int cx, cy, r;
boolean fire;

void setup() {
  size(300,300);
  colorMode(HSB,360,100,100,100);
  ellipseMode(RADIUS);

  frameRate(15);
  background(0,0,0); // black

  cx = width  / 2;
  cy = height / 2;
  fire = false;
}

void draw() {
  clearToWhite();
  if( fire ) {
    r += 10;
    drawFire();
    if( r > width / 2 + 30 ) {
      fire = false; 
    }
  } else {
    if( random(1) < 0.07 ) {
      fire = true;
      r = 0;
    }
  }
}

void clearToWhite() {
  fill(0,0,0,50); // white
  noStroke();
  rect(0,0,width,height);
}

void drawFire() {
  noStroke();
  fill(random(360),50,100,70);
  for( float angle = 0 ; angle < 360 ; angle += 10 ) {
    float rad = 2 * PI * angle / 360.0;
    float dx = r *  cos(rad);
    float dy = r * -sin(rad);
    ellipse(cx+dx,cy+dy,5,5);
  }
}