def filter_arc(w, h, aspect):
    """Aspect ratio convertor. you must specify output size and source aspect ratio (as float)"""
    taspect = float(w)/h
    if abs(taspect - aspect) < 0.01:
        return "scale=%s:%s"%(w,h)
    if taspect > aspect: # pillarbox
        pt = 0
        ph = h
        pw = int (h*aspect)
        pl = int((w - pw)/2.0)
    else: # letterbox
        pl = 0
        pw = w
        ph = int(w * (1/aspect))
        pt = int((h - ph)/2.0)
    return "scale=%s:%s[out];[out]pad=%s:%s:%s:%s:black" % (pw,ph,w,h,pl,pt)