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, 33 insertions, 0 deletions

diff --git a/doc/misc/efaq.texi b/doc/misc/efaq.texi
index f7a47f8675a..e9cfe7afce9 100644
--- a/doc/misc/efaq.texi
+++ b/doc/misc/efaq.texi
@@ -1491,6 +1491,39 @@ exhibits all the colors Emacs knows about on the current display.
 
 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.
+
+@example
+$ cat terminfo-24bit.src
+
+# Use colon separators.
+xterm-24bit|xterm with 24-bit direct color mode,
+  use=xterm-256color,
+  setb24=\E[48:2:%p1%@{65536@}%/%d:%p1%@{256@}%/%@{255@}%&%d:%p1%@{255@}%&%dm,
+  setf24=\E[38:2:%p1%@{65536@}%/%d:%p1%@{256@}%/%@{255@}%&%d:%p1%@{255@}%&%dm,
+# Use semicolon separators.
+xterm-24bits|xterm with 24-bit direct color mode,
+  use=xterm-256color,
+  setb24=\E[48;2;%p1%@{65536@}%/%d;%p1%@{256@}%/%@{255@}%&%d;%p1%@{255@}%&%dm,
+  setf24=\E[38;2;%p1%@{65536@}%/%d;%p1%@{256@}%/%@{255@}%&%d;%p1%@{255@}%&%dm,
+
+$ tic -x -o ~/.terminfo terminfo-24bit.src
+
+$ TERM=xterm-24bit 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.
+
 @node Debugging a customization file
 @section How do I debug a @file{.emacs} file?
 @cindex Debugging @file{.emacs} file