diff --git a/src/element.py b/src/element.py
index 2c3bbd2..2e81f74 100644
--- a/src/element.py
+++ b/src/element.py
@@ -4,6 +4,7 @@ import sys
 
 from point import Point,l_infinity,l_2
 from tools import isint_nonzero,sgn,in_interval
+from kivy.graphics import Rectangle
 
 # parent class of all elements
 class Element():
@@ -17,6 +18,16 @@ class Element():
         self.pos.y=y
 
 
+    # override in each subclass
+    # draw element
+    def draw(self,painter):
+        return
+
+    # override in each subclass
+    # draw boundary
+    def stroke(self,painter):
+        return
+
     # override in each subclass
     # check whether an element interacts with square
     def check_interaction(self,element):
@@ -46,6 +57,23 @@ class Element():
 
 # square elements
 class Element_square(Element):
+    # draw element
+    def draw(self,painter):
+        Rectangle(pos=(painter.pos_tocoord_x(self.pos.x-0.5*self.size),painter.pos_tocoord_y(self.pos.y-0.5*self.size)),size=(self.size*painter.base_size,self.size*painter.base_size))
+
+    # draw boundary
+    def stroke(self,painter):
+        # convert to graphical coordinates
+        coordx=painter.pos_tocoord_x(square.pos.x)
+        coordy=painter.pos_tocoord_y(square.pos.y)
+
+        Line(points=(
+        *(coordx-0.5*self.size*painter.base_size,coordy-0.5*self.size*painter.base_size),
+        *(coordx-0.5*self.size*painter.base_size,coordy+0.5*self.size*painter.base_size),
+        *(coordx+0.5*self.size*painter.base_size,coordy+0.5*self.size*painter.base_size),
+        *(coordx+0.5*self.size*painter.base_size,coordy-0.5*self.size*painter.base_size),
+        *(coordx-0.5*self.size*painter.base_size,coordy-0.5*self.size*painter.base_size)
+        ))
 
     # check whether an element interacts with square
     # TODO: this only works if element is a square!
diff --git a/src/polyomino.py b/src/polyomino.py
index 2d5c550..7487651 100644
--- a/src/polyomino.py
+++ b/src/polyomino.py
@@ -1,5 +1,5 @@
 # a polyomino is a collection of elements, defined in elements.py
-from kivy.graphics import Color,Line,Rectangle
+from kivy.graphics import Color,Line
 
 from point import l_infinity
 from element import Element_square
@@ -28,27 +28,17 @@ class Polyomino():
             Color(r/2,g/2,b/2,alpha)
 
         for element in self.elements:
-            Rectangle(pos=(painter.pos_tocoord_x(element.pos.x-0.5*element.size),painter.pos_tocoord_y(element.pos.y-0.5*element.size)),size=(element.size*painter.base_size,element.size*painter.base_size))
+            element.draw(painter)
 
         # draw boundary
         self.stroke(painter)
 
     # draw boundary (override for connected polyominos)
     def stroke(self,painter):
-        # convert to graphical coordinates
-        coordx=painter.pos_tocoord_x(square.pos.x)
-        coordy=painter.pos_tocoord_y(square.pos.y)
-
         # white
         Color(1,1,1)
         for element in self.elements:
-            Line(points=(
-            *(coordx-0.5*element.size*painter.base_size,coordy-0.5*element.size*painter.base_size),
-            *(coordx-0.5*element.size*painter.base_size,coordy+0.5*element.size*painter.base_size),
-            *(coordx+0.5*element.size*painter.base_size,coordy+0.5*element.size*painter.base_size),
-            *(coordx+0.5*element.size*painter.base_size,coordy-0.5*element.size*painter.base_size),
-            *(coordx-0.5*element.size*painter.base_size,coordy-0.5*element.size*painter.base_size)
-            ))
+            element.stroke(painter)
 
     # move by delta
     def move(self,delta):