float px, py, x, y;
float theta;
float rotation;

void setup() {
  size(300, 300);
  px = py = x = y = theta = 0;
  rotation = 0.1;
  background(255);
}

void draw() {
  translate(width/2.0, height/2.0);
  x = theta*cos(theta/PI);
  y = (theta)*sin(theta/PI);
  line(x, y, px, py);
  theta = theta + rotation;
  px = x;
  py = y;
  // check restart
  if (px>width/2.0) frameCount=-1;
}