Made the camera_transform output the point's distance.

dlib/geometry/point_transforms.h

@@ -853,13 +853,14 @@ namespace dlib
 
         inline dpoint operator() (
             const vector<double>& p,
-            double& scale
+            double& scale,
+            double& distance
         ) const
         {
             vector<double> temp = p-camera_pos;
             temp = proj*temp;
-            const double distance = temp.z()>0 ? temp.z() : 1e-9;
-            scale = dist_scale/distance;
+            distance = temp.z();
+            scale = dist_scale/(temp.z()>0 ? temp.z() : 1e-9);
             temp.x() = temp.x()*scale + width;
             temp.y() = temp.y()*scale + width;
             return temp;
@@ -869,8 +870,8 @@ namespace dlib
             const vector<double>& p
         ) const
         {
-            double scale;
-            return (*this)(p,scale);
+            double scale, distance;
+            return (*this)(p,scale,distance);
         }
 
         inline friend void serialize (const camera_transform& item, std::ostream& out)

dlib/geometry/point_transforms_abstract.h

@@ -644,7 +644,8 @@ namespace dlib
 
         dpoint operator() (
             const vector<double>& p,
-            double& scale
+            double& scale,
+            double& distance
         ) const;
         /*!
             ensures
@@ -654,6 +655,9 @@ namespace dlib
                 - #scale == a number that tells you how large things are at the point p.
                   Objects further from the camera appear smaller, in particular, they
                   appear #scale times their normal size.
+                - #distance == how far away the point is from the image plane.  Objects in
+                  front of the camera will have a positive distance and those behind a
+                  negative distance.
         !*/
 
         vector<double> get_camera_pos(