Re: how reliable is rendering of complex scripts?

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sun, 04 Oct 2015 06:39:52 +0200 (CEST)
> Cc: emacs-devel@gnu.org, handa@gnu.org
> From: Werner LEMBERG <wl@gnu.org>
> 
> >> I wonder how reliable emacs displays complex scripts like
> >> Devanagari or Arabic.
> >
> > AFAIK, no one has ever performed a study about this, let alone
> > repeated it when the relevant standards changed.
> 
> This essentially means that Emacs developers wait for users to report
> bad renderings, right?

No, it means Emacs developers lack resources, knowledge, and motivated
individuals aboard to do that.  If you or someone else volunteers,
that'd be great.

Where we do have resources, we are generally ahead of HarfBuzz: e.g.,
the implementation of UAX#9 compliant with Unicode 6.3 - 8.0 was in
Emacs many moons before it was anywhere else in the Free Software
world, including HarfBuzz.

> >> For example, the maintainers of the HarfBuzz library did extensive
> >> comparisons of the rendering results with the MS engine to iron out
> >> zillions of small buglets in OpenType handling.
> >
> > At least on MS-Windows, Emacs uses the MS engine directly, so some
> > of similar buglets should not affect us on Windows.
> 
> Well, this makes Emacs on MS-Windows really superior to other
> platforms in this area, which is less than ideal...  I mean `superior'
> in the sense that the rendering results on MS-Windows are well tested
> and can be trusted in general, something that is missing otherwise.

That's not entirely true, because, as I said, part of the data and
algorithms needed for complex script layout is in Emacs, and is used
on all supported platforms, not just by the Windows build.

> Given that HarfBuzz is very mature today, and that it has been
> extensively tested against Windows rendering results, and that it also
> contains a large corpus of test cases for complex ligatures together
> with a simple test TTY program (`hb-shape'), I suggest that someone
> (probably Ken'ichi) writes a similar test program for libm17n so that
> diffing would be possible.
> 
>   https://github.com/behdad/harfbuzz/tree/master/test/shaping
>   https://github.com/behdad/harfbuzz/tree/master/util

Thanks for the pointer.  I will see if we can extract something from
there to see how Emacs displays those scripts.

> >> are there test suites to compare the results?
> >
> > There's a test suite for bidirectional display, but it only tests
> > the reordering of characters for display, not the shaping.  There's
> > nothing else, AFAIK.  If you, or someone else, can work on adding
> > one, that'd be great.
> 
> In case someone is working on this issue, asking the HarfBuzz
> developer for assistance might be a good thing.  I guess that they
> have even larger corpora that could be probably provided for testing
> purposes.

No one is working on this, to the best of my knowledge.  Once again,
we lack individuals on board who understand these issues well enough
to do that.  A single bug in libm17n-flt reported a few months ago,
discovered in a script whose support is not even in Emacs, was fixed
by Handa-san, and that's about all the activity that I remember.