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Gradient noise can be created using the same method described above, but with 
a different filter.
The technique described above can also create noise
that is not aligned on a lattice.  To create sparse convolution noise 
[58] or spot noise [97], instead of drawing 
the entire point-sampled texture at once, draw one texel and one copy of the 
filter at a time for each random location.
David Blythe
1999-08-06