Fixed numpy_image not working correctly for grayscale images.

dlib/python/numpy_image.h

@@ -33,11 +33,10 @@ namespace dlib
     !*/
     {
         using basic_pixel_type = typename pixel_traits<pixel_type>::basic_pixel_type;
-        constexpr size_t channels = pixel_traits<pixel_type>::num;
 
         return obj.dtype().kind() == py::dtype::of<basic_pixel_type>().kind() && 
                obj.itemsize() == sizeof(basic_pixel_type) && 
-               obj.ndim() == pixel_traits<pixel_type>::num;
+               ((pixel_traits<pixel_type>::num==1) ? (obj.ndim()==2) : (obj.ndim()==3));
     }
 
 // ----------------------------------------------------------------------------------------
@@ -170,7 +169,10 @@ namespace dlib
         {
             using basic_pixel_type = typename pixel_traits<pixel_type>::basic_pixel_type;
             constexpr size_t channels = pixel_traits<pixel_type>::num;
-            *this = py::array_t<basic_pixel_type>({rows, cols, channels});
+            if (channels != 1)
+                *this = py::array_t<basic_pixel_type>({rows, cols, channels});
+            else
+                *this = py::array_t<basic_pixel_type>({rows, cols});
         }
 
     private:
@@ -256,7 +258,7 @@ namespace dlib
 
         assert_correct_num_channels_in_image<pixel_type>(img);
         using basic_pixel_type = typename pixel_traits<pixel_type>::basic_pixel_type;
-        if (img.strides(2) != sizeof(basic_pixel_type))
+        if (img.ndim()==3 && img.strides(2) != sizeof(basic_pixel_type))
             throw dlib::error("The stride of the 3rd dimension (the channel dimension) of the numpy array must be " + std::to_string(sizeof(basic_pixel_type)));
         if (img.strides(1) != sizeof(pixel_type))
             throw dlib::error("The stride of the 2nd dimension (the columns dimension) of the numpy array must be " + std::to_string(sizeof(pixel_type)));