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. <\/p>\n
Displayed here on individual lines for readability.<\/p>\n 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.<\/p>\n\\documentclass[11pt,twoside,a4paper]{book}
\n\\pdfpageheight=297mm
\n\\pdfpagewidth=210mm
\n\\usepackage{fontspec}
\n\\usepackage{bidi}
\n\\begin{document}
\n\\pagestyle{empty}
\n\\font\\scha= \"Scheherazade\" at 12bp
\n\\font\\schb= \"Scheherazade\" at 30bp
\n\\scha \\noindent Here, we compare the Arabic text contained in our \\XeTeX\\ file to the text which is
\noutput directly via a HarfBuzz pre-processor and input into our document from \"harfarab.tex\"\\par\\vskip10pt
\n\\schb
\n\\noindent \\hbox to 150pt{Actual text:\\hfill} \\RL{\u0647\u064e\u0645\u0652\u0632\u064e\u0629 \u0648\u064e\u0635\u0652\u0644}\\par
\n\\noindent \\hbox to 150pt{Processed text:\\hfill} \\input harfarab.tex
\n\\end{document}<\/code><\/p>\nharfarab.tex output via HarfBuzz<\/h2>\n
\\XeTeXglyph609
\n\\hbox to 0pt{\\vbox{\\moveright 6.53bp\\hbox{\\raise-2.71bp\\hbox{\\special{color push rgb 0 0 1}\\XeTeXglyph911 \\special{color pop}}}}}
\n\\XeTeXglyph831
\n\\hbox to 0pt{\\vbox{\\moveright 3.56bp\\hbox{\\raise-4.82bp\\hbox{\\special{color push rgb 0 0 1}\\XeTeXglyph907 \\special{color pop}}}}}
\n\\XeTeXglyph263
\n\\XeTeXglyph3
\n\\XeTeXglyph436
\n\\hbox to 0pt{\\vbox{\\moveright 1.82bp\\hbox{\\raise-3.24bp\\hbox{\\special{color push rgb 0 0 1}\\XeTeXglyph907 \\special{color pop}}}}}
\n\\XeTeXglyph489
\n\\hbox to 0pt{\\vbox{\\moveright 3.47bp\\hbox{\\raise-4.35bp\\hbox{\\special{color push rgb 0 0 1}\\XeTeXglyph911 \\special{color pop}}}}}
\n\\XeTeXglyph755
\n\\hbox to 0pt{\\vbox{\\moveright 2.20bp\\hbox{\\raise-2.64bp\\hbox{\\special{color push rgb 0 0 1}\\XeTeXglyph907 \\special{color pop}}}}}
\n\\XeTeXglyph896<\/code> <\/p>\nThe resulting PDF<\/h2>\n