
def scroll(clip, h=None, w=None, x_speed=0, y_speed=0,
           x_start=0, y_start=0, apply_to="mask"):
    """ Scrolls horizontally or vertically a clip, e.g. to make end
        credits """
    if h is None: h = clip.h
    if w is None: w = clip.w
    def f(gf,t):
        x = x_start+int(x_speed*t)
        y = y_start+ int(y_speed*t)
        return gf(t)[y:y+h, x:x+w]
    return clip.fl(f, apply_to = apply_to)
