This is part of a bigger project, so I had to peel the source code out of it - its dirty but it makes my point. The goal is an application which records a video and sends time stamps to various other machines and a PowerLab. WxPython is in the game because theres a lot of GUI and other processes. In the source code I left the video in its original size - because the effect comes earlier. (I know it would be a solution to make the video as small as possible but then the effect would simply occur later).

The capturing and writing out the frames is done by a wxpython timer. And theres no change if I remove the wxpython stuff. Thats the source code:

!/usr/bin/env python

import wx import time import cv, cv2 import sys from pylab import *

class MyFrame(wx.Frame): def __init__(self, args, *kwds):

      ############################################################################ initialize camera and capturing
      self.capture = cv2.VideoCapture(0) 
      ret, self.frame = self.capture.read()
      self.cwidth=int(self.capture.get(cv2.cv.CV_CAP_PROP_FRAME_WIDTH))
      self.cheight=int(self.capture.get(cv2.cv.CV_CAP_PROP_FRAME_HEIGHT))


      self.capSize=(self.cwidth,self.cheight)
      self.fps=12.5
      self.frames=0
      self.iterations=2000 ###################### number of frames to capture
      self.frame_rate_data=[]
      self.frame_data=[]

      print 'Camera loaded',self.cwidth,self.cheight,self.capSize

      ######################################################################## wxpython
      wx.Frame.__init__(self, *args, **kwds)
      mainSizer = wx.BoxSizer(wx.VERTICAL)

      videoWarper = wx.StaticBox(self, label="Video",size=(self.cwidth,self.cheight))
      videoBoxSizer = wx.StaticBoxSizer(videoWarper, wx.VERTICAL)
      self.videoPanel = wx.Panel(self, -1,size=(self.cwidth,self.cheight))
      videoBoxSizer.Add(self.videoPanel,0)
      mainSizer.Add(videoBoxSizer,0)
      self.Show()
      self.SetSizerAndFit(mainSizer)

      ######################################################################## convert frame to bmp for wx python
      self.frame = cv2.cvtColor(self.frame, cv2.COLOR_BGR2RGB)
      self.bmp = wx.BitmapFromBuffer(self.cwidth, self.cheight, self.frame)

      ######################################################################## initialize videowriter
      self.fourcc = cv2.cv.CV_FOURCC('m', 'p', '4', 'v') 
      self.vout = cv2.VideoWriter() ################################################### open videowriter 
  self.outmovie='test'+'.mov'
      success = self.vout.open(self.outmovie,self.fourcc,self.fps,self.capSize,True)

      ####################################################################### initialize timer
      self.timer = wx.Timer(self.videoPanel)
      self.videoPanel.Bind(wx.EVT_PAINT, self.OnPaint)
      self.videoPanel.Bind(wx.EVT_TIMER, self.NextFrame)
      self.t0 =time.time()
      self.timer.Start(int(1000.0/self.fps))

 def OnPaint(self, event):           
                dc = wx.BufferedPaintDC(self.videoPanel)
                dc.DrawBitmap(self.bmp, 0, 0)

 def NextFrame(self, event):
                 ret, self.frame = self.capture.read()
                 if ret:  
                               self.xtime= time.time() - self.t0 
                               self.frames=self.frames+1
                               self.vout.write(self.frame) ######################### this is the offending line !!!   videowriter on - off             

                               xtime= time.time() - self.t0 
                               fpa= self.frames/xtime      
                               #print  self.frames, self.xtime, fpa
                               self.frame_rate_data.append (fpa)
                               self.frame_data.append (self.frames)

                               self.frame = cv2.cvtColor(self.frame, cv2.COLOR_BGR2RGB)
                               self.bmp.CopyFromBuffer(self.frame)
                               self.Refresh()

                               if self.frames ==self.iterations: ################### plot and quit
                                    plot(self.frame_data,self.frame_rate_data)
                                    grid(True)
                                    savefig("test.png")
                                    show()
                                    self.Destroy()
                                    sys.exit()

if __name__ == "__main__":

    app=wx.App(False)
    frame_1 = MyFrame(None, wx.ID_ANY, "")
    app.SetTopWindow(frame_1)
    frame_1.Show()
    app.MainLoop()

Ok - thats the ... (more)