Untitled 1

int s;  // size
int u;  // unit
PVector p; // line position
PVector d; // direction

void setup() {

  s = 300;
  u = 2;
  p = new PVector(0, 0);
  d = new PVector(u, 0); // left to right

  size(s, s);
  background(#FFFFFF);
  colorMode(HSB, s / u);
}

void draw() {

  // direction
  if (p.x > s) {
    p.x = s;
    d.x = 0;
    d.y = u;
  }
  else if (p.y > s) {
    p.y = s;
    d.x = -u;
    d.y = 0;
  }
  else if (p.x < 0) {
    p.x = 0;
    d.x = 0;
    d.y = -u;
  }
  else if (p.y < 0) {
    p.y = 0;
    d.x = u;
    d.y = 0;
  }

  // draw
  stroke(frameCount % (s / u), 127, 127);
  line(p.x, p.y, s - p.y, p.x);

  // update
  p.add(d);
}