We employ two equivalent subscripting schemes for
referring to an element of the two-dimensional image, .
Firstly, with two subscripts, *b*_{px,py} refers to the element
that lies *p*_{x} increments along the *x*-axis, and *p*_{y}
increments along the *y*-axis.
Ranges of *p*_{x}and *p*_{y} are given by
, and
respectively.
Helical coordinates suggest an alternative subscripting scheme:
We can use a single subscript, *p*_{h}=*p*_{x} + *p*_{y} *N*_{x},
such that *b*_{px,py} = *b*_{ph} and the range of *p*_{h} is given by
.Moreover, if we impose helical boundary conditions, we can
treat as a one-dimensional function of subscript *p*_{h}.

9/5/2000