float theta;
float rotation;

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

void draw() {
  translate(width/2.0, height/2.0);
  theta += rotation;
  rotate(theta/PI);
  point(theta, 0);
  // check restart
  if (theta>width/2.0) frameCount=-1;
}