Bitmap.savePPM = function(self, filename)
  local fp = io.open(filename, "wb")
  if fp == nil then return false end
  fp:write(string.format("P6\n%d %d\n%d\n", self.width, self.height, 255))
  for y = 1, self.height do
    for x = 1, self.width do
      local pix = self.pixels[y][x]
      fp:write(string.char(pix[1]), string.char(pix[2]), string.char(pix[3]))
    end
  end
  fp:close()
  return true
end