smooth VEX function
Computes a number between zero and one.
Contexts: image3d, chop, cop, pop, sop, surface, displace, fog, light, shadow, photon, cvex

float smooth(float value1, float value2, float amount)

float smooth(float value1, float value2, float amount, float rolloff)
Computes a number between zero and one. Returns 0 if the amount passed in is less than or equal to value1, 1 if the amount is greater than or equal to value2.
If the amount is between value1 and value2, a smooth (easin/easeout) interpolation is computed. If a rolloff is specified, the inflection point of the blend will be shifted.
If the rolloff is greater than 1, the shift will be to the right, if the rolloff is less than 1 (and greater than 0), the shift will be to the left.