The subband index within its resolution level. This value uniquely identifies a subband within a resolution level and a decomposition level within it. Note that only leaf elements represent "real" subbands, while node elements represent only intermediate stages.

       <p>It is defined recursively. The root node gets a value of 0. For a
       given node, with a subband index 'b', its LL descendant gets 4*b, its
       HL descendant 4*b+1, its LH descendant 4*b+2, and its HH descendant
       4*b+3, for their subband indexes.</p>