PImage img;
  int w = 80;
  
  // It's possible to convolve the image with 
  // many different matrices
  
    float[][] matrix = { { -1, -1, -1 },
                         { -1,  9, -1 },
                         { -1, -1, -1 } }; 
  
  void setup() {
    size(200, 200);
    frameRate(30);
    img = loadImage("end.jpg");
  }
  
  void draw() {
    // We're only going to process a portion of the image
    // so let's set the whole image as the background first
    image(img,0,0);
    // Where is the small rectangle we will process
    int xstart = constrain(mouseX-w/2,0,img.width);
    int ystart = constrain(mouseY-w/2,0,img.height);
    int xend = constrain(mouseX+w/2,0,img.width);
    int yend = constrain(mouseY+w/2,0,img.height);
    int matrixsize = 3;
    loadPixels();
    // Begin our loop for every pixel
    for (int x = xstart; x < xend; x++) {
      for (int y = ystart; y < yend; y++ ) {
        color c = convolution(x,y,matrix,matrixsize,img);
        int loc = x + y*img.width;
        pixels[loc] = c;
      }
    }
    updatePixels();
  }
  
  color convolution(int x, int y, float[][] matrix,int matrixsize, PImage img)
  {
    float rtotal = 0.0;
    float gtotal = 0.0;
    float btotal = 0.0;
    int offset = matrixsize / 2;
    for (int i = 0; i < matrixsize; i++){
      for (int j= 0; j < matrixsize; j++){
        // What pixel are we testing
        int xloc = x+i-offset;
        int yloc = y+j-offset;
        int loc = xloc + img.width*yloc;
        // Make sure we haven't walked off our image, we could do better here
        loc = constrain(loc,0,img.pixels.length-1);
        // Calculate the convolution
        rtotal += (red(img.pixels[loc]) * matrix[i][j]);
        gtotal += (green(img.pixels[loc]) * matrix[i][j]);
        btotal += (blue(img.pixels[loc]) * matrix[i][j]);
      }
    }
    // Make sure RGB is within range
    rtotal = constrain(rtotal,0,255);
    gtotal = constrain(gtotal,0,255);
    btotal = constrain(btotal,0,255);
    // Return the resulting color
    return color(rtotal,gtotal,btotal);
  }