float x = 0;
float y = 50;
float vx = 2;
float vy = 2;

void setup() {
  size(200, 100);
  stroke(255);
  background(0);
  smooth();
}

void draw() {
  background(0);
  ellipse(x, y, 10, 10);

  if (x > width) {
    vx = -vx;
  }
  if (x < 0) {
    vx = -vx;
  }
  x = x + vx;

  if (y > height) {
    vy = -vy;
  }
  if (y < 0) {
    vy = -vy;
  }
  y = y + vy;
}