diff --git a/lua.lyx b/lua.lyx
index d9906147..e2f52c56 100644
--- a/lua.lyx
+++ b/lua.lyx
@@ -1480,6 +1480,31 @@ Draw string with custom font to screen.
  
 \end_layout
 
+\begin_layout Subsection
+gui.adjust_transparency: Adjust transparency of DBITMAP or PALETTE
+\end_layout
+
+\begin_layout Itemize
+Syntax: none gui.adjust_transparency(DBITMAP obj, number adj)
+\end_layout
+
+\begin_layout Itemize
+Syntax: none gui.adjust_transparency(PALETTE obj, number adj)
+\end_layout
+
+\begin_layout Standard
+Multiply alpha channel of <obj> by <adj>/256.
+ Useful for making 
+\begin_inset Quotes eld
+\end_inset
+
+ghosts
+\begin_inset Quotes erd
+\end_inset
+
+ out of solid bitmaps.
+\end_layout
+
 \begin_layout Standard
 \begin_inset Newpage pagebreak
 \end_inset
diff --git a/lua.pdf b/lua.pdf
index e08e4349..78207120 100644
Binary files a/lua.pdf and b/lua.pdf differ
diff --git a/src/lua/gui-bitmap.cpp b/src/lua/gui-bitmap.cpp
index 13935822..3c0a33dd 100644
--- a/src/lua/gui-bitmap.cpp
+++ b/src/lua/gui-bitmap.cpp
@@ -492,6 +492,37 @@ namespace
 			messages << "Color #" << (i++) << ": " << c.orig << ":" << c.origa << std::endl;
 		return 0;
 	});
+
+	inline premultiplied_color tadjust(premultiplied_color c, uint16_t adj)
+	{
+		uint32_t rgb = c.orig;
+		uint32_t a = c.origa;
+		a = (a * adj) >> 8;
+		if(a > 256)
+			a = 256;
+		if(a == 0)
+			return premultiplied_color(-1);
+		else
+			return premultiplied_color(rgb | ((uint32_t)(256 - a) << 24));
+	}
+
+	function_ptr_luafun adjust_trans(LS, "gui.adjust_transparency", [](lua_state& L, const std::string& fname)
+		-> int {
+		uint16_t tadj = L.get_numeric_argument<uint16_t>(2, fname.c_str());
+		if(lua_class<lua_dbitmap>::is(L, 1)) {
+			lua_dbitmap* b = lua_class<lua_dbitmap>::get(L, 1, fname.c_str());
+			for(auto& c : b->pixels)
+				c = tadjust(c, tadj);
+		} else if(lua_class<lua_palette>::is(L, 1)) {
+			lua_palette* p = lua_class<lua_palette>::get(L, 1, fname.c_str());
+			for(auto& c : p->colors)
+				c = tadjust(c, tadj);
+		} else {
+			throw std::runtime_error("Expected BITMAP or PALETTE as argument 1 for "
+				"gui.adjust_transparency");
+		}
+		return 2;
+	});
 }
 
 DECLARE_LUACLASS(lua_palette, "PALETTE");