diff --git a/OverLua/docs/overlua.txt b/OverLua/docs/overlua.txt
index fcf4b2df1..158d7530b 100644
--- a/OverLua/docs/overlua.txt
+++ b/OverLua/docs/overlua.txt
@@ -203,6 +203,7 @@ The following new registers are made available to the expression evaluator:
   R  In range 0..1, red component. Input and output.
   G  Ditto, for green component.
   B  Ditto, for blue component.
+  A  Ditto, for alpha component. (Only on ARGB32 surfaces.)
   X  X-coordinate of pixel being processed. Input only.
   Y  Y-coordinate of pixel being processed. Input only.
 
diff --git a/OverLua/raster_ops.cpp b/OverLua/raster_ops.cpp
index e340b890a..f78d44ea6 100644
--- a/OverLua/raster_ops.cpp
+++ b/OverLua/raster_ops.cpp
@@ -727,12 +727,14 @@ static int pixel_value_map(lua_State *L)
 				machine.registers[0] = (double)p.R() / 255.0;
 				machine.registers[1] = (double)p.G() / 255.0;
 				machine.registers[2] = (double)p.B() / 255.0;
-				machine.registers[3] = x;
-				machine.registers[4] = y;
+				machine.registers[3] = (double)p.A() / 255.0;
+				machine.registers[4] = x;
+				machine.registers[5] = y;
 				machine.Run();
 				p.R() = (uint8_t)(machine.registers[0] * 255);
 				p.G() = (uint8_t)(machine.registers[1] * 255);
 				p.B() = (uint8_t)(machine.registers[2] * 255);
+				p.A() = (uint8_t)(machine.registers[3] * 255);
 			}
 		}
 	}