diff --git a/init.lua b/init.lua
index bccc5f7..0edec00 100644
--- a/init.lua
+++ b/init.lua
@@ -1600,6 +1600,8 @@ function M.set_cell_material(layer_name, cx, cy, painted, map_id)
     end
     local cm = ensure_cells_material(layer, w, h)
     cm[cy * w + cx + 1] = painted and 1 or 0
+    m._dirty = true
+    m._opaque_ceiling = nil
 end
 
 -- 0.5.7: read the per-cell material flag. Returns false if