Added the rotate_point() function.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%402978

dlib/geometry/vector.h

@@ -1244,6 +1244,25 @@ namespace dlib
 
     typedef vector<long,2> point;
 
+// ----------------------------------------------------------------------------------------
+
+    template <typename T>
+    const dlib::vector<T,2> rotate_point (
+        const dlib::vector<T,2>& center,
+        dlib::vector<T,2> p,
+        double angle
+    )
+    {
+        p -= center;
+        dlib::vector<double,2> temp;
+        const double cos_angle = cos(angle);
+        const double sin_angle = sin(angle);
+        temp.x() = cos_angle*p.x() - sin_angle*p.y();
+        temp.y() = sin_angle*p.x() + cos_angle*p.y();
+
+        return temp + center;
+    }
+
 // ----------------------------------------------------------------------------------------
 
 }

dlib/geometry/vector_abstract.h

@@ -431,6 +431,24 @@ namespace dlib
 
     typedef vector<long,2> point;
 
+// ----------------------------------------------------------------------------------------
+
+    template <typename T>
+    const dlib::vector<T,2> rotate_point (
+        const dlib::vector<T,2> center,
+        const dlib::vector<T,2> p,
+        double angle
+    );
+    /*!
+        ensures
+            - returns a point P such that:
+                - P is the point p rotated counter-clockwise around the given
+                  center point by angle radians.
+                  (Note that this is counter clockwise with respect to the normal
+                  coordinate system with positive y going up and positive x going
+                  to the right)
+    !*/
+
 // ----------------------------------------------------------------------------------------
 
 }

dlib/test/geometry.cpp

@@ -269,6 +269,22 @@ namespace
 
         }
 
+        {
+            const double pi = 3.1415926535898;
+            point p1, center;
+
+            center = point(3,4);
+            p1 = point(10,4);
+
+            DLIB_TEST(rotate_point(center, p1, pi/2) == point(3,7+4));
+
+            center = point(3,3);
+            p1 = point(10,3);
+
+            DLIB_TEST(rotate_point(center, p1, pi/4) == point(8,8));
+            DLIB_TEST(rotate_point(center, p1, -pi/4) == point(8,-2));
+        }
+
     }