PGraphics pg; //define a buffer
  int dim = 200; //screen dimension
  void setup() {
    size(dim, dim); //size up the screen
    background(102); //set the background
    pg = createGraphics(dim, dim, P3D); //create a buffer
  }
  void draw() {
    image(pg, 0, 0); //draw the buffer
  }
  int x1,y1,x2,y2; //coordinates of a line
  void mouseDragged(){
    pg.beginDraw(); //start writing to the buffer
    pg.background(102);
    pg.stroke(255);
    for(int i=0; i<1000; i++){ //draw 1000 lines
      x1 = int(random(dim)); 
      y1 = int(random(dim));
      x2 = int(random(dim)); 
      y2 = int(random(dim));
      pg.line(x1, y1, x2, y2);
    }
    pg.endDraw(); //end writing to the buffer
  }