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.

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

etc/NEWS

@@ -77,6 +77,12 @@ modern init systems such as systemd, which manage many of the traditional
 aspects of daemon behavior themselves.  '--old-daemon' is now an alias
 for '--daemon'.
 
++++
+** Emacs now supports 24-bit colors on capable text terminals
+Terminal is automatically initialized to use 24-bit colors if the
+required capabilities are found in terminfo.  See the FAQ node
+"Colors on a TTY" for more information.
+
 
 * Changes in Emacs 26.1
 

lisp/term/tty-colors.el

@@ -824,6 +824,15 @@ A canonicalized color name is all-lower case, with any blanks removed."
 	(replace-regexp-in-string " +" "" (downcase color))
       color)))
 
+(defun tty-color-24bit (rgb)
+  "Return pixel value on 24-bit terminals. Return nil if RGB is
+nil or not on 24-bit terminal."
+  (when (and rgb (= (display-color-cells) 16777216))
+    (let ((r (lsh (car rgb) -8))
+	  (g (lsh (cadr rgb) -8))
+	  (b (lsh (nth 2 rgb) -8)))
+      (logior (lsh r 16) (lsh g 8) b))))
+
 (defun tty-color-define (name index &optional rgb frame)
   "Specify a tty color by its NAME, terminal INDEX and RGB values.
 NAME is a string, INDEX is typically a small integer used to send to
@@ -840,7 +849,10 @@ If FRAME is not specified or is nil, it defaults to the selected frame."
 	  (and rgb (or (not (listp rgb)) (/= (length rgb) 3))))
       (error "Invalid specification for tty color \"%s\"" name))
   (tty-modify-color-alist
-   (append (list (tty-color-canonicalize name) index) rgb) frame))
+   (append (list (tty-color-canonicalize name)
+		 (or (tty-color-24bit rgb) index))
+	   rgb)
+   frame))
 
 (defun tty-color-clear (&optional _frame)
   "Clear the list of supported tty colors for frame FRAME.
@@ -1013,7 +1025,10 @@ might need to be approximated if it is not supported directly."
        (let ((color (tty-color-canonicalize color)))
 	  (or (assoc color (tty-color-alist frame))
 	      (let ((rgb (tty-color-standard-values color)))
-		(and rgb (tty-color-approximate rgb frame)))))))
+		(and rgb
+		     (let ((pixel (tty-color-24bit rgb)))
+		       (or (and pixel (cons color (cons pixel rgb)))
+			   (tty-color-approximate rgb frame)))))))))
 
 (defun tty-color-gray-shades (&optional display)
   "Return the number of gray colors supported by DISPLAY's terminal.

lisp/term/xterm.el

@@ -930,6 +930,14 @@ versions of xterm."
     ;; are more colors to support, compute them now.
     (when (> ncolors 0)
       (cond
+       ((= ncolors 16777200) ; 24-bit xterm
+	;; all named tty colors
+	(let ((idx (length xterm-standard-colors)))
+	  (mapc (lambda (color)
+		  (unless (assoc (car color) xterm-standard-colors)
+		    (tty-color-define (car color) idx (cdr color))
+		    (setq idx (1+ idx))))
+		color-name-rgb-alist)))
        ((= ncolors 240)	; 256-color xterm
 	;; 216 non-gray colors first
 	(let ((r 0) (g 0) (b 0))

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;

src/tparam.h

@@ -36,4 +36,8 @@ extern char PC;
 extern char *BC;
 extern char *UP;
 
+#ifdef TERMINFO
+char *tigetstr(const char *);
+#endif
+
 #endif /* EMACS_TPARAM_H */