float ang;

void setup() {
  size (200,200);
  background (255);
  framerate (50);
  rectMode(CORNER);
}
void draw () {
  background (255); 
  stroke(0,0,0,50);                 // black lines
  translate (100,100);              //move origin of red square

  line(0,-width, 0, width);         // vertical line
  line(-height, 0, height, 0);      // horizontal line

  noFill ();
  stroke(250,0,0,50);               //red square
  rect (0,0,50,50);

  rotate (ang); 
  stroke(0,0,0);                    // black square
  noFill ();
  rect (0,0,50,50);
  ang+= TWO_PI/360;
}