bug#47235: 28.0.50; Suboptimal display of decomposed character sequences

bug#47235: 28.0.50; Suboptimal display of decomposed character sequences

[Philipp Stephani]

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On some systems (e.g. my current GNU/Linux system), "A" followed by
U+0308 COMBINING DIAERESIS isn't displayed as a single glyph.  See the
attached screenshot, which compares this character sequence with the
canonically-equivalent precomposed form "Ä".  The precomposed glyph
form looks better, and it would be great if Emacs could use it for the
composite variant as well.  Also see
https://lists.gnu.org/archive/html/help-gnu-emacs/2020-12/msg00898.html,
https://lists.gnu.org/archive/html/help-gnu-emacs/2021-01/msg00409.html,
https://lists.gnu.org/archive/html/help-gnu-emacs/2021-02/msg00808.html,
and
https://lists.gnu.org/archive/html/help-gnu-emacs/2021-03/msg00114.html
for some background discussion.


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In GNU Emacs 28.0.50 (build 10, x86_64-pc-linux-gnu, GTK+ Version 3.24.24, cairo version 1.16.0)
 of 2021-03-18
Repository revision: 52270aa0dc3313f42986a07413bf5b600d9fecbe
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12010000
System Description: Debian GNU/Linux rodete

Configured using:
 'configure --enable-gcc-warnings=warn-only
 --enable-gtk-deprecation-warnings --without-pop --with-mailutils
 --enable-checking=all --enable-check-lisp-object-type --with-modules
 'CFLAGS=-O0 -ggdb3''

Configured features:
CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GSETTINGS HARFBUZZ JPEG JSON
LIBSELINUX LIBSYSTEMD MODULES NOTIFY INOTIFY PDUMPER PNG SOUND THREADS
TIFF TOOLKIT_SCROLL_BARS X11 XDBE XIM XPM GTK3 ZLIB

Important settings:
  value of $LC_TIME: en_DK.utf8
  value of $LANG: en_US.utf8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc dired dired-loaddefs rfc822
mml mml-sec epa epg epg-config gnus-util rmail rmail-loaddefs time-date
mm-decode mm-bodies mm-encode mail-parse rfc2231 mailabbrev gmm-utils
mailheader sendmail rfc2047 rfc2045 ietf-drums mm-util mail-prsvr
mail-utils phst skeleton derived edmacro kmacro pcase ffap thingatpt url
url-proxy url-privacy url-expand url-methods url-history url-cookie
url-domsuf url-util url-parse auth-source cl-seq eieio eieio-core
cl-macs eieio-loaddefs password-cache json map url-vars mailcap rx
gnutls puny dbus xml subr-x seq byte-opt gv bytecomp byte-compile cconv
compile text-property-search comint ansi-color ring cl-loaddefs cl-lib
iso-transl tooltip eldoc electric uniquify ediff-hook vc-hooks
lisp-float-type mwheel term/x-win x-win term/common-win x-dnd tool-bar
dnd fontset image regexp-opt fringe tabulated-list replace newcomment
text-mode elisp-mode lisp-mode prog-mode register page tab-bar menu-bar
rfn-eshadow isearch easymenu timer select scroll-bar mouse jit-lock
font-lock syntax font-core term/tty-colors frame minibuffer cl-generic
cham georgian utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao
korean japanese eucjp-ms cp51932 hebrew greek romanian slovak czech
european ethiopic indian cyrillic chinese composite charscript charprop
case-table epa-hook jka-cmpr-hook help simple abbrev obarray
cl-preloaded nadvice button loaddefs faces cus-face macroexp files
window text-properties overlay sha1 md5 base64 format env code-pages
mule custom widget hashtable-print-readable backquote threads dbusbind
inotify dynamic-setting system-font-setting font-render-setting cairo
move-toolbar gtk x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 68371 3862)
 (symbols 48 8413 1)
 (strings 32 24380 1846)
 (string-bytes 1 787869)
 (vectors 16 14940)
 (vector-slots 8 195035 6583)
 (floats 8 26 32)
 (intervals 56 223 0)
 (buffers 992 11))

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bug#47235: 28.0.50; Suboptimal display of decomposed character sequences

[Eli Zaretskii]

> From: Philipp Stephani <p.stephani2@gmail.com>
> Date: Thu, 18 Mar 2021 15:35:40 +0100
> 
> On some systems (e.g. my current GNU/Linux system), "A" followed by
> U+0308 COMBINING DIAERESIS isn't displayed as a single glyph.  See the
> attached screenshot, which compares this character sequence with the
> canonically-equivalent precomposed form "Ä".  The precomposed glyph
> form looks better, and it would be great if Emacs could use it for the
> composite variant as well.

We do something like that for Hebrew, see hebrew-font-get-precomposed
and its call in hebrew-shape-gstring.  That is based on data structure
we maintain manually.  We could do that for other scripts as well,
but:

  . if the glyph for the precomposed is not available in the font, you
    will see "tofu", which is arguably worse than what happens today
  . the HarfBuzz folks were really unhappy about such "manual
    compositions", and said we should remove that code and rely on the
    shaping engine to do this