We will remind the reader that for a homogeneous random field, the mean
value η of the gray level in the picture is a constant independent of position.
Note that the autocorrelation function in (47) can be used to model pictures
with different amounts of correlation in the horizontal and vertical directions
by choosing different values of q and c
2
.
4.2.3 Examples of Sampling Using Orthonormal Expansions:
Fourier Sampling
Suppose that the region in the ;cy-plane over which the pictures are to be
sampled is a rectangle with sides A and B, and that over this region the ortho-
normal basis functions are of the form
<Pmn(x,y)
=
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