Re: Support for background transparency

["Håkon Flatval" <hkon20@hotmail.com>]

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Hi again - sorry for the long radio silence.

>Posting the resulting patch (or a series of patches, depending on how
>much code it is) is the preferred method to get code reviews.

I've attached a patch that implements basic support for background transparency. I have only considered GDK+Cairo build configurations here, as a start.

I believe the gui_set_alpha_background​ function in frame.c​ should return some error message when the frame does not support 32 bits of depth, but am not sure how to do this best. I considered adding a new error code similar to the one used by wrong_type_argument​ (also called from gui_set_...​), but it seemed to be a bit excessive to create a  specific "frame-does-not-support-adequate-bit-depth"-error type when most other error types are of the more general forms "value-out-of-range" or "wrong-length-argument". Does anyone have suggestions on how to best communicate failure in setting alpha to the user?

Newbie question: I can't guarantee that this patch works perfectly with all build variations using GDK+Cairo, nor in all (desktop) environments where Emacs might be used. How, in general, is Emacs tested before release to ensure (most) such edge cases are caught?

I'm currently working on implementing this feature for other XLib-based build configurations too, offline, planning to submit another patch for those in the not-so-distant future.

Best,
Håkon Flatval

________________________________
Fra: Lars Ingebrigtsen <larsi@gnus.org>
Sendt: torsdag 4. november 2021 21:17
Til: Håkon Flatval <hkon20@hotmail.com>
Kopi: emacs-devel@gnu.org <emacs-devel@gnu.org>
Emne: Re: Support for background transparency

Håkon Flatval <hkon20@hotmail.com> writes:

> I have implemented partial support for background transparency in Emacs.

Great!  It's a feature that has been requested many times.

> As I'm completely new to the community, I'd also like some input as to how
> one proceeds with planning and implementation given there is interest for
> this to be made reality. I would be happy to work on this, but not sure I feel
> comfortable / knowledgable enough to do everything on my own.

Posting the resulting patch (or a series of patches, depending on how
much code it is) is the preferred method to get code reviews.

--
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

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diff --git a/src/frame.c b/src/frame.c
index 33e9606e41..220e061b1e 100644
--- a/src/frame.c
+++ b/src/frame.c
@@ -3898,6 +3898,7 @@ DEFUN ("frame-scale-factor", Fframe_scale_factor, Sframe_scale_factor,
   {"z-group",			SYMBOL_INDEX (Qz_group)},
   {"override-redirect",		SYMBOL_INDEX (Qoverride_redirect)},
   {"no-special-glyphs",		SYMBOL_INDEX (Qno_special_glyphs)},
+  {"alpha-background",          SYMBOL_INDEX (Qalpha_background)},
 #ifdef NS_IMPL_COCOA
   {"ns-appearance",		SYMBOL_INDEX (Qns_appearance)},
   {"ns-transparent-titlebar",	SYMBOL_INDEX (Qns_transparent_titlebar)},
@@ -5015,6 +5016,43 @@ gui_set_alpha (struct frame *f, Lisp_Object arg, Lisp_Object oldval)
     }
 }
 
+void
+gui_set_alpha_background (struct frame *f, Lisp_Object arg, Lisp_Object oldval)
+{
+  double alpha = 1.0;
+
+  if (NILP (arg))
+    alpha = 1.0;
+  else if (FLOATP (arg))
+    {
+      alpha = XFLOAT_DATA(arg);
+      if (! (0 <= alpha && alpha <= 1.0))
+	args_out_of_range (make_float(0.0), make_float(1.0));
+    }
+  else if (FIXNUMP (arg))
+    {
+      EMACS_INT ialpha = XFIXNUM (arg);
+      if (! (0 <= ialpha && ialpha <= 100))
+	args_out_of_range (make_fixnum (0), make_fixnum (100));
+      alpha = ialpha / 100.0;
+    }
+  else
+    wrong_type_argument (Qnumberp, arg);
+
+  if (alpha != 1.0)
+    {
+      // Abort if frame does not support 32-bit color
+      if (FRAME_TERMINAL (f)->get_frame_bit_depth == NULL ||
+	  FRAME_TERMINAL (f)->get_frame_bit_depth (f) != 32)
+	{
+	  return;
+	}
+    }
+
+  f->alpha_background = alpha;
+
+  SET_FRAME_GARBAGED (f);
+}
 
 /**
  * gui_set_no_special_glyphs:
@@ -6083,6 +6121,7 @@ syms_of_frame (void)
 #endif
 
   DEFSYM (Qalpha, "alpha");
+  DEFSYM (Qalpha_background, "alpha-background");
   DEFSYM (Qauto_lower, "auto-lower");
   DEFSYM (Qauto_raise, "auto-raise");
   DEFSYM (Qborder_color, "border-color");
diff --git a/src/frame.h b/src/frame.h
index cb2bad71c5..e45e0714a1 100644
--- a/src/frame.h
+++ b/src/frame.h
@@ -636,6 +636,9 @@ #define EMACS_FRAME_H
      Negative values mean not to change alpha.  */
   double alpha[2];
 
+  /* Background opacity */
+  double alpha_background;
+
   /* Exponent for gamma correction of colors.  1/(VIEWING_GAMMA *
      SCREEN_GAMMA) where viewing_gamma is 0.4545 and SCREEN_GAMMA is a
      frame parameter.  0 means don't do gamma correction.  */
@@ -1658,6 +1661,7 @@ #define EMACS_CLASS "Emacs"
 extern long gui_figure_window_size (struct frame *, Lisp_Object, bool, bool);
 
 extern void gui_set_alpha (struct frame *, Lisp_Object, Lisp_Object);
+extern void gui_set_alpha_background (struct frame *, Lisp_Object, Lisp_Object);
 extern void gui_set_no_special_glyphs (struct frame *, Lisp_Object, Lisp_Object);
 
 extern void validate_x_resource_name (void);
diff --git a/src/gtkutil.c b/src/gtkutil.c
index 9e676cd025..bfafc0481d 100644
--- a/src/gtkutil.c
+++ b/src/gtkutil.c
@@ -1232,6 +1232,10 @@ xg_create_frame_widgets (struct frame *f)
   else
     wtop = gtk_window_new (GTK_WINDOW_TOPLEVEL);
 
+  /* This prevents GTK from painting the window's background, which
+     would interfere with transparent background in some environments */
+  gtk_widget_set_app_paintable(wtop, TRUE);
+
   /* gtk_window_set_has_resize_grip is a Gtk+ 3.0 function but Ubuntu
      has backported it to Gtk+ 2.0 and they add the resize grip for
      Gtk+ 2.0 applications also.  But it has a bug that makes Emacs loop
@@ -1339,6 +1343,16 @@ xg_create_frame_widgets (struct frame *f)
                          | GDK_STRUCTURE_MASK
                          | GDK_VISIBILITY_NOTIFY_MASK);
 
+  GdkScreen *screen = gtk_widget_get_screen (wtop);
+
+  if (FRAME_TERMINAL (f)->get_frame_bit_depth != NULL &&
+      FRAME_TERMINAL (f)->get_frame_bit_depth (f) == 32)
+    {
+      GdkVisual *visual = gdk_screen_get_rgba_visual (screen);
+      gtk_widget_set_visual(wtop, visual);
+      gtk_widget_set_visual(wfixed, visual);
+    }
+
   /* Must realize the windows so the X window gets created.  It is used
      by callers of this function.  */
   gtk_widget_realize (wfixed);
@@ -1379,7 +1393,6 @@ xg_create_frame_widgets (struct frame *f)
   g_signal_connect (wtop, "query-tooltip", G_CALLBACK (qttip_cb), f);
 
   {
-    GdkScreen *screen = gtk_widget_get_screen (wtop);
     GtkSettings *gs = gtk_settings_get_for_screen (screen);
     /* Only connect this signal once per screen.  */
     if (! g_signal_handler_find (G_OBJECT (gs),
diff --git a/src/termhooks.h b/src/termhooks.h
index 1cf9863f3a..b8572aac46 100644
--- a/src/termhooks.h
+++ b/src/termhooks.h
@@ -748,11 +748,13 @@ #define EVENT_INIT(event) memset (&(event), 0, sizeof (struct input_event))
                                             const char *name,
                                             const char *class);
 \f
-  /* Image hooks */
+  /* Window hooks */
 #ifdef HAVE_WINDOW_SYSTEM
   /* Free the pixmap PIXMAP on F.  */
   void (*free_pixmap) (struct frame *f, Emacs_Pixmap pixmap);
 
+  /* Return bit depth supported by the terminal */
+  unsigned int (*get_frame_bit_depth) (struct frame *f);
 #endif
 \f
   /* Deletion hooks */
diff --git a/src/xfns.c b/src/xfns.c
index 5eff9f5b0f..84b47d370a 100644
--- a/src/xfns.c
+++ b/src/xfns.c
@@ -4118,6 +4118,8 @@ DEFUN ("x-create-frame", Fx_create_frame, Sx_create_frame,
                          RES_TYPE_NUMBER);
   gui_default_parameter (f, parms, Qalpha, Qnil,
                          "alpha", "Alpha", RES_TYPE_NUMBER);
+  gui_default_parameter (f, parms, Qalpha_background, Qnil,
+			 "alpha_background", "AlphaBackground", RES_TYPE_NUMBER);
 
   if (!NILP (parent_frame))
     {
@@ -5731,17 +5733,33 @@ select_visual (struct x_display_info *dpyinfo)
     }
   else
     {
+      // First attempt to use 32-bit visual if available
       int n_visuals;
       XVisualInfo *vinfo, vinfo_template;
 
-      dpyinfo->visual = DefaultVisualOfScreen (screen);
+      vinfo_template.screen = XScreenNumberOfScreen (screen);
+      vinfo_template.depth = 32;
+
+      vinfo = XGetVisualInfo (dpy, VisualScreenMask | VisualDepthMask,
+			      &vinfo_template, &n_visuals);
 
+      if (n_visuals > 0)
+	{
+	  dpyinfo->n_planes = vinfo->depth;
+	  dpyinfo->visual = vinfo->visual;
+	  XFree (vinfo);
+	  return;
+	}
+
+      // 32-bit visual not available, fallback to default visual
+      dpyinfo->visual = DefaultVisualOfScreen (screen);
       vinfo_template.visualid = XVisualIDFromVisual (dpyinfo->visual);
-      vinfo_template.screen = XScreenNumberOfScreen (screen);
+
       vinfo = XGetVisualInfo (dpy, VisualIDMask | VisualScreenMask,
 			      &vinfo_template, &n_visuals);
+
       if (n_visuals <= 0)
-	fatal ("Can't get proper X visual info");
+	  fatal ("Can't get proper X visual info");
 
       dpyinfo->n_planes = vinfo->depth;
       XFree (vinfo);
@@ -6549,6 +6567,8 @@ x_create_tip_frame (struct x_display_info *dpyinfo, Lisp_Object parms)
                          "cursorType", "CursorType", RES_TYPE_SYMBOL);
   gui_default_parameter (f, parms, Qalpha, Qnil,
                          "alpha", "Alpha", RES_TYPE_NUMBER);
+  gui_default_parameter (f, parms, Qalpha_background, Qnil,
+			 "alpha_background", "AlphaBackground", RES_TYPE_NUMBER);
 
   /* Add `tooltip' frame parameter's default value. */
   if (NILP (Fframe_parameter (frame, Qtooltip)))
@@ -7878,6 +7898,7 @@ DEFUN ("x-gtk-debug", Fx_gtk_debug, Sx_gtk_debug, 1, 1, 0,
   x_set_z_group,
   x_set_override_redirect,
   gui_set_no_special_glyphs,
+  gui_set_alpha_background,
 };
 
 void
diff --git a/src/xterm.c b/src/xterm.c
index 8045470bdd..615c373cdc 100644
--- a/src/xterm.c
+++ b/src/xterm.c
@@ -684,9 +684,12 @@ x_set_cr_source_with_gc_background (struct frame *f, GC gc)
 
   XGetGCValues (FRAME_X_DISPLAY (f), gc, GCBackground, &xgcv);
   color.pixel = xgcv.background;
+
   x_query_colors (f, &color, 1);
-  cairo_set_source_rgb (FRAME_CR_CONTEXT (f), color.red / 65535.0,
-			color.green / 65535.0, color.blue / 65535.0);
+  cairo_set_source_rgba (FRAME_CR_CONTEXT (f), color.red / 65535.0,
+                         color.green / 65535.0, color.blue / 65535.0, f->alpha_background);
+
+  cairo_set_operator (FRAME_CR_CONTEXT (f), CAIRO_OPERATOR_SOURCE);
 }
 
 static const cairo_user_data_key_t xlib_surface_key, saved_drawable_key;
@@ -1096,6 +1099,29 @@ x_fill_rectangle (struct frame *f, GC gc, int x, int y, int width, int height)
 #endif
 }
 
+
+static void
+x_clear_rectangle (struct frame *f, GC gc, int x, int y, int width, int height)
+{
+  XGCValues xgcv;
+  Display *dpy = FRAME_X_DISPLAY (f);
+#ifdef USE_CAIRO
+  cairo_t *cr;
+
+  cr = x_begin_cr_clip (f, gc);
+  x_set_cr_source_with_gc_background (f, gc);
+  cairo_rectangle (cr, x, y, width, height);
+  cairo_fill (cr);
+  x_end_cr_clip (f);
+#else
+  XGetGCValues (dpy, gc, GCBackground | GCForeground, &xgcv);
+  XSetForeground (dpy, gc, argb_from_rgb (xgcv.background, f->alpha_background));
+  XFillRectangle (dpy, FRAME_X_DRAWABLE (f),
+		  gc, x, y, width, height);
+  XSetForeground (dpy, gc, xgcv.foreground);
+#endif
+}
+
 static void
 x_draw_rectangle (struct frame *f, GC gc, int x, int y, int width, int height)
 {
@@ -1975,12 +2001,7 @@ x_compute_glyph_string_overhangs (struct glyph_string *s)
 static void
 x_clear_glyph_string_rect (struct glyph_string *s, int x, int y, int w, int h)
 {
-  Display *display = FRAME_X_DISPLAY (s->f);
-  XGCValues xgcv;
-  XGetGCValues (display, s->gc, GCForeground | GCBackground, &xgcv);
-  XSetForeground (display, s->gc, xgcv.background);
-  x_fill_rectangle (s->f, s->gc, x, y, w, h);
-  XSetForeground (display, s->gc, xgcv.foreground);
+  x_clear_rectangle (s->f, s->gc, x, y, w, h);
 }
 
 
@@ -14737,6 +14758,32 @@ x_delete_display (struct x_display_info *dpyinfo)
   xfree (dpyinfo);
 }
 
+/* Returns the number of bits supported by this frame's visual */
+static unsigned int
+x_get_frame_bit_depth (struct frame *f)
+{
+  Visual *visual = FRAME_X_VISUAL (f);
+  VisualID visual_id = XVisualIDFromVisual (visual);
+
+  XVisualInfo vinfo_template;
+  vinfo_template.visualid = visual_id;
+
+  long vinfo_mask = VisualIDMask;
+  int num_items;
+
+  XVisualInfo *visual_infos = XGetVisualInfo (FRAME_X_DISPLAY (f),
+					 vinfo_mask,
+					 &vinfo_template,
+					 &num_items);
+
+  if (num_items == 0) {
+    // Assume bit depth of 24 bits is always supported
+    return 24;
+  }
+
+  return visual_infos[0].depth;
+}
+
 #ifdef USE_X_TOOLKIT
 
 /* Atimer callback function for TIMER.  Called every 0.1s to process
@@ -14978,6 +15025,7 @@ x_create_terminal (struct x_display_info *dpyinfo)
   terminal->free_pixmap = x_free_pixmap;
   terminal->delete_frame_hook = x_destroy_window;
   terminal->delete_terminal_hook = x_delete_terminal;
+  terminal->get_frame_bit_depth = x_get_frame_bit_depth;
   /* Other hooks are NULL by default.  */
 
   return terminal;