[PATCH 1/1] Detect 24-bit TTY color support with standard Terminfo capabilites.

[Rami Ylimäki]

Latest Terminfo supports RGB capability flag for direct color mode
terminals. Foreground and background colors are set with setaf and
setab capability strings.

* src/term.c (init_tty): Use standard Terminfo capabilities to detect
  24-bit TTY color support.
* src/tparam.h: Define prototype for tigetflag.

* doc/misc/efaq.texi (Colors on a TTY): Fix instructions on how to
  enable direct color TTY mode.
---
 doc/misc/efaq.texi | 34 +++++++---------------------------
 src/term.c         | 26 +++++++++++++-------------
 src/tparam.h       |  3 ++-
 3 files changed, 22 insertions(+), 41 deletions(-)

diff --git a/doc/misc/efaq.texi b/doc/misc/efaq.texi
index 8014c2b71f..0ed58eda66 100644
--- a/doc/misc/efaq.texi
+++ b/doc/misc/efaq.texi
@@ -1491,37 +1491,17 @@ Colors on a TTY
 Syntax highlighting is on by default since version 22.1.
 
 Emacs 26.1 and later support direct color mode in terminals.  If Emacs
-finds Terminfo capabilities @samp{setb24} and @samp{setf24}, 24-bit
-direct color mode is used.  The capability strings are expected to
-take one 24-bit pixel value as argument and transform the pixel to a
-string that can be used to send 24-bit colors to the terminal.
-
-There aren't yet any standard terminal type definitions that would
-support the capabilities, but Emacs can be invoked with a custom
-definition as shown below.
+finds Terminfo capability @samp{RGB}, 24-bit direct color mode is
+used.
 
 @example
-$ cat terminfo-24bit.src
-
-# Use colon separators.
-xterm-24bit|xterm with 24-bit direct color mode,
-  use=xterm-256color,
-  
setb24=\E[48:2:address@hidden@}%/%d:address@hidden@}%/address@hidden@}%&%d:address@hidden@}%&%dm,
-  
setf24=\E[38:2:address@hidden@}%/%d:address@hidden@}%/address@hidden@}%&%d:address@hidden@}%&%dm,
-# Use semicolon separators.
-xterm-24bits|xterm with 24-bit direct color mode,
-  use=xterm-256color,
-  
setb24=\E[48;2;address@hidden@}%/%d;address@hidden@}%/address@hidden@}%&%d;address@hidden@}%&%dm,
-  
setf24=\E[38;2;address@hidden@}%/%d;address@hidden@}%/address@hidden@}%&%d;address@hidden@}%&%dm,
-
-$ tic -x -o ~/.terminfo terminfo-24bit.src
-
-$ TERM=xterm-24bit emacs -nw
+$ TERM=xterm-direct2 emacs -nw
 @end example
 
-Currently there's no standard way to determine whether a terminal
-supports direct color mode.  If such standard arises later on, support
-for @samp{setb24} and @samp{setf24} may be removed.
+Currently direct color mode support is quite fragmented and therefore
+Terminfo provides various terminal specifications.  You need to make
+sure that the @samp{setab} and @samp{setaf} capabilites of your TERM
+match your terminal.
 
 @node Debugging a customization file
 @section How do I debug a @file{.emacs} file?
diff --git a/src/term.c b/src/term.c
index b3707da70a..975ba956d7 100644
--- a/src/term.c
+++ b/src/term.c
@@ -4144,19 +4144,19 @@ use the Bourne shell command 'TERM=...; export TERM' 
(C-shell:\n\
       tty->TN_max_colors = tgetnum ("Co");
 
 #ifdef TERMINFO
-      /* Non-standard support for 24-bit colors. */
-      {
-       const char *fg = tigetstr ("setf24");
-       const char *bg = tigetstr ("setb24");
-       if (fg && bg
-           && fg != (char *) (intptr_t) -1
-           && bg != (char *) (intptr_t) -1)
-         {
-           tty->TS_set_foreground = fg;
-           tty->TS_set_background = bg;
-           tty->TN_max_colors = 16777216;
-         }
-      }
+      if (tigetflag ("RGB") > 0)
+        {
+          const char *fg = tigetstr ("setaf");
+          const char *bg = tigetstr ("setab");
+          if (fg && bg
+              && fg != (char *) (intptr_t) -1
+              && bg != (char *) (intptr_t) -1)
+            {
+              tty->TS_set_foreground = fg;
+              tty->TS_set_background = bg;
+              tty->TN_max_colors = 16777216;
+            }
+        }
 #endif
 
       tty->TN_no_color_video = tgetnum ("NC");
diff --git a/src/tparam.h b/src/tparam.h
index f8fb9e0869..3a3cb52c17 100644
--- a/src/tparam.h
+++ b/src/tparam.h
@@ -37,7 +37,8 @@ extern char *BC;
 extern char *UP;
 
 #ifdef TERMINFO
-char *tigetstr(const char *);
+int tigetflag (const char *);
+char *tigetstr (const char *);
 #endif
 
 #endif /* EMACS_TPARAM_H */
-- 
2.11.0