diff --git a/src/jam b/src/jam
index 782edcb..72c340f 100755
--- a/src/jam
+++ b/src/jam
@@ -91,10 +91,6 @@ class Jam_app(App):
         # command prompt
         self.command_prompt=Command_prompt(self)
 
-        # load lattice
-        if self.lattice=="square":
-            self.painter.lattice=Square_lattice()
-
         layout.add_widget(self.painter)
         layout.add_widget(self.status_bar)
         layout.add_widget(self.command_prompt)
@@ -106,9 +102,14 @@ class Jam_app(App):
             # set readonly mode
             self.readonly=not os.access(self.openfile,os.W_OK)
 
+        # load lattice
+        if self.lattice=="square":
+            self.painter.set_lattice(Square_lattice())
+
         return layout
 
 
+
 # disable red circles on right click
 Config.set('input', 'mouse', 'mouse,disable_multitouch')
 # do not exit on escape
diff --git a/src/painter.py b/src/painter.py
index 45445d2..9055b9a 100644
--- a/src/painter.py
+++ b/src/painter.py
@@ -51,12 +51,29 @@ class Painter(Widget):
         self.keyboard = Window.request_keyboard(None,self,"text")
         self.keyboard.bind(on_key_down=self.on_key_down,on_key_up=self.on_key_up,on_textinput=self.on_textinput)
 
+        # redraw on resize
+        self.bind(size=lambda obj,value: self.draw())
+
     
     def reset(self):
         self.particles=[]
         self.undermouse=None
         self.draw()
 
+    # set lattice
+    def set_lattice(self,lattice):
+        self.lattice=lattice
+
+        # draw
+        self.draw()
+
+        # snap all existing particles to grid
+        for particle in self.particles:
+            delta=self.lattice.nearest_delta(particle.squares[0].pos)
+            if not self.check_interaction_any(particle,delta):
+                particle.move(delta)
+
+
     # convert logical coordinates (normalized and centered) to the ones that are plotted
     def pos_tocoord_x(self,x):
         return self.width/2+x*Square_element.size