void setup () {
  size(600,400);
  background(0);
  stroke(255);
}

float theta = 0.0;
float itr = (sin(theta) + 1) * width/2;

void draw () {
  
  theta += 0.01;
  itr = (sin(theta) + 1) / 2; // oscilate between 0 and 1
  // You can easiy invert it: (1-itr)
  
  background(0); // "reset" canvas

  line(itr*width,   height / 2,   width * (1-itr),   height / 2);
  line(width / 2,   itr*height,   width / 2,         height * (1-itr));
  
}