Function for drawing arrows

Porting the [PR 2970](https://github.com/Itseez/opencv/pull/2970) To the new InputOutputArray type

modules/core/doc/drawing_functions.rst

@@ -361,6 +361,37 @@ The function ``line`` draws the line segment between ``pt1`` and ``pt2`` points
 Antialiased lines are drawn using Gaussian filtering.
 
 
+arrowedLine
+----------------
+Draws a arrow segment pointing from the first point to the second one.
+
+.. ocv:function:: void arrowedLine(InputOutputArray img, Point pt1, Point pt2, const Scalar& color,          int thickness=1, int lineType=8, int shift=0, double tipLength=0.1)
+
+    :param img: Image.
+
+    :param pt1: The point the arrow starts from.
+
+    :param pt2: The point the arrow points to.
+
+    :param color: Line color.
+
+    :param thickness: Line thickness.
+
+    :param lineType: Type of the line:
+
+            * **8** (or omitted) - 8-connected line.
+
+            * **4** - 4-connected line.
+
+            * **CV_AA** - antialiased line.
+
+    :param shift: Number of fractional bits in the point coordinates.
+
+    :param tipLength: The length of the arrow tip in relation to the arrow length
+
+The function ``arrowedLine`` draws an arrow between ``pt1`` and ``pt2`` points in the image. See also :ocv:func:`line`.
+
+
 LineIterator
 ------------
 .. ocv:class:: LineIterator

modules/core/include/opencv2/core.hpp

@@ -510,6 +510,10 @@ CV_EXPORTS_W void randShuffle(InputOutputArray dst, double iterFactor = 1., RNG*
 CV_EXPORTS_W void line(InputOutputArray img, Point pt1, Point pt2, const Scalar& color,
                      int thickness = 1, int lineType = LINE_8, int shift = 0);
 
+//! draws an arrow from pt1 to pt2 in the image
+CV_EXPORTS_W void arrowedLine(InputOutputArray img, Point pt1, Point pt2, const Scalar& color,
+                     int thickness=1, int line_type=8, int shift=0, double tipLength=0.1);
+
 //! draws the rectangle outline or a solid rectangle with the opposite corners pt1 and pt2 in the image
 CV_EXPORTS_W void rectangle(InputOutputArray img, Point pt1, Point pt2,
                           const Scalar& color, int thickness = 1,

modules/core/src/drawing.cpp

@@ -1584,6 +1584,24 @@ void line( InputOutputArray _img, Point pt1, Point pt2, const Scalar& color,
     ThickLine( img, pt1, pt2, buf, thickness, line_type, 3, shift );
 }
 
+void arrowedLine(InputOutputArray img, Point pt1, Point pt2, const Scalar& color,
+           int thickness, int line_type, int shift, double tipLength)
+{
+    const double tipSize = norm(pt1-pt2)*tipLength; // Factor to normalize the size of the tip depending on the length of the arrow
+
+    line(img, pt1, pt2, color, thickness, line_type, shift);
+
+    const double angle = atan2( (double) pt1.y - pt2.y, (double) pt1.x - pt2.x );
+
+    Point p(cvRound(pt2.x + tipSize * cos(angle + CV_PI / 4)),
+        cvRound(pt2.y + tipSize * sin(angle + CV_PI / 4)));
+    line(img, p, pt2, color, thickness, line_type, shift);
+
+    p.x = cvRound(pt2.x + tipSize * cos(angle - CV_PI / 4));
+    p.y = cvRound(pt2.y + tipSize * sin(angle - CV_PI / 4));
+    line(img, p, pt2, color, thickness, line_type, shift);
+}
+
 void rectangle( InputOutputArray _img, Point pt1, Point pt2,
                 const Scalar& color, int thickness,
                 int lineType, int shift )