The rate (in [0,1]) at which the signal decays.
This term indicates how much the signal decreases at a certain
distance in x. At the distance defined by the pdist term
(see circularProjection), the signal's value will
correspond to the initial signal multiplied by 1 - decay.
shape
A parameter (>=1) of a Weibull function. When shape=1
the Weibull decay follows an exponential decay. When shape>1
the function is first convex, then concave with an inflection point.