff I want to write about, in an attempt to piece together the various concepts and ideas involved in gaining a better understanding of Unicode, OpenType and Arabic text typesetting/display. However, I'm soon to start a new job, which means I'll have less time to devote to this blog so I'll try to post as much as I can over the next couple of weeks.

Just for completeness, I should say that, for sure, you can implement Arabic layout/typesetting for LuaTeX in pure Lua code, as the ConTeXt distribution has done, through the quite incredible work of Idris Hamid and Hans Hagen.

Documentation

There is a lot to read. Here are some resources that are either essential or helpful.

Unicode

Clearly, you'll need to read relevant parts of the Unicode Standard. Here's my suggested minimal reading list.

  • Chapter 8: Middle Eastern Scripts . This gives an extremely useful description of cursive joining and a model for implementing contextual analysis.
  • Unicode ranges for Arabic (see also these posts). You'll need the Unicode code charts for Arabic (PDFs downloadable and listed under Middle Eastern Scripts, here)
  • Unicode Bidirectional Algorithm. Can't say that I've really read this properly, and certainly not yet implemented anything to handle mixed runs of text, but you certainly need it.

OpenType

Whether you are interested in eBooks, conventional typesetting or the WOFF standard, these days a working knowledge of OpenType font technology is very useful. If you want to explore typesetting Arabic then it's simply essential.

C libraries

It's always a good idea to leverage the work of true experts, especially if it is provided for free as an open source library! I spent a lot of time hunting for libraries, so here is my summary of what I found and what I eventually settled on using.

  • IBM's ICU: Initially, I looked at using IBM's International Components for Unicode but, for my requirements, it was serious overkill. It is a truly vast and powerful open source library (for C/C++ and Java) if you need the wealth of features it provides.
  • HarfBuzz: This is an interesting and ongoing development. The HarfBuzz OpenType text shaping engine looks like it will become extremely useful; although I had a mixed experience trying to build it on Windows, which is almost certainly due to my limitations, not those of the library. If you're Linux-based then no doubt it'll be fine for you. As it matures to a stable release I'll definitely take another look.
  • GNU FriBidi: As mentioned above, essential for a full implementation of displaying (eBooks, browsers etc) or typesetting mixed left-to-right and right-to-left scripts is the Unicode Bidirectional Algorithm. Fortunately, there's a free and standard implementation of this available as a C library: GNU FriBidi I've not yet reached the point of being able to use it but it's the one I'll choose.

My libraries of choice

Eventually, I settled on FreeType and libotf. You need to use them together because libotf depends on FreeType. Both libraries are mature and easy to use and I simply cannot praise these libraries too highly. Clearly, this is my own personal bias and preference but ease of use rates extremely highly on my list of requirements. FreeType has superb documentation whereas libotf does not, although it has some detailed comments within the main #include file. I'll definitely post a short "getting started with libotf" because it is not difficult to use (when you've worked it out!).

libotf: words are not enough!

Mindful that I've not yet explained how all these libraries work together, or what they do, but I just have to say that libotf is utterly superb. libotf provides a set of functions which "drive" the features and lookups contained in an OpenType font, allowing you to pass in a Unicode string and apply OpenType tables to generate the corresponding sequence of glyphs which you can subsequently render. Of course, for Arabic you also need to perform contextual analysis to select the appropriate joining forms but once that is done then libotf lets you take full advantage of any advanced typesetting features present in the font.

UTF-8 encoding/decoding

To pass Unicode strings between your C code and LuaTeX you'll be using UTF-8 so you will need to encode and decode UTF-8 from within your C. Encoding is easy and has been covered elsewhere on this site. For decoding UTF-8 into codepoints I use the The Flexible and Economical UTF-8 Decoder.

Desktop software

In piecing together my current understanding of Unicode and OpenType I found the following software to be indespensible. Some of these are Windows-only applications.

TIP: Microsoft VOLT and the Arabic Typesetting or Scheherazade fonts. I'll talk about VOLT in more detail later but Microsoft and SIL provide "VOLT versions" of their respective Arabic fonts. These are absolutely invaluable resources for understanding advanced OpenType concepts and if you are interested to learn more I strongly recommend taking a look at them.

  • The VOLT version of the Arabic Typesetting font is shipped with the VOLT installer and is contained within a file called "VoltSupplementalFiles.exe", so just run that to extract the VOLT version.
  • The VOLT version of Scheherazade is made available as a download from SIL.

I can only offer my humble thanks to the people who created these resources and made them available for free: a truly substantial amount of work is involved in creating them.

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