docs: 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)