size(200, 200);    
background(0);
stroke(255);
strokeWeight(1);
beginShape(POINTS);
for ( int i = 0; i < 100; i=i+10 ) {
  vertex(50, 50+i); 
  vertex(50+i, 150); 
  vertex(150, 150-i); 
  vertex(150-i, 50);
}
endShape(CLOSE);