finite difference stencil derivation for staggered positions

pystencils/fd/derivation.py

@@ -236,13 +236,14 @@ class FiniteDifferenceStaggeredStencilDerivation:
     """Derives a finite difference stencil for application at a staggered position
 
     Args:
-        neighbor: the neighbor direction string at whose staggered index to calculate the derivative
+        neighbor: the neighbor direction string or vector at whose staggered position to calculate the derivative
         dim: how many dimensions (2 or 3)
         derivative: a tuple of directions over which to perform derivatives
     """
 
     def __init__(self, neighbor, dim, derivative=tuple()):
-        neighbor = direction_string_to_offset(neighbor)
+        if type(neighbor) is str:
+            neighbor = direction_string_to_offset(neighbor)
         if dim == 2:
             assert neighbor[dim:] == 0
         assert derivative is tuple() or max(derivative) < dim
@@ -339,4 +340,10 @@ class FiniteDifferenceStaggeredStencilDerivation:
         plot(pts, data=ws)
 
     def apply(self, field):
-        return sum([field.__getitem__(point) * weight for point, weight in zip(self.points, self.weights)])
+        if field.index_dimensions == 0:
+            return sum([field.__getitem__(point) * weight for point, weight in zip(self.points, self.weights)])
+        else:
+            total = field.neighbor_vector(self.points[0]) * self.weights[0]
+            for point, weight in zip(self.points[1:], self.weights[1:]):
+                total += field.neighbor_vector(point) * weight
+            return total