Support 24-bit direct colors on text terminals

* src/term.c (init_tty): Use 24-bit terminal colors if corresponding
foreground and background functions are present in terminal type
definition.
* src/tparam.h: Define prototype for tigetstr.

* lisp/term/tty-colors.el (tty-color-define): Convert color palette
index to pixel value on 16.7M color terminals.
(tty-color-24bit): New function to convert color palette index to
pixel value on 16.7M color terminals.
(tty-color-desc): Don't approximate colors on 16.7M color terminals.
* lisp/term/xterm.el (xterm-register-default-colors): Define all named
TTY colors on 16.7M color terminals.

* doc/misc/efaq.texi (Colors on a TTY): Add instructions on how to
enable direct color TTY mode.
* etc/NEWS: Mention direct color TTY mode and point to FAQ.

1 files changed, 14 insertions, 0 deletions

diff --git a/src/term.c b/src/term.c
index b0ff9cb546d..35fa8c931c1 100644
--- a/src/term.c
+++ b/src/term.c
@@ -4131,6 +4131,20 @@ 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 *)-1 && bg != (char *)-1)
+          {
+            tty->TS_set_foreground = fg;
+            tty->TS_set_background = bg;
+            tty->TN_max_colors = 16777216;
+          }
+      }
+#endif
+
       tty->TN_no_color_video = tgetnum ("NC");
       if (tty->TN_no_color_video == -1)
         tty->TN_no_color_video = 0;