Synopsis
		sfgradient < vel.rsf > time.rsf s1=o[0] s2=o[1] s3=o[2] v0=1. b=0. s=0.

Parameters
		float b=0.   float s=0.   float s1=o[0]   float s2=o[1]   float s3=o[2]   float v0=1.