Why does my buffered GraphicsContext application have a flickering problem?

Question

import wx

class MainFrame(wx.Frame):
    def __init__(self,parent,title):

        wx.Frame.__init__(self, parent, title=title, size=(640,480))
        self.mainPanel=DoubleBufferTest(self,-1)

        self.Show(True)

class DoubleBufferTest(wx.Panel):
    def __init__(self,parent=None,id=-1):
        wx.Panel.__init__(self,parent,id,style=wx.FULL_REPAINT_ON_RESIZE)

        self.SetBackgroundColour("#FFFFFF")

        self.timer = wx.Timer(self)
        self.timer.Start(100)        
        self.Bind(wx.EVT_TIMER, self.update, self.timer)
        self.Bind(wx.EVT_PAINT,self.onPaint)


    def onPaint(self,event):
        event.Skip()
        dc = wx.MemoryDC()
        dc.SelectObject(wx.EmptyBitmap(640, 480))
        gc = wx.GraphicsContext.Create(dc)
        gc.PushState()
        gc.SetBrush(wx.Brush("#CFCFCF"))
        bgRect=gc.CreatePath()
        bgRect.AddRectangle(0,0,640,480)
        gc.FillPath(bgRect)    
        gc.PopState()

        dc2=wx.PaintDC(self)
        dc2.Blit(0,0,640,480,dc,0,0)
    def update(self,event):
        self.Refresh()

app = wx.App(False)
f=MainFrame(None,"Test")
app.MainLoop()

I've come up with this code to draw double buffered GraphicsContext content onto a panel, but there's a constant flickering across the window. I've tried different kinds of paths, like lines and curves but it's still there and I don't know what's causing it.

Answer 1

If you're using a relatively modern wxWidgets, you can use wx.BufferedPaintDC and avoid having to muck around with the memory DC and painting and blitting on your own. Also, on windows, FULL_REPAINT_ON_RESIZE often causes flickering even when you're not resizing the window due to funny things going on under the covers - if you don't need it, going with NO_FULL_REPAINT_ON_RESIZE may help. Otherwise, you'll want to simplify your code some to make sure you can get the simplest thing to work, and perhaps take a look at the DoubleBufferedDrawing wiki page at wxpython.org.

Answer 2

You get flicker because each Refresh() causes the background to get erased before calling onPaint. You need to bind to EVT_ERASE_BACKGROUND and make it a no-op.

class DoubleBufferTest(wx.Panel):
    def __init__(self,parent=None,id=-1):
        # ... existing code ...
        self.Bind(wx.EVT_ERASE_BACKGROUND, self.onErase)
    def onErase(self, event):
        pass
    # ... existing code ...