[HarfBuzz] Question about zero width glyphs in shaping output

[Khaled Hosny]

On Mon, Dec 07, 2015 at 09:13:23PM +0530, Deepak Jois wrote:
> Maybe this is a bit related to Khaled’s question earlier about control
> characters inside ligatures, but I wanted to start a new thread.
> 
> When I shape text with Noto Nastaliq, I notice a bunch of zero-width
> glyphs generated
> 
> $> hb-unicode-encode U+06CC,U+06C1 |  hb-shape notonastaliq.ttf
> [HehFin=1+472|TwoDotsBelowNS=0 at 310,-383+0|sp2=0+0|BehxIni.outS1=0 at 0,-68+731]
> 
> 1. What is the purpose of these zero-width glyphs?

That is something internal to the font, they are not glyphs inserted by
HarfBuzz.

> 2. If I am rendering the shaped output to a PDF file (for e.g. when
> using Harfbuzz with LuaTeX), do I need to care about these zero-width
> glyphs at all? How will they affect rendering

You should just output the glyph as returned by HarfBuzz,
second-guessing it is likely to be wrong. If a character should be
invisible HarfBuzz will replace it by the space glyph, so you need not
worry about this, unless you really know what you are doing.

Regards,
Khaled