Median filter

The median filter takes in the neighbourhood the median color.

(to test the function below, you can use these input and output solutions)

OCaml

<ocaml>let color_add (r1,g1,b1) (r2,g2,b2) =

 ( (r1 + r2),
   (g1 + g2),
   (b1 + b2) )

let color_div (r,g,b) d =

 ( (r / d),
   (g / d),
   (b / d) )

let compare_as_grayscale (r1,g1,b1) (r2,g2,b2) =

 let v1 = (2_126 * r1 +  7_152 * g1 + 722 * b1)
 and v2 = (2_126 * r2 +  7_152 * g2 + 722 * b2) in
 (Pervasives.compare v1 v2)

let get_rgb img x y =

 let _, r_channel,_,_ = img in
 let width = Bigarray.Array2.dim1 r_channel
 and height = Bigarray.Array2.dim2 r_channel in
 if (x < 0) || (x >= width) then (0,0,0) else
 if (y < 0) || (y >= height) then (0,0,0) else  (* feed borders with black *)
 (get_pixel img x y)

let median_value img radius =

 let samples = (radius*2+1) * (radius*2+1) in
 fun x y ->
   let sample = ref [] in

   for _x = (x - radius) to (x + radius) do
     for _y = (y - radius) to (y + radius) do
     
       let v = get_rgb img _x _y in

       sample := v :: !sample;
     done;
   done;

   let ssample = List.sort compare_as_grayscale !sample in
   let mid = (samples / 2) in

   if (samples mod 2) = 1
   then List.nth ssample (mid+1)
   else
     let median1 = List.nth ssample (mid)
     and median2 = List.nth ssample (mid+1) in
     (color_div (color_add median1 median2) 2)

let median img radius =

 let _, r_channel,_,_ = img in
 let width = Bigarray.Array2.dim1 r_channel
 and height = Bigarray.Array2.dim2 r_channel in

 let _median_value = median_value img radius in

 let res = new_img ~width ~height in
 for y = 0 to pred height do
   for x = 0 to pred width do
     let color = _median_value x y in
     put_pixel res color x y;
   done;
 done;
 (res)</ocaml>

an alternate version of the function median_value using arrays instead of lists: <ocaml>let median_value img radius =

 let samples = (radius*2+1) * (radius*2+1) in
 let sample = Array.make samples (0,0,0) in
 fun x y ->
   let i = ref 0 in
   for _x = (x - radius) to (x + radius) do
     for _y = (y - radius) to (y + radius) do
       let v = get_rgb img _x _y in
       sample.(!i) <- v;
       incr i;
     done;
   done;

   Array.sort compare_as_grayscale sample;
   let mid = (samples / 2) in

   if (samples mod 2) = 1
   then sample.(mid+1)
   else (color_div (color_add sample.(mid)
                              sample.(mid+1)) 2)</ocaml>