Bubble without objects

float x;
float y;

void setup(){
 size(640,480);
 x = width/2;
 y = height/2;
}

void draw(){
  background(255);
  display();
  ascend();
  top();
}

void display(){
  stroke(0);
  strokeWeight(2);
  fill(127);
  ellipse(x, y, 50, 50);
}

void ascend(){
  y--;
}

void top(){
  if( y <= -20 ) y = height+20;
}