Emacs 28: bitmap fonts, vector font character spacing

[Christoph Groth <christoph@grothesque.org>]

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Hello,

I’m having some trouble trying to set up a pleasant font after upgrading
to Emacs 28.  Would someone be so kind to help?

For many years I was happily using the “Terminus” bitmap font in Emacs
and terminal windows on “regular density” displays.  On “HiDPI” displays
I switched to vector fonts, but for low density displays I preferred the
crisp Terminus.

I recently upgraded Emacs from 27.1 to 28.2 as part of upgrading to the
new Debian stable and this broke my font setup.  I have something like
the following in my emacs config.

(add-to-list 'default-frame-alist '(font . "Terminus-12"))

With Emacs 28.2 the default font is now much too small (I’m not even
sure whether it’s still Terminus).  It is possible to choose Terminus
with M-x menu-set-font, but this shows the same tiny font independently
of the chosen size.

I noticed the following entry in NEWS:

> ** The Cairo graphics library is now used by default if present.
> '--with-cairo' is now the default, if the appropriate development
> files are found by 'configure'.  Building with Cairo is known to cause
> some problems with bitmap fonts.  This may require you to adjust your
> font settings, or to build with Xft support instead.

Unfortunately, this is not very specific.  Do “some problems” mean that
fonts like Terminus no longer work and that I have to recompile Emacs if
I want to use them, or is there some workaround?  I tried to find one,
but so far without success.

----------------------------------------------------------------

So, OK, I thought I’d go with the time and switch to a vector font on
low-density displays as well.  I like how Fira Code Light looks in
xfce4-terminal.

I managed to set it to the size I like through

(add-to-list
 'default-frame-alist
 '(font . "-CTDB-Fira Code-light-normal-normal-*-14-*-*-*-m-0-iso10646-1"))

(Interestingly, I am not able to obtain this size by simply using “Fira
Code-N”.  The font is either too small or too large!)

Anyway, while with the above the vertical line spacing matches the one in
xfce4-terminal, the horizontal inter-character distance is somewhat
wider.  Is there a way to render the font like in xfce4-terminal?

(I believe that this issue could be related to kerning being handled
differently by Emacs/Cairo, but I did not manage to find any information
on how to adjust it.)

For reference:

----------------------------------------------------------------
(frame-parameter nil 'font-backend)
(ftcrhb x)
system-configuration-features
"ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG JSON LCMS2 LIBOTF LIBSELINUX LIBSYSTEMD LIBXML2 M17N_FLT MODULES NATIVE_COMP NOTIFY INOTIFY PDUMPER PNG RSVG SECCOMP SOUND THREADS TIFF TOOLKIT_SCROLL_BARS X11 XDBE XIM XPM GTK3 ZLIB"
----------------------------------------------------------------

My ~/.Xresources contains

----------------------------------------------------------------
Xft.antialias: 0
Xft.hinting: 0
Xft.hintstyle: hintfull
Xft.lcdfilter: lcddefault
Xft.rgba: rgb
----------------------------------------------------------------

Cheers
Christoph

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