size (200,200);
background(255);
int y0=0;//line1
int y1=0;//line2
int s=255;//stroke color
while (y0<200) {
  stroke(s,0,0);
  s=s-5;
  smooth(); 
  line(0,0,200,y0);
  line(200,0,0,y1);
  y0=y0+4;
  y1=y1+4;
}