bug#33729: 27.0.50; Partial glyphs not rendered for Gujarati with Harfbuzz enabled (renders fine using m17n)

[Khaled Hosny]

On Sat, Dec 29, 2018 at 04:49:23PM +0200, Eli Zaretskii wrote:
> > Date: Mon, 24 Dec 2018 19:37:23 +0200
> > From: Khaled Hosny <dr.khaled.hosny@gmail.com>
> > Cc: rgm@gnu.org, far.nasiri.m@gmail.com, behdad@behdad.org,
> >     33729@debbugs.gnu.org, kaushal.modi@gmail.com
> > 
> > > > We need to know, for a given lgstring we are shaping:
> > > > * Its direction (from applying bidi algorithm). Each lgstring we are
> > > >   shaping must be of a single direction.
> > > 
> > > Communicating this to ftfont_shape_by_hb will need changes in a couple
> > > of interfaces (the existing shaping engines didn't need this
> > > information).  I will work on this soon.
> > 
> > Great.
> 
> Done.  Please test.  I made sure it compiles, but I couldn't actually
> test the results, as I don't have access to a GNU/Linux system with
> GUI display.  So it could be that I misunderstood the Harfbuzz APIs,
> as I was essentially flying blind, guided only by the Harfbuzz docs.

It seems to work, but still not quite right. You seem to be passing the
paragraph direction, but what HarfBuzz needs is resolved direction of
the text (i.e. the bidi embedding level of the run). In other words, if
Emacs is going to draw this text from right to left, then HarfBuzz must
shape it in right to left direction. Both should use the same direction
all the time and HarfBuzz direction guessing should never be used (i.e.
always pass to it an explicit direction).

Regards,
Khaled