each [] is a bit.  most significant on left, least significant on right

       ------- Y --------   ------- X --------
byte2  [--][--][+81][-81] | [-81][+81][--][--]
byte1  [+3][-3][-27][+27] | [-27][+27][-3][+3]
byte0  [+1][-1][ -9][ +9] | [ -9][ +9][-1][+1]