[RFC] Add :invisible face attribute

[Michal Nazarewicz <mina86@mina86.com>]

Introduce :invisible face attribute which makes foreground to be the
same as background rendering the text invisible; or when :invert-video
is also in effect, background is the same as foreground.

Use it in Org mode for org-hide face eliminting the need for
org-find-invisible-foreground function.  This also simplifies
auto-dim-other-buffers NonGNU ELPA package removing the need to
configure a separate hide face for org-hide remap.

To observe the atribute in action, set ‘org-hide-leading-stars’ option,
open NEWS file and enable org-mode.  The initial stars in section
headings are rendered using the new attribute rather than Org mode
needing to explicitly match foregroun to background.

---
 lisp/cus-face.el      |  6 ++++
 lisp/faces.el         | 32 +++++++++++++++--
 lisp/org/org-faces.el |  4 +--
 lisp/org/org.el       | 18 ----------
 src/dispextern.h      |  1 +
 src/xfaces.c          | 80 +++++++++++++++++++++++++++++++++++++------
 6 files changed, 108 insertions(+), 33 deletions(-)

diff --git a/lisp/cus-face.el b/lisp/cus-face.el
index 478092c30cb..b7000747393 100644
--- a/lisp/cus-face.el
+++ b/lisp/cus-face.el
@@ -251,6 +251,12 @@ custom-face-attributes
 	     (const :tag "Off" nil)
 	     (const :tag "On" t)))
 
+    (:invisible
+     (choice :tag "Invisible"
+	     :help-echo "Control whether text should be rendered in the same color as background."
+	     (const :tag "Off" nil)
+	     (const :tag "On" t)))
+
     (:foreground
      (color :tag "Foreground"
 	    :help-echo "Set foreground color (name or #RRGGBB hex spec)."))
diff --git a/lisp/faces.el b/lisp/faces.el
index 05df685c679..6451664d791 100644
--- a/lisp/faces.el
+++ b/lisp/faces.el
@@ -375,6 +375,7 @@ face-x-resources
     (:underline (".attributeUnderline" . "Face.AttributeUnderline"))
     (:inverse-video (".attributeInverse" . "Face.AttributeInverse"))
     (:extend (".attributeExtend" . "Face.AttributeExtend"))
+    (:invisible (".attributeInvisible" . "Face.AttributeInvisible"))
     (:stipple
      (".attributeStipple" . "Face.AttributeStipple")
      (".attributeBackgroundPixmap" . "Face.AttributeBackgroundPixmap"))
@@ -615,6 +616,15 @@ face-inverse-video-p
  (eq (face-attribute face :inverse-video frame inherit) t))
 
 
+(defun face-invisible-video-p (face &optional frame inherit)
+ "Return non-nil if FACE specifies a non-nil invisible.
+If the optional argument FRAME is given, report on face FACE in that frame.
+If FRAME is t, report on the defaults for face FACE (for new frames).
+If FRAME is omitted or nil, use the selected frame.
+Optional argument INHERIT is passed to `face-attribute'."
+ (eq (face-attribute face :invisible frame inherit) t))
+
+
 (defun face-bold-p (face &optional frame inherit)
   "Return non-nil if the font of FACE is bold on FRAME.
 If the optional argument FRAME is given, report on face FACE in that frame.
@@ -822,6 +832,12 @@ set-face-attribute
 VALUE specifies whether characters in FACE should be displayed in
 inverse video.  VALUE must be one of t or nil.
 
+`:invisible'
+
+VALUE specifies whether characters in FACE should be displayed in
+foreground color the same as background color rendering them
+invisible.  VALUE must be one of t or nil.
+
 `:stipple'
 
 If VALUE is a string, it must be the name of a file of pixmap data.
@@ -1038,6 +1054,17 @@ set-face-inverse-video
 (define-obsolete-function-alias 'set-face-inverse-video-p
                                 'set-face-inverse-video "24.4")
 
+(defun set-face-invisible (face invisible-p &optional frame)
+  "Specify whether face FACE is in invisible.
+INVISIBLE-P non-nil means FACE displays explicitly in invisible.
+INVISIBLE-P nil means FACE explicitly is not in invisible.
+FRAME nil or not specified means change face on all frames.
+Use `set-face-attribute' to \"unspecify\" the invisible attribute."
+  (interactive
+   (let ((list (read-face-and-attribute :invisible)))
+     (list (car list) (if (cadr list) t))))
+  (set-face-attribute face frame :invisible invisible-p))
+
 (defun set-face-bold (face bold-p &optional frame)
   "Specify whether face FACE is bold.
 BOLD-P non-nil means FACE should explicitly display bold.
@@ -1215,7 +1242,7 @@ face-valid-attribute-values
 	   (:slant
             (mapcar (lambda (x) (cons (symbol-name (aref x 1)) (aref x 1)))
 		    font-slant-table))
-	   ((or :inverse-video :extend)
+	   ((or :inverse-video :extend :invisible)
             (mapcar (lambda (x) (cons (symbol-name x) x))
 		    (internal-lisp-face-attribute-values attribute)))
            ((or :underline :overline :strike-through :box)
@@ -1265,6 +1292,7 @@ face-attribute-name-alist
     (:strike-through . "strike-through")
     (:box . "box")
     (:inverse-video . "inverse-video display")
+    (:invisible . "invisible text")
     (:foreground . "foreground color")
     (:background . "background color")
     (:stipple . "background stipple")
@@ -1668,7 +1696,7 @@ face-spec-reset-face
 	     (append
 	      '(:underline nil :overline nil :strike-through nil
 		:box nil :inverse-video nil :stipple nil :inherit nil
-                :extend nil)
+                :extend nil :invisible nil)
 	      ;; `display-graphic-p' is unavailable when running
 	      ;; temacs, prior to loading frame.el.
 	      (when (fboundp 'display-graphic-p)
diff --git a/lisp/org/org-faces.el b/lisp/org/org-faces.el
index 21b23b641ca..ad6af62526d 100644
--- a/lisp/org/org-faces.el
+++ b/lisp/org/org-faces.el
@@ -40,9 +40,7 @@ org-default
   "Face used for default text."
   :group 'org-faces)
 
-(defface org-hide
-  '((((background light)) (:foreground "white"))
-    (((background dark)) (:foreground "black")))
+(defface org-hide '((t :invisible t))
   "Face used to hide leading stars in headlines.
 The foreground color of this face should be equal to the background
 color of the frame."
diff --git a/lisp/org/org.el b/lisp/org/org.el
index 4166738c162..782c2818c82 100644
--- a/lisp/org/org.el
+++ b/lisp/org/org.el
@@ -5100,10 +5100,6 @@ org-mode
   ;; Activate `org-table-header-line-mode'
   (when org-table-header-line-p
     (org-table-header-line-mode 1))
-  ;; Try to set `org-hide' face correctly.
-  (let ((foreground (org-find-invisible-foreground)))
-    (when foreground
-      (set-face-foreground 'org-hide foreground)))
   ;; Set face extension as requested.
   (org--set-faces-extend '(org-block-begin-line org-block-end-line)
                          org-fontify-whole-block-delimiter-line)
@@ -5138,20 +5134,6 @@ org-mode-transpose-word-syntax-table
   (abbrev-table-put org-mode-abbrev-table
 		    :parents (list text-mode-abbrev-table)))
 
-(defun org-find-invisible-foreground ()
-  (let ((candidates (remove
-		     "unspecified-bg"
-		     (nconc
-		      (list (face-background 'default)
-			    (face-background 'org-default))
-		      (mapcar
-		       (lambda (alist)
-			 (when (boundp alist)
-			   (cdr (assq 'background-color (symbol-value alist)))))
-		       '(default-frame-alist initial-frame-alist window-system-default-frame-alist))
-		      (list (face-foreground 'org-hide))))))
-    (car (remove nil candidates))))
-
 (defun org-current-time (&optional rounding-minutes past)
   "Current time, possibly rounded to ROUNDING-MINUTES.
 When ROUNDING-MINUTES is not an integer, fall back on the car of
diff --git a/src/dispextern.h b/src/dispextern.h
index 9df6eaf623a..2adb2686188 100644
--- a/src/dispextern.h
+++ b/src/dispextern.h
@@ -1700,6 +1700,7 @@ #define FONT_TOO_HIGH(ft)						\
   LFACE_FONTSET_INDEX,
   LFACE_DISTANT_FOREGROUND_INDEX,
   LFACE_EXTEND_INDEX,
+  LFACE_INVISIBLE_INDEX,
   LFACE_VECTOR_SIZE
 };
 
diff --git a/src/xfaces.c b/src/xfaces.c
index f6264802fa4..1b89bd4eb04 100644
--- a/src/xfaces.c
+++ b/src/xfaces.c
@@ -1378,6 +1378,7 @@ load_face_colors (struct frame *f, struct face *face,
 {
   Lisp_Object fg, bg, dfg;
   Emacs_Color xfg, xbg;
+  bool invisible;
 
   bg = attrs[LFACE_BACKGROUND_INDEX];
   fg = attrs[LFACE_FOREGROUND_INDEX];
@@ -1391,6 +1392,11 @@ load_face_colors (struct frame *f, struct face *face,
       bg = tmp;
     }
 
+  /* Set foreground to background if invisible.  */
+  invisible = EQ (attrs[LFACE_INVISIBLE_INDEX], Qt);
+  if (invisible)
+    fg = bg;
+
   /* Check for support for foreground, not for background because
      face_color_supported_p is smart enough to know that grays are
      "supported" as background because we are supposed to use stipple
@@ -1405,14 +1411,17 @@ load_face_colors (struct frame *f, struct face *face,
   face->background = load_color2 (f, face, bg, LFACE_BACKGROUND_INDEX, &xbg);
   face->foreground = load_color2 (f, face, fg, LFACE_FOREGROUND_INDEX, &xfg);
 
-  dfg = attrs[LFACE_DISTANT_FOREGROUND_INDEX];
-  if (!NILP (dfg) && !UNSPECIFIEDP (dfg)
-      && color_distance (&xbg, &xfg) < face_near_same_color_threshold)
+  if (!invisible)
     {
-      if (EQ (attrs[LFACE_INVERSE_INDEX], Qt))
-        face->background = load_color (f, face, dfg, LFACE_BACKGROUND_INDEX);
-      else
-        face->foreground = load_color (f, face, dfg, LFACE_FOREGROUND_INDEX);
+      dfg = attrs[LFACE_DISTANT_FOREGROUND_INDEX];
+      if (!NILP (dfg) && !UNSPECIFIEDP (dfg)
+          && color_distance (&xbg, &xfg) < face_near_same_color_threshold)
+        {
+          if (EQ (attrs[LFACE_INVERSE_INDEX], Qt))
+            face->background = load_color (f, face, dfg, LFACE_BACKGROUND_INDEX);
+          else
+            face->foreground = load_color (f, face, dfg, LFACE_FOREGROUND_INDEX);
+        }
     }
 }
 
@@ -1803,6 +1812,7 @@ #define LFACE_FONTSET(LFACE)	    AREF (LFACE, LFACE_FONTSET_INDEX)
 #define LFACE_EXTEND(LFACE)	    AREF (LFACE, LFACE_EXTEND_INDEX)
 #define LFACE_DISTANT_FOREGROUND(LFACE) \
   AREF (LFACE, LFACE_DISTANT_FOREGROUND_INDEX)
+#define LFACE_INVISIBLE(LFACE)	    AREF (LFACE, LFACE_INVISIBLE_INDEX)
 
 /* True if LFACE is a Lisp face.  A Lisp face is a vector of size
    LFACE_VECTOR_SIZE which has the symbol `face' in slot 0.  */
@@ -1912,6 +1922,10 @@ check_lface_attrs (Lisp_Object attrs[LFACE_VECTOR_SIZE])
 	   || RESET_P (attrs[LFACE_FONTSET_INDEX])
 	   || NILP (attrs[LFACE_FONTSET_INDEX]));
 #endif
+  eassert (UNSPECIFIEDP (attrs[LFACE_INVISIBLE_INDEX])
+	   || IGNORE_DEFFACE_P (attrs[LFACE_INVISIBLE_INDEX])
+	   || RESET_P (attrs[LFACE_INVISIBLE_INDEX])
+	   || SYMBOLP (attrs[LFACE_INVISIBLE_INDEX]));
 }
 
 
@@ -2911,6 +2925,13 @@ merge_face_ref (struct window *w,
 		  else
 		    err = true;
 		}
+	      else if (EQ (keyword, QCinvisible))
+		{
+		  if (EQ (value, Qt) || NILP (value))
+		    to[LFACE_INVISIBLE_INDEX] = value;
+		  else
+		    err = true;
+		}
 	      else
 		err = true;
 
@@ -3484,6 +3505,19 @@ DEFUN ("internal-set-lisp-face-attribute", Finternal_set_lisp_face_attribute,
       old_value = LFACE_EXTEND (lface);
       ASET (lface, LFACE_EXTEND_INDEX, value);
     }
+  else if (EQ (attr, QCinvisible))
+    {
+      if (!UNSPECIFIEDP (value)
+	  && !IGNORE_DEFFACE_P (value)
+	  && !RESET_P (value))
+	{
+	  CHECK_SYMBOL (value);
+	  if (!EQ (value, Qt) && !NILP (value))
+	    signal_error ("Invalid invisible face attribute value", value);
+	}
+      old_value = LFACE_INVISIBLE (lface);
+      ASET (lface, LFACE_INVISIBLE_INDEX, value);
+    }
   else if (EQ (attr, QCforeground))
     {
       HANDLE_INVALID_NIL_VALUE (QCforeground, face);
@@ -3977,7 +4011,8 @@ DEFUN ("internal-set-lisp-face-attribute-from-resource",
   else if (EQ (attr, QCweight) || EQ (attr, QCslant) || EQ (attr, QCwidth))
     value = intern (SSDATA (value));
   else if (EQ (attr, QCinverse_video)
-           || EQ (attr, QCextend))
+           || EQ (attr, QCextend)
+           || EQ (attr, QCinvisible))
     value = face_boolean_x_resource_value (value, true);
   else if (EQ (attr, QCunderline)
 	   || EQ (attr, QCoverline)
@@ -4203,6 +4238,8 @@ DEFUN ("internal-get-lisp-face-attribute", Finternal_get_lisp_face_attribute,
     value = LFACE_INHERIT (lface);
   else if (EQ (keyword, QCextend))
     value = LFACE_EXTEND (lface);
+  else if (EQ (keyword, QCinvisible))
+    value = LFACE_INVISIBLE (lface);
   else if (EQ (keyword, QCfont))
     value = LFACE_FONT (lface);
   else if (EQ (keyword, QCfontset))
@@ -4231,7 +4268,8 @@ DEFUN ("internal-lisp-face-attribute-values",
   if (EQ (attr, QCunderline) || EQ (attr, QCoverline)
       || EQ (attr, QCstrike_through)
       || EQ (attr, QCinverse_video)
-      || EQ (attr, QCextend))
+      || EQ (attr, QCextend)
+      || EQ (attr, QCinvisible))
     result = list2 (Qt, Qnil);
 
   return result;
@@ -5360,7 +5398,10 @@ gui_supports_face_attributes_p (struct frame *f,
 				def_attrs[LFACE_STRIKE_THROUGH_INDEX]))
       || (!UNSPECIFIEDP (lattrs[LFACE_BOX_INDEX])
 	  && face_attr_equal_p (lattrs[LFACE_BOX_INDEX],
-				def_attrs[LFACE_BOX_INDEX])))
+				def_attrs[LFACE_BOX_INDEX]))
+      || (!UNSPECIFIEDP (lattrs[LFACE_INVISIBLE_INDEX])
+	  && face_attr_equal_p (lattrs[LFACE_INVISIBLE_INDEX],
+				def_attrs[LFACE_INVISIBLE_INDEX])))
     return false;
 
   /* Check font-related attributes, as those are the most commonly
@@ -5549,6 +5590,17 @@ tty_supports_face_attributes_p (struct frame *f,
 	test_caps |= TTY_CAP_STRIKE_THROUGH;
     }
 
+  /* invisible video */
+  val = attrs[LFACE_INVERSE_INDEX];
+  if (!UNSPECIFIEDP (val))
+    {
+      if (face_attr_equal_p (val, def_attrs[LFACE_INVERSE_INDEX]))
+	return false;		/* same as default */
+      /* Always supported since it just copies bg to fg.  XXX: Is this
+	 true?  I actually have no idea what this function is supposed
+	 to do. */
+    }
+
   /* Color testing.  */
 
   /* Check if foreground color is close enough.  */
@@ -5968,6 +6020,9 @@ realize_default_face (struct frame *f)
   if (UNSPECIFIEDP (LFACE_INVERSE (lface)))
     ASET (lface, LFACE_INVERSE_INDEX, Qnil);
 
+  if (UNSPECIFIEDP (LFACE_INVISIBLE (lface)))
+    ASET (lface, LFACE_INVISIBLE_INDEX, Qnil);
+
   if (UNSPECIFIEDP (LFACE_FOREGROUND (lface)))
     {
       /* This function is called so early that colors are not yet
@@ -6716,6 +6771,9 @@ realize_tty_face (struct face_cache *cache,
       face->background = tem;
     }
 
+  if (!NILP (attrs[LFACE_INVISIBLE_INDEX]))
+    face->foreground = face->background;
+
   if (tty_suppress_bold_inverse_default_colors_p
       && face->tty_bold_p
       && face->background == FACE_TTY_DEFAULT_FG_COLOR
@@ -7342,6 +7400,7 @@ init_xfaces (void)
   face_attr_sym[LFACE_FONTSET_INDEX] = QCfontset;
   face_attr_sym[LFACE_DISTANT_FOREGROUND_INDEX] = QCdistant_foreground;
   face_attr_sym[LFACE_EXTEND_INDEX] = QCextend;
+  face_attr_sym[LFACE_INVISIBLE_INDEX] = QCinvisible;
 }
 
 void
@@ -7381,6 +7440,7 @@ syms_of_xfaces (void)
   DEFSYM (QCbox, ":box");
   DEFSYM (QCinherit, ":inherit");
   DEFSYM (QCextend, ":extend");
+  DEFSYM (QCinvisible, ":invisible");
 
   /* Symbols used for Lisp face attribute values.  */
   DEFSYM (QCcolor, ":color");
-- 
2.45.2