frameDifference with Mat OpenCV library for Processing

frameDifferenceMat.pde

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

Capture video;

OpenCV opencv;                      
    
Mat cvLastFrameMat;
Mat cvThisFrameMat;

void setup()
{
  size ( 640, 480 );                     
  
  video = new Capture(this, width, height);
  video.start();  
  
  opencv = new OpenCV( this, 640, 480);            //  Initialises the OpenCV library
  
  // Any idea how to create an empty Mat?
  // new Mat(640, 480, 0); gives an error however cols and rows seem correct
  cvLastFrameMat = opencv.getGray(); 
  //println(cvLastFrame.cols());
  
  frameRate(25);

}

void draw()
{
  if (video.available()) {
    video.read();
    
    opencv.loadImage(video);
    
    cvThisFrameMat = opencv.getGray();
  
    // difference with last Mat and this Mat
    // result is stored in this Mat
    OpenCV.diff(cvThisFrameMat, cvLastFrameMat);
    
    image( opencv.getOutput(), 0, 0 );  
    
    // this gives a strange grid, so we do load image again
    //OpenCV.toCv(video, cvLastFrameMat);   
       
    opencv.loadImage(video);
    cvLastFrameMat = opencv.getGray();
  }

}