bug#50951: 28.0.50; Urdu text is not displayed correctly

[Rah Guzar]

I finally tested the patches and both of them improve the situation by a
lot but problems still remain. One word that is not rendered by
accurately by them is

ہنگام

Where is problem is fourth character which is
        character: گ‎ (displayed as گ‎) (codepoint 1711, #o3257, #x6af)
        charset: unicode-bmp (Unicode Basic Multilingual Plane (U+0000..U+FFFF))
        code point in charset: 0x06AF
        script: arabic

This character should be rendered as a circle and two slanted lines
which seem to get clipped.

Both the patches seem to have the same effect, except that with the
first one above a threshold I have seen a newline not getting rendered
so what should be two lines appear as one. This doesn't happen with the
second patch.


Rah Guzar

Rah Guzar <rahguzar@zohomail.eu> writes:

> Unfortunately I have very limited experience with building software and
> none at all with applying patches so testing this will be a non-trivial
> undertaking. I will try to do this next weekend.
>
> But what Visuwesh describes seem exactly the symptoms I was experiencing
> so I am pretty positive that it will work for me too. So thank you very
> much indeed!
>
> Rah Guzar
>
> Visuwesh <visuweshm@gmail.com> writes:
>
>> [புதன் செப்டம்பர் 07, 2022] Eli Zaretskii wrote:
>>
>>>> Date: Wed, 07 Sep 2022 15:18:21 +0900
>>>> From: YAMAMOTO Mitsuharu <mituharu@math.s.chiba-u.ac.jp>
>>>> Cc: Eli Zaretskii <eliz@gnu.org>,
>>>>    larsi@gnus.org,
>>>>    50951@debbugs.gnu.org
>>>>
>>>> Probably the following change will fix the problem:
>>>
>>> Thanks.  Rah Guzar, can you try one of these changes and see if they
>>> fix the problem?
>>
>> Not sure about Rah Guzar, but it squashes bug#54646 for me.  Thank you
>> so very much, Yamamoto Mitsuharu!
>>
>> With regards to the performance, my eyes say that patch no. 1 (i.e., the
>> one that patches src/hbfont.c) is slightly faster than patch no. 2 when
>> I continually zoomed to reproduce bug#54646.  The test material was this
>> random webpage https://www.dinamalar.com/news_detail.asp?id=3117719,
>> and M-x report-emacs-bug report on the Emacs with the patch is at the
>> end.
>> The test that I used is the same as I outlined in bug#54646, namely,
>> visit that webpage in eww, zoom in and out with C-scroll-wheel.  The
>> glyphs _do not_ bump into each other or get weirdly placed whereas
>> without the patch, glyphs can be oddly spaced.
>>
>> In GNU Emacs 29.0.50 (build 6, x86_64-pc-linux-gnu, X toolkit, cairo
>>  version 1.16.0, Xaw scroll bars) of 2022-09-08 built on astatine
>> Repository revision: 9219e83b3c0ef53df02caf4c8ba38f482937ab50
>> Repository branch: master
>> Windowing system distributor 'The X.Org Foundation', version 11.0.12101004
>> System Description: Debian GNU/Linux bookworm/sid
>>
>> Configured using:
>>  'configure --with-sound=alsa --with-x-toolkit=lucid --with-json
>>  --without-xaw3d --without-gconf --without-libsystemd'
>>
>> Configured features:
>> ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
>> JSON LIBOTF LIBSELINUX LIBXML2 MODULES NOTIFY INOTIFY PDUMPER PNG RSVG
>> SECCOMP SOUND SQLITE3 THREADS TIFF TOOLKIT_SCROLL_BARS WEBP X11 XDBE XIM
>> XINPUT2 XPM LUCID ZLIB
>>
>> Important settings:
>>   value of $LC_MONETARY: ta_IN.UTF-8
>>   value of $LC_NUMERIC: ta_IN.UTF-8
>>   value of $LANG: en_GB.UTF-8
>>   locale-coding-system: utf-8-unix