Use the CSS convention for #RGB colors (bug#36304)

* src/xterm.c (x_parse_color): Change interpretation of #RGB color
triplets to match CSS rather than X conventions.

* lisp/term/tty-colors.el (tty-color-standard-values): Change
interpretation of #RGB color triplets to match CSS rather than X
conventions.  Allow upper-case digits.  Fix rgb:R/G/B
interpretation.

* doc/emacs/display.texi (Colors): Specify the convention used for
"#RGB" color triplets.

* test/lisp/tty-colors-tests.el: New file.

* etc/NEWS: Mention the change.

4 files changed, 88 insertions, 44 deletions

diff --git a/doc/emacs/display.texi b/doc/emacs/display.texi
index 0ce291335a9..8e842bea179 100644
--- a/doc/emacs/display.texi
+++ b/doc/emacs/display.texi
@@ -556,14 +556,14 @@ Font Lock mode.
 
 @node Colors
 @section Colors for Faces
-@cindex color name
-@cindex RGB triplet
 
   Faces can have various foreground and background colors.  When you
 specify a color for a face---for instance, when customizing the face
 (@pxref{Face Customization})---you can use either a @dfn{color name}
 or an @dfn{RGB triplet}.
 
+@subsection Color Names
+@cindex color name
 @findex list-colors-display
 @vindex list-colors-sort
   A color name is a pre-defined name, such as @samp{dark orange} or
@@ -578,12 +578,16 @@ such terminals.  However, Emacs understands X11 color names even on
 text terminals; if a face is given a color specified by an X11 color
 name, it is displayed using the closest-matching terminal color.
 
+@subsection RGB Triplets
+@cindex RGB triplet
   An RGB triplet is a string of the form @samp{#RRGGBB}.  Each of the
-R, G, and B components is a hexadecimal number specifying the
-component's relative intensity, one to four digits long (usually two
-digits are used).  The components must have the same number of digits.
-For hexadecimal values A to F, either upper or lower case are
-acceptable.
+primary color components is represented by a hexadecimal number
+between @samp{00} (intensity 0) and @samp{FF} (the maximum intensity).
+It is also possible to use one, three, or four hex digits for each
+component, so @samp{red} can be represented as @samp{#F00},
+@samp{#fff000000}, or @samp{#ffff00000000}.  The components must have
+the same number of digits.  For hexadecimal values A to F, either
+upper or lower case are acceptable.
 
   The @kbd{M-x list-colors-display} command also shows the equivalent
 RGB triplet for each named color.  For instance, @samp{medium sea
diff --git a/etc/NEWS b/etc/NEWS
index 021e84c9a43..c970a3c55d1 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -407,6 +407,12 @@ names in xref buffers.
 
 ** New variable `file-size-function' controls how file sizes are displayed.
 
++++
+** Emacs now interprets RGB triplets like HTML, SVG, and CSS do.
+
+The X convention previously used differed slightly, particularly for
+RGB triplets with a single hexadecimal digit per component.
+
 
 * Editing Changes in Emacs 27.1
 
@@ -1509,7 +1515,6 @@ automatically updates.  In the buffer, you can use 's q' or 's e' to
 signal a thread with quit or error respectively, or get a snapshot
 backtrace with 'b'.
 
-
 ** thingatpt.el
 
 ---
diff --git a/lisp/term/tty-colors.el b/lisp/term/tty-colors.el
index 5af8170203e..43c1071ceb7 100644
--- a/lisp/term/tty-colors.el
+++ b/lisp/term/tty-colors.el
@@ -919,57 +919,63 @@ FRAME defaults to the selected frame."
 The result is a list of integer RGB values--(RED GREEN BLUE).
 These values range from 0 to 65535; white is (65535 65535 65535).
 
-The returned value reflects the standard X definition of COLOR,
-regardless of whether the terminal can display it, so the return value
-should be the same regardless of what display is being used."
+The returned value reflects the standard Emacs definition of
+COLOR (see the info node `(emacs) Colors'), regardless of whether
+the terminal can display it, so the return value should be the
+same regardless of what display is being used."
   (let ((len (length color)))
-    (cond ((and (>= len 4) ;; X-style "#XXYYZZ" color spec
+    (cond ((and (>= len 4) ;; HTML/CSS/SVG-style "#XXYYZZ" color spec
                 (eq (aref color 0) ?#)
                 (member (aref color 1)
                         '(?0 ?1 ?2 ?3 ?4 ?5 ?6 ?7 ?8 ?9
-                             ?a ?b ?c ?d ?e ?f)))
-           ;; Translate the string "#XXYYZZ" into a list
-           ;; of numbers (XX YY ZZ).  If the primary colors
-           ;; are specified with less than 4 hex digits,
-           ;; the used digits represent the most significant
-           ;; bits of the value (e.g. #XYZ = #X000Y000Z000).
+                             ?a ?b ?c ?d ?e ?f
+                             ?A ?B ?C ?D ?E ?F)))
+           ;; Translate the string "#XXYYZZ" into a list of numbers
+           ;; (XX YY ZZ), scaling each to the {0..65535} range.  This
+           ;; follows the HTML color convention, where both "#fff" and
+           ;; "#ffffff" represent the same color, white.
            (let* ((ndig (/ (- len 1) 3))
+                  (maxval (1- (ash 1 (* 4 ndig))))
                   (i1 1)
                   (i2 (+ i1 ndig))
-                  (i3 (+ i2 ndig)))
+                  (i3 (+ i2 ndig))
+                  (i4 (+ i3 ndig)))
              (list
-              (ash
-               (string-to-number (substring color i1 i2) 16)
-               (* 4 (- 4 ndig)))
-              (ash
-               (string-to-number (substring color i2 i3) 16)
-               (* 4 (- 4 ndig)))
-              (ash
-               (string-to-number (substring color i3) 16)
-               (* 4 (- 4 ndig))))))
-          ((and (>= len 9) ;; X-style RGB:xx/yy/zz color spec
+              (/ (* (string-to-number
+                     (substring color i1 i2) 16)
+                    65535)
+                 maxval)
+              (/ (* (string-to-number
+                     (substring color i2 i3) 16)
+                    65535)
+                 maxval)
+              (/ (* (string-to-number
+                     (substring color i3 i4) 16)
+                    65535)
+                 maxval))))
+          ((and (>= len 9) ;; X-style rgb:xx/yy/zz color spec
                 (string= (substring color 0 4) "rgb:"))
-           ;; Translate the string "RGB:XX/YY/ZZ" into a list
-           ;; of numbers (XX YY ZZ).  If fewer than 4 hex
-           ;; digits are used, they represent the fraction
-           ;; of the maximum value (RGB:X/Y/Z = #XXXXYYYYZZZZ).
+           ;; Translate the string "rgb:XX/YY/ZZ" into a list of
+           ;; numbers (XX YY ZZ), scaling each to the {0..65535}
+           ;; range.  "rgb:F/F/F" is white.
            (let* ((ndig (/ (- len 3) 3))
                   (maxval (1- (ash 1 (* 4 (- ndig 1)))))
                   (i1 4)
                   (i2 (+ i1 ndig))
-                  (i3 (+ i2 ndig)))
+                  (i3 (+ i2 ndig))
+                  (i4 (+ i3 ndig)))
              (list
               (/ (* (string-to-number
                      (substring color i1 (- i2 1)) 16)
-                    255)
+                    65535)
                  maxval)
               (/ (* (string-to-number
                      (substring color i2 (- i3 1)) 16)
-                    255)
+                    65535)
                  maxval)
               (/ (* (string-to-number
-                     (substring color i3) 16)
-                    255)
+                     (substring color i3 (1- i4)) 16)
+                    65535)
                  maxval))))
           (t
            (cdr (assoc color color-name-rgb-alist))))))
@@ -977,9 +983,9 @@ should be the same regardless of what display is being used."
 (defun tty-color-translate (color &optional frame)
   "Given a color COLOR, return the index of the corresponding TTY color.
 
-COLOR must be a string that is either the color's name, or its X-style
-specification like \"#RRGGBB\" or \"RGB:rr/gg/bb\", where each primary.
-color can be given with 1 to 4 hex digits.
+COLOR must be a string that is either the color's name, or its
+color triplet specification like \"#RRGGBB\" or \"rgb:RR/GG/BB\",
+where each primary color can be given with 1 to 4 hex digits.
 
 If COLOR is a color name that is found among supported colors in
 `tty-color-alist', the associated index is returned.  Otherwise, the
@@ -987,7 +993,7 @@ RGB values of the color, either as given by the argument or from
 looking up the name in `color-name-rgb-alist', are used to find the
 supported color that is the best approximation for COLOR in the RGB
 space.
-If COLOR is neither a valid X RGB specification of the color, nor a
+If COLOR is neither a valid RGB specification of the color, nor a
 name of a color in `color-name-rgb-alist', the returned value is nil.
 
 If FRAME is unspecified or nil, it defaults to the selected frame."
diff --git a/src/xterm.c b/src/xterm.c
index c96aa74a7a6..75568a82a18 100644
--- a/src/xterm.c
+++ b/src/xterm.c
@@ -2381,6 +2381,8 @@ x_query_frame_background_color (struct frame *f, XColor *bgcolor)
   x_query_colors (f, bgcolor, 1);
 }
 
+#define HEX_COLOR_NAME_LENGTH 32
+
 /* On frame F, translate the color name to RGB values.  Use cached
    information, if possible.
 
@@ -2398,9 +2400,36 @@ Status x_parse_color (struct frame *f, const char *color_name,
 
   if (color_name[0] == '#')
     {
-      /* The hex form is parsed directly by XParseColor without
+      /* Don't pass #RGB strings directly to XParseColor, because that
+         follows the X convention of zero-extending each channel
+         value: #f00 means #f00000.  We want the convention of scaling
+         channel values, so #f00 means #ff0000, just as it does for
+         HTML, SVG, and CSS.
+
+         So we translate #f00 to rgb:f/0/0, which X handles
+         differently. */
+      char rgb_color_name[HEX_COLOR_NAME_LENGTH];
+      int len = strlen (color_name);
+      int digits_per_channel;
+      if (len == 4)
+        digits_per_channel = 1;
+      else if (len == 7)
+        digits_per_channel = 2;
+      else if (len == 10)
+        digits_per_channel = 3;
+      else if (len == 13)
+        digits_per_channel = 4;
+      else
+        return 0;
+
+      snprintf (rgb_color_name, sizeof rgb_color_name, "rgb:%.*s/%.*s/%.*s",
+                digits_per_channel, color_name + 1,
+                digits_per_channel, color_name + digits_per_channel + 1,
+                digits_per_channel, color_name + 2 * digits_per_channel + 1);
+
+      /* The rgb form is parsed directly by XParseColor without
          talking to the X server.  No need for caching.  */
-      return XParseColor (dpy, cmap, color_name, color);
+      return XParseColor (dpy, cmap, rgb_color_name, color);
     }
 
   for (cache_entry = FRAME_DISPLAY_INFO (f)->color_names; cache_entry;