Index: src/frame.c
===================================================================
RCS file: /sources/emacs/emacs/src/frame.c,v
retrieving revision 1.385
diff -u -r1.385 frame.c
--- src/frame.c	20 Jul 2008 00:47:28 -0000	1.385
+++ src/frame.c	22 Jul 2008 03:17:18 -0000
@@ -3688,7 +3688,7 @@
   for (i = 0; i < 2; i++)
     f->alpha[i] = newval[i];
 
-#if defined (HAVE_X_WINDOWS) || defined (HAVE_NTGUI)
+#if defined (HAVE_X_WINDOWS) || defined (HAVE_NTGUI) || defined (NS_IMPL_COCOA)
   BLOCK_INPUT;
   x_set_frame_alpha (f);
   UNBLOCK_INPUT;
Index: src/nsfns.m
===================================================================
RCS file: /sources/emacs/emacs/src/nsfns.m,v
retrieving revision 1.10
diff -u -r1.10 nsfns.m
--- src/nsfns.m	21 Jul 2008 20:27:57 -0000	1.10
+++ src/nsfns.m	22 Jul 2008 03:17:19 -0000
@@ -1040,7 +1040,8 @@
   x_set_fringe_width, /* generic OK */
   0, /* x_set_wait_for_wm, will ignore */
   0,  /* x_set_fullscreen will ignore */
-  x_set_font_backend /* generic OK */
+  x_set_font_backend, /* generic OK */
+  x_set_alpha
 };
 
 
@@ -1303,6 +1304,8 @@
                       RES_TYPE_BOOLEAN);
   x_default_parameter (f, parms, Qbuffered, Qt, "buffered", "Buffered",
                       RES_TYPE_BOOLEAN);
+  x_default_parameter (f, parms, Qalpha, Qnil,
+		       "alpha", "Alpha", RES_TYPE_NUMBER);
 
   width = FRAME_COLS (f);
   height = FRAME_LINES (f);
Index: src/nsterm.m
===================================================================
RCS file: /sources/emacs/emacs/src/nsterm.m,v
retrieving revision 1.11
diff -u -r1.11 nsterm.m
--- src/nsterm.m	21 Jul 2008 17:47:24 -0000	1.11
+++ src/nsterm.m	22 Jul 2008 03:17:20 -0000
@@ -970,11 +970,13 @@
         {
           /*ns_lower_frame (old_highlight); */
           x_update_cursor (old_highlight, 1);
+          x_set_frame_alpha (old_highlight);
         }
       if (dpyinfo->x_highlight_frame)
         {
           /*ns_raise_frame (dpyinfo->x_highlight_frame); */
           x_update_cursor (dpyinfo->x_highlight_frame, 1);
+          x_set_frame_alpha (dpyinfo->x_highlight_frame);
         }
     }
 }
@@ -1234,6 +1236,35 @@
   UNBLOCK_INPUT;
 }
 
+void
+x_set_frame_alpha (struct frame *f)
+/* --------------------------------------------------------------------------
+     Experimental: 
+   -------------------------------------------------------------------------- */
+{
+  NSTRACE (x_set_frame_alpha);
+  struct ns_display_info *dpyinfo = FRAME_NS_DISPLAY_INFO (f);
+  EmacsView *view = FRAME_NS_VIEW (f);
+  double alpha = 1.0;
+  double alpha_min = 1.0;
+
+  if (dpyinfo->x_highlight_frame == f)
+    alpha = f->alpha[0];
+  else
+    alpha = f->alpha[1]; // doesn't work yet
+
+  if (FLOATP (Vframe_alpha_lower_limit))
+    alpha_min = XFLOAT_DATA (Vframe_alpha_lower_limit);
+  else if (INTEGERP (Vframe_alpha_lower_limit))
+    alpha_min = (XINT (Vframe_alpha_lower_limit)) / 100.0;
+
+  if (alpha < 0.0 || 1.0 < alpha)
+    alpha = 1.0;
+  else if (0.0 <= alpha && alpha < alpha_min && alpha_min <= 1.0)
+    alpha = alpha_min;
+  
+  [[view window] setAlphaValue: alpha];
+}
 
 /* ==========================================================================
 
@@ -5128,6 +5159,7 @@
     {
       FRAME_NEW_CURSOR (emacsframe) = hollow_box;
       x_update_cursor (emacsframe, 1);
+      x_set_frame_alpha (emacsframe);
       FRAME_LAST_INACTIVE (emacsframe) = YES;
     }