float y = 50.0;
float speed = 1.0;
float radius = 15.0;
int direction = 1;

void setup() {
  size(100, 100);
  smooth();
  noStroke();
  ellipseMode(RADIUS);
}

void draw() {
  fill(0, 12);
  rect(0, 0, width, height);
  fill(255);
  ellipse(33, y, radius, radius);
  y += speed * direction;
  if ((y > height-radius) || (y < radius)) {
    direction = -direction;
  }
}