Grayscale image: Difference between revisions

Content added Content deleted

Inline

@@ Line 395: / Line 395: @@
 implement {}
 rgb24_to_gray8 rgb =
+  (* There is no need for floating point here, although equivalent
+     integer calculations are a bit longer to write out. *)
   let
-    extern castfn u8_to_dbl : uint8 -<> double
+    extern castfn i2u32 : int -<> uint32
-    extern castfn dbl_to_u8 : double -<> uint8
+    extern castfn u8_to_u32 : uint8 -<> uint32
+    extern castfn u32_to_u8 : uint32 -<> uint8
     val @(r, g, b) = rgb24_values rgb
-    val r : double = u8_to_dbl r
+    val r = u8_to_u32 r
-    and g : double = u8_to_dbl g
+    and g = u8_to_u32 g
-    and b : double = u8_to_dbl b
+    and b = u8_to_u32 b
-    val Y = (0.2126 * r) + (0.7152 * g) + (0.0722 * b)
+    val Y = (i2u32 2126 * r) + (i2u32 7152 * g) + (i2u32 722 * b)
-    val Yi = dbl_to_u8 ($extfcall (double, "rint", Y))
+    val Y1 = Y / i2u32 10000
+    and Y0 = Y mod (i2u32 10000)
   in
+    if Y0 < i2u32 5000 then
-    gray8_make Yi
+      gray8_make (u32_to_u8 Y1)
+    else if i2u32 5000 < Y0 then
+      gray8_make (succ (u32_to_u8 Y1))
+    else if Y0 mod (i2u32 2) = i2u32 0 then
+      gray8_make (u32_to_u8 Y1)
+    else
+      gray8_make (succ (u32_to_u8 Y1))
   end