bug#33648: 25.2; feature request: configurable position of strike-through line

bug#33648: 25.2; feature request: configurable position of strike-through line

[Robert Samal]

When I use set-face-attribute to give some text a strike-through
attribute, I see that emacs behaves differently from other pieces of
software: Apparently, the line is drawn at the level of half the glyph's
height -- which causes the line to be just touching letters like "a",
"c", "e", ... and it isn't perceived as striking the text out (at least
not by me).

In comparison, web browsers interpreting html strike tag make the
line at the level of half of the "small lowercase letters" -- basically
at the level of the line in lowercase "e". The same is done by Libre
Office ... and others.

My suggestion is to make this configurable in some way. I'm not sure,
if it would be better to have a global option for all files, or
if "(set-face-attribute ... :strike-through" should accept more choices
to make each line configurable. (I personally would change all of my
striking lines, if possible.)

Thanks in advance for considering this, for some uses (in particular
org-mode) this would help a lot.

Robert Samal, samal@iuuk.mff.cuni.cz


In GNU Emacs 25.2.2 (x86_64-pc-linux-gnu, GTK+ Version 3.22.21)
of 2017-09-22, modified by Debian built on lgw01-amd64-050
Windowing system distributor 'The X.Org Foundation', version 11.0.11906000
System Description: Ubuntu 18.04.1 LTS

Configured using:
'configure --build x86_64-linux-gnu --prefix=/usr
--sharedstatedir=/var/lib --libexecdir=/usr/lib
--localstatedir=/var/lib --infodir=/usr/share/info
--mandir=/usr/share/man --with-pop=yes
--enable-locallisppath=/etc/emacs25:/etc/emacs:/usr/local/share/emacs/25.2/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/25.2/site-lisp:/usr/share/emacs/site-lisp 

--with-sound=alsa --without-gconf --build x86_64-linux-gnu
--prefix=/usr --sharedstatedir=/var/lib --libexecdir=/usr/lib
--localstatedir=/var/lib --infodir=/usr/share/info
--mandir=/usr/share/man --with-pop=yes
--enable-locallisppath=/etc/emacs25:/etc/emacs:/usr/local/share/emacs/25.2/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/25.2/site-lisp:/usr/share/emacs/site-lisp 

--with-sound=alsa --without-gconf --with-x=yes --with-x-toolkit=gtk3
--with-toolkit-scroll-bars 'CFLAGS=-g -O2
-fdebug-prefix-map=/build/emacs25-jYekUr/emacs25-25.2+1=.
-fstack-protector-strong -Wformat -Werror=format-security -Wall'
'CPPFLAGS=-Wdate-time -D_FORTIFY_SOURCE=2'
'LDFLAGS=-Wl,-Bsymbolic-functions -Wl,-z,relro''

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GSETTINGS NOTIFY
ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11

Important settings:
value of $LC_ALL:
value of $LC_CTYPE: en_GB.utf8
value of $LC_MESSAGES: en_GB.utf8
value of $LC_MONETARY: cs_CZ.utf8
value of $LC_NUMERIC: cs_CZ.utf8
value of $LC_TIME: en_GB.utf8
value of $LANG: cs_CZ.utf8
value of $XMODIFIERS: @im=ibus
locale-coding-system: utf-8-unix

Major mode: Info

Minor modes in effect:
projectile-mode: t
recentf-mode: t
global-magit-file-mode: t
diff-auto-refine-mode: t
global-git-commit-mode: t
async-bytecomp-package-mode: t
winum-mode: t
winner-mode: t
volatile-highlights-mode: t
global-vi-tilde-fringe-mode: t
spaceline-info-mode: t
spaceline-helm-mode: t
save-place-mode: t
savehist-mode: t
popwin-mode: t
persp-mode: t
Info-breadcrumbs-in-mode-line-mode: t
flx-ido-mode: t
eyebrowse-mode: t
global-evil-surround-mode: t
evil-surround-mode: t
global-evil-search-highlight-persist: t
evil-search-highlight-persist: t
show-smartparens-global-mode: t
evil-escape-mode: t
global-anzu-mode: t
anzu-mode: t
eval-sexp-fu-flash-mode: t
counsel-mode: t
ivy-mode: t
clean-aindent-mode: t
which-key-mode: t
override-global-mode: t
global-undo-tree-mode: t
undo-tree-mode: t
shell-dirtrack-mode: t
evil-mode: t
evil-local-mode: t
spacemacs-leader-override-mode: t
global-spacemacs-leader-override-mode: t
global-hl-line-mode: t
xterm-mouse-mode: t
global-auto-revert-mode: t
ido-vertical-mode: t
global-page-break-lines-mode: t
global-eldoc-mode: t
electric-indent-mode: t
mouse-wheel-mode: t
file-name-shadow-mode: t
global-font-lock-mode: t
font-lock-mode: t
auto-composition-mode: t
auto-encryption-mode: t
auto-compression-mode: t
buffer-read-only: t
column-number-mode: t
line-number-mode: t
transient-mark-mode: t

Recent messages:
user-error: This is the last Info node you looked at [8 times]
evil-line-move: End of buffer [2 times]
Mark saved where search started [2 times]
user-error: Point neither on reference nor in menu item description
user-error: Node has no Previous [2 times]
user-error: Node has no Next [3 times]
user-error: This is the last Info node you looked at [2 times]
C-c y is undefined
Saving file /home/samal/.emacs.d/.cache/recentf...
Wrote /home/samal/.emacs.d/.cache/recentf

Load-path shadows:
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Features:
(shadow sort mail-extr emacsbug sendmail smex vc-git misearch
multi-isearch jka-compr diminish projectile grep ibuf-ext ibuffer
recentf tree-widget smartparens-org smartparens-text orgit org-element
avl-tree git-rebase evil-magit magit-obsolete magit-blame magit-stash
magit-bisect magit-remote magit-commit magit-sequence magit-notes
magit-worktree magit-tag magit-merge magit-branch magit-reset
magit-collab ghub-graphql treepy graphql ghub url-http tls gnutls url-gw
url-auth url url-proxy url-privacy url-expand url-methods url-history
url-cookie url-domsuf url-util mailcap let-alist magit-files magit-refs
magit-status magit magit-repos magit-apply magit-wip magit-log
which-func imenu magit-diff smerge-mode diff-mode magit-core
magit-autorevert magit-process magit-margin magit-mode git-commit
magit-git magit-section magit-utils crm magit-popup log-edit message
rfc822 mml mml-sec epg mailabbrev mail-utils gmm-utils mailheader
pcvs-util add-log with-editor async-bytecomp async org org-macro
org-footnote org-pcomplete org-list org-faces org-entities org-version
ob-emacs-lisp ob ob-tangle org-src ob-ref ob-lob ob-table ob-keys ob-exp
ob-comint ob-core ob-eval org-compat org-macs org-loaddefs find-func
cal-menu calendar cal-loaddefs hl-todo server ws-butler winum winner
etags volatile-highlights vi-tilde-fringe spaceline-config
spaceline-segments spaceline powerline powerline-separators
powerline-themes smartparens-config saveplace savehist popwin persp-mode
linum ivy-hydra info+ image-mode flx-ido flx eyebrowse format-spec
evil-unimpaired f s evil-surround evil-search-highlight-persist
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bug#33648: 25.2; feature request: configurable position of strike-through line

[Stefan Kangas]

[-- Attachment #1: Type: text/plain, Size: 1155 bytes --]

tags 33648 + confirmed
thanks

Robert Samal <samal@iuuk.mff.cuni.cz> writes:

> When I use set-face-attribute to give some text a strike-through
> attribute, I see that emacs behaves differently from other pieces of
> software: Apparently, the line is drawn at the level of half the glyph's
> height -- which causes the line to be just touching letters like "a",
> "c", "e", ... and it isn't perceived as striking the text out (at least
> not by me).
>
> In comparison, web browsers interpreting html strike tag make the
> line at the level of half of the "small lowercase letters" -- basically
> at the level of the line in lowercase "e". The same is done by Libre
> Office ... and others.
>
> My suggestion is to make this configurable in some way. I'm not sure,
> if it would be better to have a global option for all files, or
> if "(set-face-attribute ... :strike-through" should accept more choices
> to make each line configurable. (I personally would change all of my
> striking lines, if possible.)
>
> Thanks in advance for considering this, for some uses (in particular
> org-mode) this would help a lot.

This is what I see on current master:

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This is the same text rendered in LibreOffice:


[-- Attachment #3: bug-33648-emacs-orig.png --]
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[-- Attachment #4: bug-33648-libreoffice.png --]
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I see two problems here:

1. The strike-through in our case is placed at half the height of an
   upper-case glyph.  LibreOffice seems to have a smarter positioning.

2. The strike-through line is hard coded to 1 pixel in our case, whereas
   LibreOffice scales it up as the font size increases.

I'm have no idea how to fix it; the font code is pretty opaque to me.

I mucked around a bit and came up with the below diff.  It is probably
not the correct fix, but does make it slightly better for me.  Someone
who understands this better would need to investigate this properly.

FWIW, here is what it looks like with the below patch:


[-- Attachment #6: bug-33648-with-patch.png --]
[-- Type: image/png, Size: 2288 bytes --]

[-- Attachment #7: Type: text/plain, Size: 513 bytes --]


diff --git a/src/xterm.c b/src/xterm.c
index 2e0407aff4..5792574e05 100644
--- a/src/xterm.c
+++ b/src/xterm.c
@@ -3932,7 +3932,7 @@ x_draw_glyph_string (struct glyph_string *s)
 	  /* Strike-through width and offset from the glyph string's
 	     top edge.  */
           unsigned long h = 1;
-          unsigned long dy = (glyph_height - h) / 2;
+          unsigned long dy = (glyph_height - h) / 3;
 
 	  if (s->face->strike_through_color_defaulted_p)
 	    x_fill_rectangle (s->f, s->gc, s->x, glyph_y + dy,

bug#33648: 25.2; feature request: configurable position of strike-through line

[Stefan Kangas]

Stefan Kangas <stefan@marxist.se> writes:

> I mucked around a bit and came up with the below diff.  It is probably
> not the correct fix, but does make it slightly better for me.  Someone
> who understands this better would need to investigate this properly.

Actually, looking at this more, I'm starting to get convinced that
LibreOffice is actually just positioning the line at the upper case
glyph height divided by 3 instead of 2.

I had a look also at Firefox, and, if I'm not mistaken, they place the
line at half the height of the lower-case glyph.  (Perhaps that makes
even more sense?)  I used the following link for testing:

https://developer.mozilla.org/en-US/docs/Web/HTML/Element/strike#Example

Best regards,
Stefan Kangas

bug#33648: 25.2; feature request: configurable position of strike-through line

[Eli Zaretskii]

> From: Stefan Kangas <stefan@marxist.se>
> Date: Mon, 31 Aug 2020 11:21:09 +0000
> Cc: 33648@debbugs.gnu.org
> 
> > When I use set-face-attribute to give some text a strike-through
> > attribute, I see that emacs behaves differently from other pieces of
> > software: Apparently, the line is drawn at the level of half the glyph's
> > height -- which causes the line to be just touching letters like "a",
> > "c", "e", ... and it isn't perceived as striking the text out (at least
> > not by me).
> >
> > In comparison, web browsers interpreting html strike tag make the
> > line at the level of half of the "small lowercase letters" -- basically
> > at the level of the line in lowercase "e". The same is done by Libre
> > Office ... and others.

Web browsers and the rest normally don't have to support display
elements of different height on the same screen line.

> > My suggestion is to make this configurable in some way. I'm not sure,
> > if it would be better to have a global option for all files, or
> > if "(set-face-attribute ... :strike-through" should accept more choices
> > to make each line configurable. (I personally would change all of my
> > striking lines, if possible.)

Patches to make the offset configurable via a Lisp variable are
welcome.  (I'm not sure the result will be prettier, except in some
very special cases, but if someone wants to shoot themselves in the
foot, why not?)

> I'm have no idea how to fix it; the font code is pretty opaque to me.

The code to handle this is quite simple: search for strike_through_p
in xterm.c.  (We have similar code in w32term.c and nsterm.m.)

> I mucked around a bit and came up with the below diff.

I found no diffs in your message.

bug#33648: 25.2; feature request: configurable position of strike-through line

[Eli Zaretskii]

> From: Stefan Kangas <stefan@marxist.se>
> Date: Mon, 31 Aug 2020 11:35:50 +0000
> Cc: 33648@debbugs.gnu.org
> 
> Actually, looking at this more, I'm starting to get convinced that
> LibreOffice is actually just positioning the line at the upper case
> glyph height divided by 3 instead of 2.
> 
> I had a look also at Firefox, and, if I'm not mistaken, they place the
> line at half the height of the lower-case glyph.

What happens/will happen if the same screen line has characters from
fonts of different height, all of them with strike-through? doesn't
the strike-through line break?

bug#33648: 25.2; feature request: configurable position of strike-through line

[Eli Zaretskii]

> From: Stefan Kangas <stefan@marxist.se>
> Date: Mon, 31 Aug 2020 11:35:50 +0000
> Cc: 33648@debbugs.gnu.org
> 
> Actually, looking at this more, I'm starting to get convinced that
> LibreOffice is actually just positioning the line at the upper case
> glyph height divided by 3 instead of 2.
> 
> I had a look also at Firefox, and, if I'm not mistaken, they place the
> line at half the height of the lower-case glyph.

What happens/will happen if the same screen line has characters from
fonts of different height, all of them with strike-through? doesn't
the strike-through line break in Firefox and the likes?

bug#33648: 25.2; feature request: configurable position of strike-through line

[Stefan Kangas]

[-- Attachment #1: Type: text/plain, Size: 389 bytes --]

Stefan Kangas <stefan@marxist.se> writes:

> 2. The strike-through line is hard coded to 1 pixel in our case, whereas
>    LibreOffice scales it up as the font size increases.

I've attached a suggested patch to change the strike-through line
depending on the glyph size.  The constant factor was found by
experimentation and produces reasonable results here.

Best regards,
Stefan Kangas

[-- Attachment #2: 0001-Make-strike-through-thicker-for-larger-font-sizes.patch --]
[-- Type: text/x-diff, Size: 2625 bytes --]

From 74b5091161890bc145034c5f2fd0c7e615b21d65 Mon Sep 17 00:00:00 2001
From: Stefan Kangas <stefankangas@gmail.com>
Date: Mon, 31 Aug 2020 13:39:51 +0200
Subject: [PATCH] Make strike-through thicker for larger font sizes

* src/nsterm.m (ns_draw_text_decoration):
* src/w32term.c (w32_draw_glyph_string):
* src/xterm.c (x_draw_glyph_string): Make strike-through thicker
for larger font sizes.  (Bug#33648)
* src/dispextern.h (STRIKE_THROUGH_WIDTH_FACTOR): New constant.
---
 src/dispextern.h | 4 ++++
 src/nsterm.m     | 2 +-
 src/w32term.c    | 2 +-
 src/xterm.c      | 2 +-
 4 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/src/dispextern.h b/src/dispextern.h
index 956ca96eb6..461526a7f8 100644
--- a/src/dispextern.h
+++ b/src/dispextern.h
@@ -656,6 +656,10 @@ #define GLYPH_INVALID_P(GLYPH) (GLYPH_CHAR (GLYPH) < 0)
 
 #define CHAR_GLYPH_PADDING_P(GLYPH) (GLYPH).padding_p
 
+/* Calculate the width of the strike-through line.  */
+
+#define STRIKE_THROUGH_WIDTH(x) ceilf (x / 15.0)
+
 
 
 \f
diff --git a/src/nsterm.m b/src/nsterm.m
index 26059ab67c..16575c066b 100644
--- a/src/nsterm.m
+++ b/src/nsterm.m
@@ -3706,7 +3706,7 @@ larger if there are taller display elements (e.g., characters
       int glyph_height = s->first_glyph->ascent + s->first_glyph->descent;
       /* Strike-through width and offset from the glyph string's
 	 top edge.  */
-      unsigned long h = 1;
+      unsigned long h = STRIKE_THROUGH_WIDTH (glyph_height);
       unsigned long dy;
 
       dy = lrint ((glyph_height - h) / 2);
diff --git a/src/w32term.c b/src/w32term.c
index 1766b32514..1b8d9b811b 100644
--- a/src/w32term.c
+++ b/src/w32term.c
@@ -2636,7 +2636,7 @@ w32_draw_glyph_string (struct glyph_string *s)
 	  int glyph_height = s->first_glyph->ascent + s->first_glyph->descent;
 	  /* Strike-through width and offset from the glyph string's
 	     top edge.  */
-          unsigned long h = 1;
+	  unsigned long h = STRIKE_THROUGH_WIDTH (glyph_height);
           unsigned long dy = (glyph_height - h) / 2;
 
           if (s->face->strike_through_color_defaulted_p)
diff --git a/src/xterm.c b/src/xterm.c
index 2e0407aff4..b17afdf225 100644
--- a/src/xterm.c
+++ b/src/xterm.c
@@ -3931,7 +3931,7 @@ x_draw_glyph_string (struct glyph_string *s)
 	  int glyph_height = s->first_glyph->ascent + s->first_glyph->descent;
 	  /* Strike-through width and offset from the glyph string's
 	     top edge.  */
-          unsigned long h = 1;
+	  unsigned long h = STRIKE_THROUGH_WIDTH (glyph_height);
           unsigned long dy = (glyph_height - h) / 2;
 
 	  if (s->face->strike_through_color_defaulted_p)
-- 
2.28.0

bug#33648: 25.2; feature request: configurable position of strike-through line

[Eli Zaretskii]

> From: Stefan Kangas <stefan@marxist.se>
> Date: Mon, 31 Aug 2020 17:42:43 +0000
> Cc: 33648@debbugs.gnu.org
> 
> I've attached a suggested patch to change the strike-through line
> depending on the glyph size.  The constant factor was found by
> experimentation and produces reasonable results here.

Which glyph, though?  Emacs draws glyphs in series, called "glyph
string".  All the glyphs in a glyph string share the same face (and
thus the same font), but that doesn't mean all the glyphs are of the
same height.  And here...

> --- a/src/xterm.c
> +++ b/src/xterm.c
> @@ -3931,7 +3931,7 @@ x_draw_glyph_string (struct glyph_string *s)
>  	  int glyph_height = s->first_glyph->ascent + s->first_glyph->descent;
>  	  /* Strike-through width and offset from the glyph string's
>  	     top edge.  */
> -          unsigned long h = 1;
> +	  unsigned long h = STRIKE_THROUGH_WIDTH (glyph_height);
>            unsigned long dy = (glyph_height - h) / 2;

..glyph_height is simply the height of the first glyph in the glyph
string.  Which could be anything, whereas you seem to assume that all
of the glyphs in a glyph string are of the same height.  IOW, you are
repeating the same "mistake" we do now, just with a different fraction
value.  Wanna bet that it will break with some fonts?

That is why I suggested a Lisp-exposed variable that could be set to
the offset in pixels of the strike-through line from the base.  (If
you want to be fancy, the Lisp variable could also be a float, in
which case it will be interpreted as a fraction of the first glyph's
height.)  This should be user-controlled, which will allow users (or
Lisp programs) to set that as they see fit, without requiring us to
second-guess up front what they would like.

Thanks.