Re: Compositions and bidi display (was: bug#5977: 24.0.50; Lao HELLO is incorrectly displayed)

[Kenichi Handa <handa@m17n.org>]

In article <8339yhziyj.fsf@gnu.org>, Eli Zaretskii <eliz@gnu.org> writes:
> > All composition-related functions expect characters are in
> > logical order.  The bottom-most library for OTF handling
> > (libotf) requires it because OpenType tables expect
> > characters in logical order.

> Btw, where does libotf come into this picture?  That is, which libotf
> functions we use for composite characters, and at what stage in the
> redisplay process?

For instance, an OpenType font may have independent glyphs
for Hebrew consontants and diacriticals, and provide GPOS
(glyph positioning) table to tell where to place a specific
diacritical glyph on a specific consontant.  To utilize such
a font, a composition function calls libotf's OTF_drive_gpos
in this calling sequence.

CHAR_COMPOSED_P
 -> composition_reseat_it
   -> autocmp_chars
     -> a Lisp function in `composition-function-table
       -> font-shape-gstring
         -> font_driver->shape
           -> ftfont_shape_by_flt
             -> mflt_run (of libm17n-flt)
               -> ftfont_drive_otf (ftfont.c) as a callback rountine
                 -> OTF_drive_gpos (of libotf)

---
Kenichi Handa
handa@m17n.org