[PATCH] TTY Support for ECMA-48 strike-through graphic rendition

[Mike Hamrick]

Hello,

In 1984, the third edition of ECMA-48 "Control Functions for Coded
Character Sets" was published. In section 8.3.117 under "Select Graphic
Rendition" a new character attribute for "crossed-out" was added.

As Thomas Dickey notes, many escape sequences in ECMA-48 were in fact
created by a committee, and did not exist in hardware terminals of the
time, but were rather a framework for proposed terminal implementations.

Some 30 years on, many terminal programs such as xterm, gnome-terminal,
termite, st, konsole, and others have implemented the ECMA-48
strike-through escape sequences, however there wasn't a terminfo(5)
capability defined for the purpose until a few years ago.

Here is my patch which adds Emacs TTY support for strike-through
rendering, which I personally use in org-mode for done TODO items. In
order for this to work for you, your terminfo database entry needs to have the
"smxx" capability defined, which I believe more recent versions[0] of the
xterm entry have.

[0]: https://invisible-island.net/xterm/terminfo.html

* term.c: Support strike-through in capable terminals.
(no_color_bit): Replace unused NC_INVIS with NC_STRIKE_THROUGH.
(turn_on_face): Output via TS_enter_strike_through_mode if available.
(turn_off_face): Handle strike-through case.
(tty_capable_p, init_tty): Support strike-through.

* termchar.h (struct tty_display_info): Add field for strike-through.

* xfaces.c (tty_supports_face_attributes_p, realize_tty_face):
Handle strike-through case.

* dispextern.h: Add TTY_CAP_STRIKE_THROUGH definition.
(struct face): Add field tty_strike_through_p.
---
 src/dispextern.h |  2 ++
 src/term.c       | 34 ++++++++++++++++++++--------------
 src/termchar.h   |  1 +
 src/xfaces.c     | 12 +++++++++++-
 4 files changed, 34 insertions(+), 15 deletions(-)

diff --git a/src/dispextern.h b/src/dispextern.h
index e1d6eddc41..082a345441 100644
--- a/src/dispextern.h
+++ b/src/dispextern.h
@@ -1744,6 +1744,7 @@ #define FONT_TOO_HIGH(ft)                                 
        \
   bool_bf tty_italic_p : 1;
   bool_bf tty_underline_p : 1;
   bool_bf tty_reverse_p : 1;
+  bool_bf tty_strike_through_p : 1;

   /* True means that colors of this face may not be freed because they
      have been copied bitwise from a base face (see
@@ -3290,6 +3291,7 @@ #define TTY_CAP_UNDERLINE 0x02
 #define TTY_CAP_BOLD           0x04
 #define TTY_CAP_DIM            0x08
 #define TTY_CAP_ITALIC         0x10
+#define TTY_CAP_STRIKE_THROUGH 0x20

 
 /***********************************************************************
diff --git a/src/term.c b/src/term.c
index 5cbb092ad1..bf5e339d2f 100644
--- a/src/term.c
+++ b/src/term.c
@@ -105,14 +105,14 @@ #define OUTPUT1_IF(tty, a) do { if (a) emacs_tputs 
((tty), a, 1, cmputc); } whil

 enum no_color_bit
 {
-  NC_STANDOUT   = 1 << 0,
-  NC_UNDERLINE  = 1 << 1,
-  NC_REVERSE    = 1 << 2,
-  NC_ITALIC     = 1 << 3,
-  NC_DIM        = 1 << 4,
-  NC_BOLD       = 1 << 5,
-  NC_INVIS      = 1 << 6,
-  NC_PROTECT    = 1 << 7
+  NC_STANDOUT           = 1 << 0,
+  NC_UNDERLINE          = 1 << 1,
+  NC_REVERSE            = 1 << 2,
+  NC_ITALIC             = 1 << 3,
+  NC_DIM                = 1 << 4,
+  NC_BOLD               = 1 << 5,
+  NC_STRIKE_THROUGH     = 1 << 6,
+  NC_PROTECT            = 1 << 7
 };

 /* internal state */
@@ -1931,6 +1931,9 @@ turn_on_face (struct frame *f, int face_id)
   if (face->tty_underline_p && MAY_USE_WITH_COLORS_P (tty, NC_UNDERLINE))
     OUTPUT1_IF (tty, tty->TS_enter_underline_mode);

+  if (face->tty_strike_through_p && MAY_USE_WITH_COLORS_P (tty, 
NC_STRIKE_THROUGH))
+    OUTPUT1_IF (tty, tty->TS_enter_strike_through_mode);
+
   if (tty->TN_max_colors > 0)
     {
       const char *ts;
@@ -1971,7 +1974,8 @@ turn_off_face (struct frame *f, int face_id)
       if (face->tty_bold_p
          || face->tty_italic_p
          || face->tty_reverse_p
-         || face->tty_underline_p)
+         || face->tty_underline_p
+          || face->tty_strike_through_p)
        {
          OUTPUT1_IF (tty, tty->TS_exit_attribute_mode);
          if (strcmp (tty->TS_exit_attribute_mode, tty->TS_end_standout_mode) 
== 0)
@@ -2006,11 +2010,12 @@ #define TTY_CAPABLE_P_TRY(tty, cap, TS, NC_bit)         
                \
   if ((caps & (cap)) && (!(TS) || !MAY_USE_WITH_COLORS_P(tty, NC_bit)))        
\
     return 0;

-  TTY_CAPABLE_P_TRY (tty, TTY_CAP_INVERSE,     tty->TS_standout_mode,          
NC_REVERSE);
-  TTY_CAPABLE_P_TRY (tty, TTY_CAP_UNDERLINE,   tty->TS_enter_underline_mode,   
NC_UNDERLINE);
-  TTY_CAPABLE_P_TRY (tty, TTY_CAP_BOLD,        tty->TS_enter_bold_mode,        
NC_BOLD);
-  TTY_CAPABLE_P_TRY (tty, TTY_CAP_DIM,                 tty->TS_enter_dim_mode, 
        NC_DIM);
-  TTY_CAPABLE_P_TRY (tty, TTY_CAP_ITALIC,      tty->TS_enter_italic_mode,      
NC_ITALIC);
+  TTY_CAPABLE_P_TRY (tty, TTY_CAP_INVERSE,       tty->TS_standout_mode,        
        NC_REVERSE);
+  TTY_CAPABLE_P_TRY (tty, TTY_CAP_UNDERLINE,     tty->TS_enter_underline_mode, 
        NC_UNDERLINE);
+  TTY_CAPABLE_P_TRY (tty, TTY_CAP_BOLD,          tty->TS_enter_bold_mode,      
        NC_BOLD);
+  TTY_CAPABLE_P_TRY (tty, TTY_CAP_DIM,                   
tty->TS_enter_dim_mode,               NC_DIM);
+  TTY_CAPABLE_P_TRY (tty, TTY_CAP_ITALIC,        tty->TS_enter_italic_mode,    
        NC_ITALIC);
+  TTY_CAPABLE_P_TRY (tty, TTY_CAP_STRIKE_THROUGH, 
tty->TS_enter_strike_through_mode,   NC_STRIKE_THROUGH);

   /* We can do it!  */
   return 1;
@@ -4124,6 +4129,7 @@ init_tty (const char *name, const char *terminal_type, 
bool must_succeed)
   tty->TS_enter_alt_charset_mode = tgetstr ("as", address);
   tty->TS_exit_alt_charset_mode = tgetstr ("ae", address);
   tty->TS_exit_attribute_mode = tgetstr ("me", address);
+  tty->TS_enter_strike_through_mode = tgetstr ("smxx", address);

   MultiUp (tty) = tgetstr ("UP", address);
   MultiDown (tty) = tgetstr ("DO", address);
diff --git a/src/termchar.h b/src/termchar.h
index c96b81913b..a8b3051767 100644
--- a/src/termchar.h
+++ b/src/termchar.h
@@ -136,6 +136,7 @@ #define EMACS_TERMCHAR_H
   const char *TS_enter_reverse_mode; /* "mr" -- enter reverse video mode.  */
   const char *TS_exit_underline_mode; /* "us" -- start underlining.  */
   const char *TS_enter_underline_mode; /* "ue" -- end underlining.  */
+  const char *TS_enter_strike_through_mode; /* "smxx" -- turn on 
strike-through mode.  */

   /* "as"/"ae" -- start/end alternate character set.  Not really
      supported, yet.  */
diff --git a/src/xfaces.c b/src/xfaces.c
index c4a4e1c94f..dc6ea5eacf 100644
--- a/src/xfaces.c
+++ b/src/xfaces.c
@@ -5207,7 +5207,6 @@ tty_supports_face_attributes_p (struct frame *f,
       || !UNSPECIFIEDP (attrs[LFACE_HEIGHT_INDEX])
       || !UNSPECIFIEDP (attrs[LFACE_SWIDTH_INDEX])
       || !UNSPECIFIEDP (attrs[LFACE_OVERLINE_INDEX])
-      || !UNSPECIFIEDP (attrs[LFACE_STRIKE_THROUGH_INDEX])
       || !UNSPECIFIEDP (attrs[LFACE_BOX_INDEX]))
     return false;

@@ -5272,6 +5271,15 @@ tty_supports_face_attributes_p (struct frame *f,
        test_caps |= TTY_CAP_INVERSE;
     }

+  /* strike through */
+  val = attrs[LFACE_STRIKE_THROUGH_INDEX];
+  if (!UNSPECIFIEDP (val))
+    {
+      if (face_attr_equal_p (val, def_attrs[LFACE_STRIKE_THROUGH_INDEX]))
+       return false;           /* same as default */
+      else
+       test_caps |= TTY_CAP_STRIKE_THROUGH;
+    }

   /* Color testing.  */

@@ -6234,6 +6242,8 @@ realize_tty_face (struct face_cache *cache,
     face->tty_underline_p = true;
   if (!NILP (attrs[LFACE_INVERSE_INDEX]))
     face->tty_reverse_p = true;
+  if (!NILP (attrs[LFACE_STRIKE_THROUGH_INDEX]))
+    face->tty_strike_through_p = true;

   /* Map color names to color indices.  */
   map_tty_color (f, face, LFACE_FOREGROUND_INDEX, &face_colors_defaulted);
--
2.24.0