for y = 0, height - 1 do
  for x = 0, width - 1 do
    r1, g1, b1 = get_rgb(x, y)
    r2, g2, b2 = get_rgb(width - 1 - x, y)
    r = (r1 + r2) / 2
    g = (g1 + g2) / 2
    b = (b1 + b2) / 2
    set_rgb(x, y, r, g, b)
  end
  progress(y / height)
end
