FrameDifference problem Processing OpenCV library

frameDifferenceProblem.pde

/*
Problem. Why I see a light image of me, while I do a difference on exactly the same
images.

OpenCV for Processing 0.5.2 by Greg Borenstein http://gregborenstein.com
Using Java OpenCV 2.4.5.0

Processing 3.0a4
*/

import org.opencv.core.Mat;
import gab.opencv.*;
import processing.video.*;

Capture video;

OpenCV opencv;                     
 
PImage videoFrame;
PImage lastImg;

void setup()
{
  size ( 640, 480 );                     
  
  video = new Capture(this, width, height);
  video.start();  
  
  opencv = new OpenCV( this, 640, 480);
  videoFrame = new PImage( 640, 480 );
  lastImg = new PImage( 640, 480 );
  
  frameRate(30);
}

void draw()
{
  //bubbles.add(new Bubble( (int)random( 0, width - 40), -bubblePNG.height, bubblePNG.width, bubblePNG.height));   //  Adds a new bubble to the array with a random x position
  
  if (video.available()) {
    
    video.read();
    
    // just store it in a PImage videoFrame
    // to really make sure we use to save frame
    // as lastImg. However this is not needed, just an extra check
    videoFrame = video;
    
    // load the frame in opencv.
    // I suppose this overwrites the old image. However I'm not sure, since
    // some noise is visible.
    // I can't find something to empty it though. 
    opencv.loadImage(videoFrame);
    
    // normally we do this here
    // but to show the problem I want to find the difference between
    // two the same images. There should be a differce. 
    // but still there is.....
    lastImg = videoFrame;
    
    // difference with the lastImg (actually the videoFrame)
    opencv.diff(lastImg); 
    
    // display the opencv output
    image( opencv.getOutput(), 0, 0 );  
    
    // set the last Image to this videoFrame
    // normal location
    //lastImg = videoFrame;

  }
  
}