Question

I’ve got a problem with DCs. I’m trying to make an application that will draw many lines on the screens and needs to update really fast, and since I don’t want flickering, I decided to give buffered dcs a shot. But when I run this code, it doesn’t draw anything. What am I doing wrong?

import wx

class MainFrame(wx.Frame):
    def __init__(self):
        screensize = wx.GetDisplaySize()
        self.framesize = (screensize[0]/4*3, screensize[1]/4*3)
        wx.Frame.__init__(self, None, -1, "CursorTracker", size=self.framesize,
                          style=wx.SYSTEM_MENU|
                          wx.CAPTION|
                          wx.CLOSE_BOX|
                          wx.MINIMIZE_BOX)
        self.dc = wx.ClientDC(self)
        self.bdc = wx.BufferedDC(self.dc)
        self.SetBackgroundColour(wx.WHITE)
        self.Timer = wx.Timer(self)
        self.Bind(wx.EVT_TIMER, self.OnTimer)
        self.Timer.Start(100)

    def OnTimer(self, event):

        self.bdc.DrawLine(1,1,100,100)


class App(wx.App):
    def OnInit(self):
        frame = MainFrame()
        frame.Show()
        return True

app = App(redirect=False)
app.MainLoop()

Answer 1

I’ve used AutoBufferedPaintDC, but I’ve found doing my own double-buffering with a MemoryDC to be more flexible. Here’s a template for you.

import wx

class Frame(wx.Frame):
    def __init__(self):
        super(Frame, self).__init__(None, -1, 'CursorTracker')
        self.mdc = None # memory dc to draw off-screen
        self.Bind(wx.EVT_SIZE, self.on_size)
        self.Bind(wx.EVT_ERASE_BACKGROUND, self.on_erase)
        self.Bind(wx.EVT_PAINT, self.on_paint)
        w, h = wx.GetDisplaySize()
        w, h = w * 3 / 4, h * 3 / 4
        self.SetSize((w, h))
        self.Center()
        self.on_timer()
    def on_size(self, event):
        # re-create memory dc to fill window
        w, h = self.GetClientSize()
        self.mdc = wx.MemoryDC(wx.EmptyBitmap(w, h))
        self.redraw()
    def on_erase(self, event):
        pass # don't do any erasing to avoid flicker
    def on_paint(self, event):
        # just blit the memory dc
        dc = wx.PaintDC(self)
        if not self.mdc:
            return
        w, h = self.mdc.GetSize()
        dc.Blit(0, 0, w, h, self.mdc, 0, 0)
    def on_timer(self):
        # refresh every N milliseconds
        self.redraw()
        wx.CallLater(100, self.on_timer)
    def redraw(self):
        # do the actual drawing on the memory dc here
        dc = self.mdc
        w, h = dc.GetSize()
        dc.Clear()
        dc.DrawLine(0, 0, w, h)
        self.Refresh()

if __name__ == '__main__':
    app = wx.PySimpleApp()
    frame = Frame()
    frame.Show()
    app.MainLoop()

The basic approach is:

• create a memory dc for off-screen drawing
• if the window is resized, resize the memory dc and redraw
• when a paint event occurs, just blit the memory dc onto the paint dc
• do nothing on a erase background event to avoid flicker
• call redraw when and only when you actually need to change what’s on the screen

If you store a reference to that EmptyBitmap that’s created in on_size, you can even save the window contents to an image file with wxBitmap.SaveFile()