size (200,200);
background(255);
int s=255;
int s1=255;

for (int  y0=0; y0<200; y0=y0+4) {
  stroke(s,0,0);
  s=s-5;          
  line(0,0,200,y0);
}
for (int  y1=0; y1<200; y1=y1+4) {
  stroke(s1,0,0);
  s1=s1-5;  
  line(200,0,0,y1);                              
}