int space = (10), step = 10;
int i;
boolean b;

void setup() {
  size(500, 500 );
  background(0);
  stroke(255);
}

void draw() {
  stroke (  random(256), random(256),random(256));

  if ( i+space < width/2)
  {
    b = true;
  } else
  {
    b = false;
  }

  // kortere versie b = i+space, space, i+space, height-space);

  if ( b) //b == true
  {
    //vertical lines
    line(i+space, space, i+space, height-space);
  } else
  {
    //horizontal line
    line( width/2, i-width/2 + space*2, width-space, i - width/2 + space*2);
  }

  i+=step;

  if ( i + space > height + width/2 - 2*space)
  {
    i = 0;
  }
}