The two simplest rasters useful for the rectangular views are the row and column
rasters, which are actually the same raster but placed on the screen rotated 90
degrees from each other. These rasters are genuinely two-dimensional: they only
use two dimensions in the ZigZag structure to find cells to display.
The row/column raster starts from the cursor and moves along one ZigZag
dimension and place cells on the center column/row of the grid. After that
column/row is full, the raster starts from the cells placed and from each, moves
along the other dimension and places the cells found along the other dimension
there.
These rasters are called hard rasters: the arrangement of cells is fixed so that
there is only one possible path from the cursor to a cell to be placed in a
given position on the screen. If there is no cell in the structure along such a
path, then that location is left empty.
The converse of hard rasters are the soft rasters where there can be several
different paths for each cell of the on-screen grid. The point of soft rasters
is that since ZigZag structures are often relatively sparse in terms of
connections, the hard row and column rasters may show relatively few cells at a
time. Soft rasters are able to show more of the structure at the same time.
There are many different ways to define soft rasters for the rectangular grid.
One fairly useful way is to specify the soft raster so that all the cells that a
certain hard raster would show are shown, but additionally, if there is space
left, more cells are shown along the dimensions.