int i;
int direction;
void setup(){
  size(300,300);
  direction=1;
}
void loop(){
  background(i,0,0);
  stroke(0,i,0);
 strokeWeight(i/5);
  line(0,i,300,i);
  i+=direction;
  if(i>300){
    direction=-1;
    }
    if(i<0){
    direction=1;
    }
}