The billow fractal is a slight modification of the fbm()
fractal. Before
adding the new layer onto the last, the new layer is modified by taking the
absolute value, multiplying by 2, and subtracting one. The result is that the
new value will not contain negative values and so will always add on top of
the old values. This function is intended to be used in conjunction with
fracture()
billow(base, new, strength, ...)
The prior values to modify
The new values to modify base
with
A value to modify new
with before applying it to base
ignored