glitch in RubyMotion (memo)

glitch_motion.rb

  def glitchnize(image_data)
    bytes = image_data.bytes
    length = image_data.length
    d = Pointer.new(:uchar, length)
    length.times do |i|
      c = bytes[i]
      if c == 42 && rand > 0.8
        d[i] = rand(255)
      else
        d[i] = c
      end
    end
    NSData.dataWithBytes(d, length: length)
  end