Added a missing assert and also added a function to convert from HOG

block coordinates back into the original image coordinates.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%403465

dlib/image_keypoint/hog.h

@@ -7,6 +7,7 @@
 #include "../algs.h"
 #include "../matrix.h"
 #include "../array2d.h"
+#include "../geometry.h"
 #include <cmath>
 
 namespace dlib
@@ -87,6 +88,18 @@ namespace dlib
             long col
         ) const
         {
+            // make sure requires clause is not broken
+            DLIB_ASSERT( 0 <= row && row < nr() &&
+                         0 <= col && col < nc(),
+                "\t descriptor_type hog_image::operator()()"
+                << "\n\t invalid row or col argument"
+                << "\n\t row:  " << row
+                << "\n\t col:  " << col 
+                << "\n\t nr(): " << nr() 
+                << "\n\t nc(): " << nc() 
+                << "\n\t this: " << this
+                );
+
             row *= cell_stride;
             col *= cell_stride;
             ++row;
@@ -109,6 +122,36 @@ namespace dlib
             return des;
         }
 
+        const rectangle get_block_rect (
+            long row,
+            long col
+        ) const
+        {
+            // make sure requires clause is not broken
+            DLIB_ASSERT( 0 <= row && row < nr() &&
+                         0 <= col && col < nc(),
+                "\t rectangle hog_image::get_block_rect()"
+                << "\n\t invalid row or col argument"
+                << "\n\t row:  " << row
+                << "\n\t col:  " << col 
+                << "\n\t nr(): " << nr() 
+                << "\n\t nc(): " << nc() 
+                << "\n\t this: " << this
+                );
+
+            row *= cell_stride;
+            col *= cell_stride;
+
+            row *= cell_size;
+            col *= cell_size;
+
+            // do this to account for the 1 pixel padding we use all around the image
+            ++row;
+            ++col;
+
+            return rectangle(col, row, col+cell_size*block_size-1, row+cell_size*block_size-1);
+        }
+
     private:
 
         template <

dlib/image_keypoint/hog_abstract.h

@@ -6,6 +6,7 @@
 #include "../algs.h"
 #include "../matrix.h"
 #include "../array2d.h"
+#include "../geometry.h"
 #include <cmath>
 
 namespace dlib
@@ -170,9 +171,24 @@ namespace dlib
                 - 0 <= col < nc()
             ensures
                 - returns the descriptor for the HOG block at the given row and column.  This descriptor 
-                  will include information from a window that is cell_size*block_size pixels wide and tall.
+                  will include information from a window that is located at get_block_rect(row,col) in
+                  the original image given to load().
                 - The returned descriptor vector will have block_size*block_size*num_orientation_bins elements.
         !*/
+
+        const rectangle get_block_rect (
+            long row,
+            long col
+        ) const;
+        /*!
+            requires
+                - 0 <= row < nr()
+                - 0 <= col < nc()
+            ensures
+                - returns a rectangle that tells you what part of the original image is associated
+                  with a particular HOG block.
+                - The returned rectangle will be cell_size*block_size pixels wide and tall.
+        !*/
     };
 
 // ----------------------------------------------------------------------------------------