Clarify why a self.dc is needed in CanvasView.py

diff --git a/CanvasView.py b/CanvasView.py
index e91902e..bbac60b 100755 (executable)
--- a/CanvasView.py
+++ b/CanvasView.py
@@ -73,12 +73,10 @@ class CanvasView(wx.Window):
 
         self.Bind(wx.EVT_PAINT, self.OnPaint)
 
-        # make a DC to draw into...
-        self.buffer = wx.EmptyBitmap(w, h)
-        self.dc = wx.BufferedDC(None, self.buffer)
-
-        # Because we want the position of the mouse pointer
-        # relative to the center of the canvas
+        # Have a reference DC with the desired coordinate system.
+        # Because we want the position of the mouse pointer relative to the
+        # center of this canvas.
+        self.dc = wx.MemoryDC(wx.EmptyBitmap(w, h))
         self.dc.SetDeviceOrigin(w/2., h/2.)
 
         self.draw_grid = True