Added comments clarifying the thread safety requirements of the new

object detection tools.

dlib/image_keypoint/hashed_feature_image_abstract.h

@@ -37,6 +37,14 @@ namespace dlib
                 creating an indicator vector with multiple non-zero indicator features.
 
 
+            THREAD SAFETY
+                Concurrent access to an instance of this object is not safe and should be protected
+                by a mutex lock except for the case where you are copying the configuration 
+                (via copy_configuration()) of a hashed_feature_image object to many other threads.  
+                In this case, it is safe to copy the configuration of a shared object so long
+                as no other operations are performed on it.
+
+
             NOTATION 
                 let BASE_FE denote the base feature_extractor object contained inside 
                 the hashed_feature_image.

dlib/image_keypoint/hog_abstract.h

@@ -98,6 +98,14 @@ namespace dlib
                 Finally, the cell_stride parameter controls how much overlap you get between
                 blocks.  The maximum amount of overlap is obtained when cell_stride == 1.
                 At the other extreme, you would have no overlap if cell_stride == block_size. 
+
+
+            THREAD SAFETY
+                Concurrent access to an instance of this object is not safe and should be protected
+                by a mutex lock except for the case where you are copying the configuration 
+                (via copy_configuration()) of a hog_image object to many other threads.  
+                In this case, it is safe to copy the configuration of a shared object so long
+                as no other operations are performed on it.
         !*/
 
     public:

dlib/image_processing/box_overlap_testing_abstract.h

@@ -16,6 +16,11 @@ namespace dlib
             WHAT THIS OBJECT REPRESENTS
                 This object is a simple function object for determining if two rectangles
                 overlap.  
+
+            THREAD SAFETY
+                Concurrent access to an instance of this object is safe provided that 
+                only const member functions are invoked.  Otherwise, access must be
+                protected by a mutex lock.
         !*/
 
     public:

dlib/image_processing/scan_image_pyramid_abstract.h

@@ -76,6 +76,13 @@ namespace dlib
 
                 Finally, the sliding window classifiers described above are applied to every level 
                 of an image pyramid.  
+
+            THREAD SAFETY
+                Concurrent access to an instance of this object is not safe and should be protected
+                by a mutex lock except for the case where you are copying the configuration 
+                (via copy_configuration()) of a scan_image_pyramid object to many other threads.  
+                In this case, it is safe to copy the configuration of a shared object so long
+                as no other operations are performed on it.
         !*/
     public: