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.

2 files changed, 25 insertions, 2 deletions

diff --git a/lisp/term/tty-colors.el b/lisp/term/tty-colors.el
index 252a4301295..9cfe30a4630 100644
--- a/lisp/term/tty-colors.el
+++ b/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.
diff --git a/lisp/term/xterm.el b/lisp/term/xterm.el
index 339d05d9728..e6d224dd3de 100644
--- a/lisp/term/xterm.el
+++ b/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))