Added angle_between_lines()

dlib/geometry/line.h

@@ -6,6 +6,7 @@
 #include "line_abstract.h"
 #include "vector.h"
 #include <utility>
+#include "../numeric_constants.h"
 
 namespace dlib
 {
@@ -172,6 +173,17 @@ namespace dlib
         return cnt;
     }
 
+// ----------------------------------------------------------------------------------------
+
+    inline double angle_between_lines (
+        const line& a,
+        const line& b
+    )
+    {
+        auto tmp = put_in_range(0.0, 1.0, std::abs(dot(a.normal(),b.normal()))); 
+        return std::acos(tmp)*180/pi;
+    }
+
 // ----------------------------------------------------------------------------------------
 
 }

dlib/geometry/line_abstract.h

@@ -145,6 +145,18 @@ namespace dlib
               exists then this function returns a point with Inf values in it.
     !*/
 
+// ----------------------------------------------------------------------------------------
+
+    double angle_between_lines (
+        const line& a,
+        const line& b
+    );
+    /*!
+        ensures
+            - returns the angle, in degrees, between the given lines.  This is a number in
+              the range [0 90].
+    !*/
+
 // ----------------------------------------------------------------------------------------
 
     template <typename T>