Houdini 12 Reference   This cookbook refers to the old
expression language. For information on Python expressions, see
the Houdini Object Model section.  Choosing a random number seed
The rand(seed) function always returns the same number when
given the same seed value. This makes “random” values repeatable
between playbacks. However, you must be careful to supply a seed
value that varies.  The current frame number ($F) is usually a
good seed value, however:    • If you need random values for a
sub-frame process, use the fractional frame number $FF instead
of $F.    • If you need more than one random value, such as R,
G, and B values for a random color, give each call to rand a
different seed value by modifying the $F, otherwise each call
will return the same number and the color will be gray.
rand($F * 10)      rand($F * 100)    • If you need random values
that vary across geometry, use a variable like $PT (the current
point number) as the seed. If the value needs to be random
across the geometry a