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

[Eli Zaretskii]

> 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
> 
> On Mon, Dec 24, 2018 at 06:10:49PM +0200, Eli Zaretskii wrote:
> > > Date: Mon, 24 Dec 2018 04:08:47 +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
> > > 
> > > I think we are almost good now. There is only one serious FIXME left:
> > > 
> > >   /* FIXME: guess_segment_properties is BAD BAD BAD.
> > >    * we need to get these properties with the LGSTRING. */
> > > #if 1
> > >   hb_buffer_guess_segment_properties (hb_buffer);
> > > #else
> > >   hb_buffer_set_direction (hb_buffer, XXX);
> > >   hb_buffer_set_script (hb_buffer, XXX);
> > >   hb_buffer_set_language (hb_buffer, XXX);
> > > #endif
> > > 
> > > 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.

In particularly, I hope I understood correctly the way we should leave
to Harfbuzz guess the properties not explicitly provided by the Emacs
context, both for the direction of the text and its script.

> > > * Its script, possibly after applying something like:
> > >   http://unicode.org/reports/tr24/#Common
> > 
> > Per previous discussions, we decided to use the Harfbuzz built-in
> > methods for determining the script, since Emacs doesn't have this
> > information, and adding it will just do the same as Harfbuzz does,
> > i.e. find the first character whose script is not Common etc., using
> > the UCD database.  I think it was you who suggested to use the
> > Harfbuzz built-ins in this case.
> 
> The built-in HarfBuzz code is for getting the script for a given
> character, but resolving characters with Common script is left to the
> client. Suppose you have this string (upper case for RTL) ABC 123 DEF,
> what HarfBuzz sees during shaping is three separate chunks of text ABC,
> 123, DEF. The 123 part is all Common script characters and thus
> hb_buffer_guess_segment_properties won’t be able to guess anything (and
> based on the font and the script, this can cause rendering differences).
> Emacs will have to resolve the script of Common characters before
> applying bidi algorithm and pass that down to HarfBuzz.

See my followup questions about this.  For now, I left this aspect to
HarfBuzz.

> > > * Its language, is Emacs allows setting text language (my understand is
> > >   that it doesn’t). Some languages really need this for applying
> > >   language-specfic features (Urdu digits, Serbian alternate glyphs, etc.).
> > 
> > We don't currently have a language property for chunks of text, we
> > only have the current global language setting determined from the
> > locale (and there's a command to change that for Emacs, should the
> > user want it).  This is not really appropriate for multilingual
> > buffers, but we will have to use that for now, and hope that in the
> > future, infrastructure will be added to allow more flexible
> > determination of the language of each run of text.  (I see that
> > Harfbuzz already looks a the locale for its default language, but
> > since Emacs allows user control of this, however unlikely, I think
> > it's best to use the value Emacs uses.)  I will work on this as well.
> 
> Yes, better pass that from Emacs to HarfBuzz.

Done, but please see the FIXME I left behind.  For testing purposes,
you can change the current language like this:

  M-x set-locale-environment RET xx_YY.CODESET RET

For example:

  M-x set-locale-environment RET sr_RS.UTF-8 RET

for the Cyrillic Serbian locale.  This should change the value of
current-iso639-language to the symbol 'sr'.

Please tell if you encounter any difficulties with the code I added,
or if you need any further help.

Thanks.