### 14-6 Other Space-Filling
Curves

As was mentioned, Peano was first, in 1890,
to discover a space-filling curve. The many variations discovered since then
are often called "Peano curves." One interesting variation of
Hilbert's curve was discovered by Eliakim Hastings Moore in 1900. It is
"cyclic" in the sense that the end point is one step away from the
starting point. The Peano curve of order 3, and the Moore curve of order 4, are shown in Figure 14-13. Moore's curve has an irregularity in that
the order 1 curve is up-right-down but
this shape does not appear in the higher-order curves. Except for this minor
exception, the algorithms for dealing with Moore's curve are very similar to
those for the Hilbert curve.

##### Figure 14-13. Peano
(left) and Moore (right) curves.

The Hilbert curve has been generalized to
arbitrary rectangles and to three and higher dimensions. The basic building
block for a 3-dimensional Hilbert curve is shown below. It hits all eight
points of a 2x2x2 cube. These and many other space-filling curves are discussed
in [Sagan].