void draw() {
  /* ... */

  // Doesn't work for moving left (negative
  // on the x axis) or up (negative on the
  // y axis).
  //heroX = heroX % width;
  //heroY = heroY % height;

  // Handles negative and positive.
  heroX = floorMod(heroX, width);
  heroY = floorMod(heroY, height);

  // Draw avatar.
  fill(0.0, 127.0, 255.0);
}

float floorMod(float a, float b) {
  return a - b * floor(a / b);
}