Changed fhog coordinate transforms so that they are properly invertible.

dlib/image_transforms/fhog.h

@@ -434,7 +434,13 @@ namespace dlib
     )
     {
         // Convert to image space and then set to the center of the cell.
-        return (p+point(1,1))*cell_size + point(1,1) + point(cell_size/2,cell_size/2);
+        point offset;
+        
+        if (p.x() >= 0 && p.y() >= 0) offset = point(cell_size/2,cell_size/2);
+        if (p.x() <  0 && p.y() >= 0) offset = point(-cell_size/2,cell_size/2);
+        if (p.x() >= 0 && p.y() <  0) offset = point(cell_size/2,-cell_size/2);
+        if (p.x() <  0 && p.y() <  0) offset = point(-cell_size/2,-cell_size/2);
+        return (p+point(1,1))*cell_size + point(1,1) + offset;
     }
 
 // ----------------------------------------------------------------------------------------

dlib/image_transforms/fhog_abstract.h

@@ -130,6 +130,8 @@ namespace dlib
               invertible.  Therefore, the returned location will be the center of the cell
               in the original image that contained the FHOG vector at position p.  Moreover,
               cell_size should be set to the value used by the call to extract_fhog_features().
+            - Mapping from fhog space to image space is an invertible transformation.  That
+              is, for any point P we have P == image_to_fhog(fhog_to_image(P,cell_size),cell_size).
     !*/
 
 // ----------------------------------------------------------------------------------------
@@ -144,6 +146,8 @@ namespace dlib
         ensures
             - maps a rectangle from fhog space to image space.  In particular this function returns:
               rectangle(fhog_to_image(rect.tl_corner(),cell_size), fhog_to_image(rect.br_corner(),cell_size))
+            - Mapping from fhog space to image space is an invertible transformation.  That
+              is, for any rectangle R we have R == image_to_fhog(fhog_to_image(R,cell_size),cell_size).
     !*/
 
 // ----------------------------------------------------------------------------------------

dlib/test/fhog.cpp

@@ -103,9 +103,33 @@ namespace
             }
         }
 
+        void test_point_transforms()
+        {
+            for (int cell_size = 1; cell_size < 10; ++cell_size)
+            {
+                print_spinner();
+                for (long i = -10; i <= 10; ++i)
+                {
+                    for (long j = -10; j <= 10; ++j)
+                    {
+                        for (long k = -10; k <= 10; ++k)
+                        {
+                            for (long l = -10; l <= 10; ++l)
+                            {
+                                rectangle rect(point(i,j), point(k,l));
+                                DLIB_TEST(rect == image_to_fhog(fhog_to_image(rect,cell_size),cell_size));
+                            }
+                        }
+                    }
+                }
+            }
+        }
+
+
         void perform_test (
         )
         {
+            test_point_transforms();
             test_on_small();
 
             print_spinner();