Markus Holzer
--- a/lbmpy/sparse/mapping.py
+++ b/lbmpy/sparse/mapping.py

 class SparseLbBoundaryMapper:
    NEIGHBOR_IDX_NAME = 'nidx{}'
-    DIR_SYMBOL = TypedSymbol("dir", np.uint32)
+    DIR_SYMBOL = TypedSymbol("dir", np.int64)
--- a/lbmpy/sparse/mapping.py
+++ b/lbmpy/sparse/mapping.py
+            if fa.field == f_out:
+                f_dir = 'out'
+            elif fa.field == f_in:
+                f_dir = 'in'
+            else:
+                continue
+            i = 1 if f_dir == "in" else 2
+            accessor_subs[fa] = pdf_field_sparse.absolute_access((index_field(i),), ())
+
+        self.boundary_eqs = assignments.new_with_substitutions(accessor_subs)
+        self.boundary_eqs_orig = assignments
        self.method = method
        self.index_field_dtype = index_field_dtype
        self.neighbor_offsets = neighbor_offsets
        self.index_field = index_field
+        self.timesptep_indexing = indexing