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
}