/**
 * Rotate. 
 * 
 * Rotating a square around the Z axis. To get the results
 * you expect, send the rotate function angle parameters that are
 * values between 0 and PI*2 (TWO_PI which is roughly 6.28). If you prefer to 
 * think about angles as degrees (0-360), you can use the radians() 
 * method to convert your values. For example: scale(radians(90))
 * is identical to the statement scale(PI/2). 
 */

float angle;
float jitter;

void setup() {
  size(640, 360);
  noStroke();
  fill(255);
  rectMode(CENTER);
}

void draw() {
  background(51);

  // during even-numbered seconds (0, 2, 4, 6...)
  if (second() % 2 == 0) {  
    jitter = random(-0.1, 0.1);
  }
  angle = angle + jitter;
  float c = cos(angle);
  translate(width/2, height/2);
  rotate(c);
  rect(0, 0, 180, 180);   
}