Add convex hull util.

CV.cabal

@@ -174,6 +174,7 @@ Library
                        ,Utils.Pointer                         
                        ,Utils.Rectangle
                        ,Utils.Stream
+                       ,Utils.ConvexHull
     Other-modules:     CV.Bindings.Matrix,
                        CV.Bindings.Calibrate,
                        CV.Bindings.Fittings,

Utils/ConvexHull.hs

@@ -0,0 +1,42 @@
+module Utils.ConvexHull
+    ( convexHull
+    ) where
+
+import Data.List
+import Data.Ord
+
+convexHull :: [(Double,Double)] -> [(Double,Double)]
+convexHull lst =
+    let frst = minPoint lst
+    in case frst of
+        Nothing -> []
+        Just f ->
+            let sorted = sortBy (comparing (heading f)) lst
+            in case sorted of
+                   (a:b:cs) -> grahamScan (b:a:f:[]) cs
+                   cs       -> f : cs
+  where
+    grahamScan [] _  = []
+    grahamScan ps [] = ps
+    grahamScan (x:[]) _ = [x]
+    grahamScan (p2:p1:ps) (x:xs) =
+        case turn p1 p2 x of
+            LeftTurn -> grahamScan (x:p2:p1:ps) xs
+            Straight -> grahamScan (x:p2:p1:ps) xs
+            _        -> grahamScan (p1:ps) (x:xs)
+
+heading :: (Double,Double) -> (Double,Double) -> Double
+heading (x1,y1) (x2,y2) = atan2 (y2-y1) (x2-x1)
+
+minPoint :: [(Double,Double)] -> Maybe (Double,Double)
+minPoint [] = Nothing
+minPoint xs = Just $ minimumBy (comparing snd) xs
+
+data Turn = LeftTurn | RightTurn | Straight deriving (Eq, Ord, Show, Read)
+
+turn :: (Double,Double) -> (Double,Double) -> (Double,Double) -> Turn
+turn a b c =
+    let h1 = heading a b
+        h2 = heading b c
+        d = h2 - h1
+    in if d >= 0 && d < pi then LeftTurn else RightTurn