def getGraphicsControlExt(self, duration=0.1, dispose=2):
        """ getGraphicsControlExt(duration=0.1, dispose=2)

        Graphics Control Extension. A sort of header at the start of
        each image. Specifies duration and transparancy.

        Dispose
        -------
          * 0 - No disposal specified.
          * 1 - Do not dispose. The graphic is to be left in place.
          * 2 - Restore to background color. The area used by the graphic
            must be restored to the background color.
          * 3 - Restore to previous. The decoder is required to restore the
            area overwritten by the graphic with what was there prior to
            rendering the graphic.
          * 4-7 -To be defined.

        """

        bb = '\x21\xF9\x04'
        bb += chr((dispose & 3) << 2)  # low bit 1 == transparency,
        # 2nd bit 1 == user input , next 3 bits, the low two of which are used,
        # are dispose.
        bb += intToBin(int(duration * 100))  # in 100th of seconds
        bb += '\x00'  # no transparant color
        bb += '\x00'  # end
        return bb