How To Fix Rendering of Persian Font With XeLaTeX?

Question

I am trying to typeset a poem with XeLaTeX using the IranNastaliq font, and I am running into problems with the glyphs rendered for many words. I have a PDF file I rendered from the same source tex file several years ago that is free of these problems.

A minimal example is shown below:

%!TEX TS-program-xelatex
%!TEX encoding = UTF-8 Unicode
\documentclass{article}
\usepackage{fontspec}
\newfontfamily\farsifont[Script=Arabic,Scale=10]{IranNastaliq}

\begin{document}
\farsifont
{\huge نگار}
\end{document}

This produces the image on the left on my current setup (MacTeX BasicTeX 2014). The image on the right, on the other hand, is what gets rendered in other programs, such as TextEdit, and what I had rendered in the past using XeLaTeX on an older computer (also OS X).

What could be causing this problem? How would I debug this?

Answer 1

The change does indeed seem to be due to the HarfBuzz library. I noticed the same "bad" version of the font's rendering in Chrome and Firefox, which both use HarfBuzz, while I saw the "good" version in applications that used Apple's native rendering.

I filed a bug with HarfBuzz, and the lead developer explained that this font makes use of contextual swashes cswh, which are not enabled by default in OpenType. More can be read about this and other OpenType features as they apply to Arabic fonts here: https://www.microsoft.com/typography/OpenTypeDev/arabic/intro.htm.

In the end I found I could fix the problem by manually enabling contextual swashes in my fontspec. A good guide to working with OpenType features in LaTeX can be found here: http://nitens.org/taraborelli/TeXOpenType.

\newfontfamily\farsifont[Script=Arabic,Scale=10,Contextuals=Swash]{IranNastaliq}

The good news is that as far as I can tell, there doesn't seem to be any bug here.