bug#35781: 27.0.50; [PATCH] Improve font display on Cairo builds

[Kévin Le Gouguec]

Hi,

The attached patch makes fonts in the Cairo build more readable[1] on
some setups I use.

diff --git a/src/ftcrfont.c b/src/ftcrfont.c
index e7c73eac4d..51890dc18c 100644
--- a/src/ftcrfont.c
+++ b/src/ftcrfont.c
@@ -146,6 +146,7 @@ ftcrfont_open (struct frame *f, Lisp_Object entity, int 
pixel_size)
       cairo_matrix_init_scale (&font_matrix, pixel_size, pixel_size);
       cairo_matrix_init_identity (&ctm);
       cairo_font_options_t *options = cairo_font_options_create ();
+      cairo_font_options_set_hint_style (options, CAIRO_HINT_STYLE_SLIGHT);
       ftcrfont_info->cr_scaled_font =
        cairo_scaled_font_create (font_face, &font_matrix, &ctm, options);
       cairo_font_face_destroy (font_face);

More context: I started trying out builds --with-cairo recently to
check out Emacs's support of multicolor fonts.  While these fonts seem
to work fine (emojis! 🙌), I found that the fonts for regular text
look… sharper?  In an aggressive-and-too-contrasted kind of way?

This screenshot shows the difference on one of my desktops[2]:

desktop.png
Description: PNG image

Each frame is a different instance of 'emacs -Q'.  The left one is
configured without Cairo, the middle one --with-cairo *with my patch
applied*, the right one --with-cairo without my patch.

Same visual difference on my laptop[3]:

laptop.png
Description: PNG image

Each frame is a different instance of 'emacs -Q -fn "DejaVu Sans
Mono-9"'[4].  The left frame is configured --with-cairo, without my
patch; the top-right one is without cairo, the bottom-right
--with-cairo with my patch.

On the third setup I could test[5], all three builds look the same
AFAICT; no idea what this can be attributed to.


Does this patch look like something that can be applied
unconditionally?

- CAIRO_HINT_STYLE_SLIGHT has been available from version 1.0, and we
  require 1.12, so AFAICT the setting can be used without checking for
  any CAIRO_… feature.

- On the other hand, maybe this setting should be conditioned by a
  user option: as far as I could test, …_SLIGHT gives the "best"
  visual result[6], but maybe this is specific to my setups.

  I guess that a customizable option would need to specify a :set
  function tasked with making a few Cairo calls for the change to be
  effective without restarting; I have not spent much time looking at
  ftcrfont.c and its users yet, but I imagine that this hypothetical
  setter function should do something along the lines of:

  1. retrieve all 'struct font_info' objects created with
     ftcrfont_open,
  2. call cairo_scaled_font_get_font_options on their 'cr_scaled_font'
     field,
  3. call …set_hint_style on the returned options.

  (… This is absolutely untested guesswork.)


Caveat: I do not know the first thing about font rendering (or any
kind of rendering for that matter), nor font hinting.  This patch is
the result of me googling around for "cairo smooth font" and trying
out random snippets to tweak cairo_font_options_* knobs.  Some
snippets I have seen call …set_{antialias,subpixel_order} in
conjunction with …set_hint_style; as far as I tested, the former two
have no impact, so I left them out of the patch.


Let me know if I should provide more information (font configuration,
results after tweaking other settings… though I cannot access my
desktops until Monday), and thank you for your time!


[1] Objectively-speaking, the patch merely makes fonts "visually
    equivalent to the non-Cairo build"; whether it makes them "more
    readable" is a matter of taste I guess.

[2] OS: Xubuntu 16.04 (libcairo2: 1.14.6)
    xrandr digest: DP-2 1920x1080 509mm x 286mm
    emacs-repository-version: … probably a commit on master from these
                              past two weeks

[3] OS: Debian stretch (libcairo2: 1.14.8)
    xrandr digest: LVDS1 1024x600 230mm x 140mm
    emacs-repository-version: e0ee41d155b210327eb9c9ad5334f80ed59439f4

[4] emacs -Q defaults to a bigger font size than what I use, and the
    "sharpness" difference is less visible then.

[5] OS: openSUSE Tumbleweed (cairo: 1.16.0)
    xrandr digest: DVI-I-0 1680x1050 474mm x 296mm
    emacs-repository-version: … probably a commit on master from these
                              past two weeks

[6] Compiling with any other enum value looked equivalent to not
    setting this option at all IIRC.



In GNU Emacs 27.0.50 (build 3, i686-pc-linux-gnu, GTK+ Version 3.22.11)
 of 2019-05-13 built on nc10-laptop
Repository revision: d2d4916046e31e46598f0a0edbc65e75b8cb4cc3
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Windowing system distributor 'The X.Org Foundation', version 11.0.11902000
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