int i;
int direction;

void setup(){

  size(300,300);
  direction = 1;
  
  }
  
void loop(){
  background(255-i,i,255);
  
  strokeWeight(i/10);
  stroke(255,i,70);
  line(0,300-i,300,i);
  i += direction;
  
   if(i > 300 || i < 0){
   direction = direction *-1;
   }
   
}