In the forward wrap-around, cells 1 to C -1 are translated by the box size R, to belong to the α 4 group. In the reverse wrap-around, cells C + 1 to N cell are translated backward to become α 2 to α 4 groups in a reverse manner. (B) Repeating boxes. Illustration is for forward repeat. Vertical arrows mark boundary between image boxes. Due to the wrap-around, the repeating unit differs for each cell. The first repeating unit for cell C is shown as Repeat 1 (dashed arrow). r c and r f in this case are also shown.