diff --git a/docs/source/preprocess.rst b/docs/source/preprocess.rst
index 0cdab27..5ff4859 100644
--- a/docs/source/preprocess.rst
+++ b/docs/source/preprocess.rst
@@ -8,7 +8,7 @@ Preprocess
 Building preprocess
 =============================
 
-bd_preprocess(buildings):
+.. function:: pybdshadow.bd_preprocess(buildings)
 
 Preprocess building data, so that we can perform shadow calculation.
 Remove empty polygons and convert multipolygons into polygons.