Refined the definition of the drectangle a little. In particular,

now it behaves more like the rectangle in terms of how it measures
width and height.

dlib/geometry/drectangle.h

@@ -56,18 +56,18 @@ namespace dlib
 
         drectangle (
             const rectangle& rect
-        ) : l(rect.left()-0.5), 
-            t(rect.top()-0.5),
-            r(rect.right()+0.5),
-            b(rect.bottom()+0.5) {}
+        ) : l(rect.left()), 
+            t(rect.top()),
+            r(rect.right()),
+            b(rect.bottom()) {}
 
         operator rectangle (
         ) const
         {
-            return rectangle((long)std::ceil(l), 
-                             (long)std::ceil(t),
-                             (long)std::floor(r),
-                             (long)std::floor(b));
+            return rectangle((long)std::floor(l+0.5), 
+                             (long)std::floor(t+0.5),
+                             (long)std::floor(r+0.5),
+                             (long)std::floor(b+0.5));
         }
 
         double left()   const { return l; }
@@ -95,21 +95,19 @@ namespace dlib
         double width (
         ) const 
         { 
-            // if either the width or height would be 0.
-            if (t >= b || l >= r)
+            if (is_empty())
                 return 0;
             else
-                return r - l; 
+                return r - l + 1; 
         }
 
         double height (
         ) const 
         { 
-            // if either the width or height would be 0.
-            if (t >= b || l >= r)
+            if (is_empty())
                 return 0;
             else
-                return b - t; 
+                return b - t + 1; 
         }
 
         double area (
@@ -189,6 +187,15 @@ namespace dlib
             return *this;
         }
 
+        drectangle& operator += (
+            const dlib::vector<double,2>& p
+        )
+        {
+            *this = *this + drectangle(p);
+            return *this;
+        }
+
+
     private:
         double l;
         double t;
@@ -298,10 +305,17 @@ namespace dlib
         const double& scale 
     )
     {
-        const double width = rect.width()*scale;
-        const double height = rect.height()*scale;
-        const dlib::vector<double,2> p = center(rect);
-        return drectangle(p.x()-width/2, p.y()-height/2, p.x()+width/2, p.y()+height/2);
+        if (!rect.is_empty())
+        {
+            const double width = (rect.right()-rect.left())*scale;
+            const double height = (rect.bottom()-rect.top())*scale;
+            const dlib::vector<double,2> p = center(rect);
+            return drectangle(p.x()-width/2, p.y()-height/2, p.x()+width/2, p.y()+height/2);
+        }
+        else
+        {
+            return rect;
+        }
     }
 
     inline drectangle operator* (
@@ -361,10 +375,13 @@ namespace dlib
 
     inline drectangle centered_drect (
         const dlib::vector<double,2>& p,
-        const double& width,
-        const double& height
+        double width,
+        double height
     )
     {
+        width--;
+        height--;
+
         return drectangle(p.x()-width/2, p.y()-height/2, p.x()+width/2, p.y()+height/2);
     }
 

dlib/geometry/drectangle_abstract.h

@@ -19,9 +19,9 @@ namespace dlib
             WHAT THIS OBJECT REPRESENTS
                 This object is just like dlib::rectangle except that it stores the
                 coordinates of the rectangle using double rather than long variables.  As
-                such, this object represents a rectangular region inside a Cartesian
-                coordinate system.  The region is the rectangle with its top left corner at
-                position (left(),top()) and its bottom right corner at (right(),bottom()).
+                such, this object represents a rectangular region inside an image.  The
+                region is the rectangle with its top left corner at position (left(),top())
+                and its bottom right corner at (right(),bottom()).
 
                 Note that the origin of the coordinate system, i.e. (0,0), is located at
                 the upper left corner.  That is, points such as (1,1) or (3,5) represent
@@ -91,10 +91,10 @@ namespace dlib
         );
         /*!
             ensures
-                - left()   == rect.left()-0.5
-                - top()    == rect.top()-0.5
-                - right()  == rect.right()+0.5
-                - bottom() == rect.bottom()+0.5
+                - left()   == rect.left()
+                - top()    == rect.top()
+                - right()  == rect.right()
+                - bottom() == rect.bottom()
                 - dcenter(*this) == dcenter(rect)
                 - width() == rect.width()
                 - height() == rect.height()
@@ -104,14 +104,8 @@ namespace dlib
         ) const;
         /*!
             ensures
-                - returns a rectangle R such that:
-                    - R.left()   == ceil(left())
-                    - R.top()    == ceil(top())
-                    - R.right()  == floor(right())
-                    - R.bottom() == floor(bottom())
-                - This function is designed this way so that converting a rectangle to a
-                  drectangle and then back to a rectangle again results in the original
-                  rectangle.
+                - returns a rectangle where left(), top(), right(), and bottom() have been
+                  rounded to the nearest integer values.
         !*/
 
         double left (
@@ -214,7 +208,7 @@ namespace dlib
                     - returns 0
                 - else
                     - returns the width of this rectangle.
-                      (i.e. right() - left())
+                      (i.e. right() - left() + 1)
         !*/
 
         double height (
@@ -225,7 +219,7 @@ namespace dlib
                     - returns 0
                 - else
                     - returns the height of this rectangle.
-                      (i.e. bottom() - top())
+                      (i.e. bottom() - top() + 1)
         !*/
 
         double area (
@@ -316,6 +310,15 @@ namespace dlib
                 - returns #*this
         !*/
 
+        drectangle& operator += (
+            const dlib::vector<double,2>& p
+        );
+        /*!
+            ensures
+                - performs: *this = *this + drectangle(p)
+                - returns #*this
+        !*/
+
     };
 
 // ----------------------------------------------------------------------------------------
@@ -388,12 +391,12 @@ namespace dlib
     );
     /*!
         ensures
-            - This function returns a rectangle that has the same center as rect but with a
-              width and height that are scale times larger.  That is, we return a new
-              rectangle R such that:
+            - This function returns a rectangle that has the same center as rect but with
+              dimensions that are scale times larger.  That is, we return a new rectangle R
+              such that:
                 - center(R) == center(rect)
-                - R.width() == rect.width()*scale
-                - R.height() == rect.height()*scale
+                - R.right()-R.left() == (rect.right()-rect.left())*scale
+                - R.bottom()-R.top() == (rect.bottom()-rect.top())*scale
     !*/
 
 // ----------------------------------------------------------------------------------------
@@ -484,20 +487,20 @@ namespace dlib
 
     drectangle centered_drect (
         const vector<double,2>& p,
-        const double& width,
-        const double& height
+        double width,
+        double height
     );
     /*!
         ensures
             - returns a rectangle R such that:
                 - center(R) == p
-                - if (width == 0 || height == 0)
+                - if (width < 1 || height < 1)
                     - R.width() == 0 
                     - R.height() == 0 
+                    - R.is_empty() == true
                 - else
                     - R.width() == width
                     - R.height() == height 
-                - R.tl_corner() == point(p.x()-width/2, p.y()-height/2)
     !*/
 
 // ----------------------------------------------------------------------------------------