Added a version of set_aspect_ratio() that works for drectangle.

dlib/geometry/drectangle.h

@@ -385,6 +385,15 @@ namespace dlib
         return drectangle(p.x()-width/2, p.y()-height/2, p.x()+width/2, p.y()+height/2);
     }
 
+    inline drectangle centered_drect (
+        const drectangle& rect,
+        double width,
+        double height
+    )
+    {
+        return centered_drect(dcenter(rect), width, height);
+    }
+
     inline const drectangle shrink_rect (
         const drectangle& rect,
         double num 
@@ -419,6 +428,21 @@ namespace dlib
         return shrink_rect(rect, -width, -height);
     }
 
+    inline drectangle set_aspect_ratio (
+        const drectangle& rect,
+        double ratio
+    )
+    {
+        DLIB_ASSERT(ratio > 0,
+            "\t drectangle set_aspect_ratio()"
+            << "\n\t ratio: " << ratio 
+            );
+
+        const double h = std::sqrt(rect.area()/ratio);
+        const double w = h*ratio;
+        return centered_drect(rect, w, h);
+    }
+
 // ----------------------------------------------------------------------------------------
 
 }

dlib/geometry/drectangle_abstract.h

@@ -503,6 +503,18 @@ namespace dlib
                     - R.height() == height 
     !*/
 
+// ----------------------------------------------------------------------------------------
+
+    drectangle centered_drect (
+        const drectangle& rect,
+        double width,
+        double height
+    );
+    /*!
+        ensures
+            - returns centered_drect(center(rect), width, height)
+    !*/
+
 // ----------------------------------------------------------------------------------------
 
     const drectangle shrink_rect (
@@ -554,6 +566,24 @@ namespace dlib
               (i.e. grows the given drectangle by expanding its border)
     !*/
 
+// ----------------------------------------------------------------------------------------
+
+    drectangle set_aspect_ratio (
+        const drectangle& rect,
+        double ratio
+    );
+    /*!
+        requires
+            - ratio > 0
+        ensures
+            - This function reshapes the given rectangle so that it has the given aspect
+              ratio.  In particular, this means we return a rectangle R such that the
+              following equations are true:
+                - R.width()/R.height() == ratio
+                - R.area() == rect.area()
+                - dcenter(rect) == dcenter(R)
+    !*/
+
 // ----------------------------------------------------------------------------------------
 
 }