what is the outputof this line step(opticalFlow,I1,I2) ?

hi everyone i'm learning about motion estimation and i'm trying to detect the motion of cloud through optical flow between two sequential images of the sky. i want to use this line : [...] = step(opticalFlow,I1,I2) my question is : what i have to put instead of the points as output ?how many ?what kind of variable? i though to put [u ,v] put i get this error : ""Error using vision.OpticalFlow/step Too many input arguments; expected 1 (in addition to the object handle), got 2."" i hope that i've explained what i need well.