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15Aug/13Off

Colouring Arabic vowels with XeTeX and a HarfBuzz pre-processor

Introduction

Using an external pre-processor (built using HarfBuzz) you can achieve affects that are not possible (or, at least, not easy) directly with XeTeX. Here's a simple example of colouring Arabic vowels – this example is likely to be possible with XeTeX alone, but it's just a quick demo – many other interesting possibilities come to mind. At the moment the Arabic string is hardcoded into the pre-processor, just for testing, but I plan to make it read from files output by XeTeX – it's just a proof of concept. The vowel positioning was achieved by putting the vowel glyphs in boxes and shifting them according to the anchor point data provided by HarfBuzz.

My test document

\documentclass[11pt,twoside,a4paper]{book}
\pdfpageheight=297mm
\pdfpagewidth=210mm
\usepackage{fontspec}
\usepackage{bidi}
\begin{document}
\pagestyle{empty}
\font\scha= "Scheherazade" at 12bp
\font\schb= "Scheherazade" at 30bp
\scha \noindent Here, we compare the Arabic text contained in our \XeTeX\ file to the text which is
output directly via a HarfBuzz pre-processor and input into our document from "harfarab.tex"\par\vskip10pt
\schb
\noindent \hbox to 150pt{Actual text:\hfill} \RL{هَمْزَة وَصْل}\par
\noindent \hbox to 150pt{Processed text:\hfill} \input harfarab.tex
\end{document}

harfarab.tex output via HarfBuzz

Displayed here on individual lines for readability.

\XeTeXglyph609
\hbox to 0pt{\vbox{\moveright 6.53bp\hbox{\raise-2.71bp\hbox{\special{color push rgb 0 0 1}\XeTeXglyph911 \special{color pop}}}}}
\XeTeXglyph831
\hbox to 0pt{\vbox{\moveright 3.56bp\hbox{\raise-4.82bp\hbox{\special{color push rgb 0 0 1}\XeTeXglyph907 \special{color pop}}}}}
\XeTeXglyph263
\XeTeXglyph3
\XeTeXglyph436
\hbox to 0pt{\vbox{\moveright 1.82bp\hbox{\raise-3.24bp\hbox{\special{color push rgb 0 0 1}\XeTeXglyph907 \special{color pop}}}}}
\XeTeXglyph489
\hbox to 0pt{\vbox{\moveright 3.47bp\hbox{\raise-4.35bp\hbox{\special{color push rgb 0 0 1}\XeTeXglyph911 \special{color pop}}}}}
\XeTeXglyph755
\hbox to 0pt{\vbox{\moveright 2.20bp\hbox{\raise-2.64bp\hbox{\special{color push rgb 0 0 1}\XeTeXglyph907 \special{color pop}}}}}
\XeTeXglyph896

The resulting PDF

As you can see, the results are identical – as you'd expect since they both use the HarfBuzz engine, one internally to XeTeX, the other externally in a pre-processor.

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