1. A computer-implemented method of forming an optical flow vector describing estimated displacement of a subject pixel from an initial image to a subsequent image, the method comprising: obtaining four input color vectors respectively associated with four input pixels at four known input positions of the initial image; obtaining a subject color vector associated with the subject pixel, the subject pixel residing at a known subject position of the subsequent image; performing inverse bilinear interpolation for the subject color vector with respect to the four input color vectors to obtain an estimated position; and forming the optical flow vector from the estimated position to the known subject position.