Python Zlib：第一次压缩后压缩尺寸减小

我正在尝试压缩OpenCV库捕获的图像帧。然而，在第一次迭代之后，压缩因子（diff_in_len/original_len）显著降低。为什么我会看到这种奇怪的行为

压缩系数从84%降至54%，然后稳定在约40%

我先酸洗框架，压缩它，然后去压缩，再去酸洗。我在任何一次迭代中都没有例外，这意味着压缩或初始帧没有任何错误，而且在所有迭代中原始的ByTestStream长度都是完全相同的。前两帧的压缩因子仍然比其他帧小

while True:
   check,frame=vc.read()
   print("Frame: {}".format(frame_no))

   bytestream = pickle.dumps(frame)
   print("Original len: {}".format(len(bytestream)))

   c_bytestream = zlib.compress(bytestream, 6)
   print("Compressed len: {}".format(len(c_bytestream)))

   u_bytestream = zlib.decompress(c_bytestream)

   compression_factor = ((len(bytestream)-len(c_bytestream))/len(bytestream))*100
   print("Compression factor: {}".format(compression_factor))

   try:
      cv2.imshow('client', pickle.loads(u_bytestream))
      print("Displayed")
   except Exception as e:
      print(e)

   cv2.waitKey(10)
   frame_no+=1
   print("++++++++++++++++++++++++=")

输出：

Frame: 0
Original len: 921764
Compressed len: 142641
Compression factor: 84.52521469703743
Displayed
++++++++++++++++++++++++=
Frame: 1
Original len: 921764
Compressed len: 421162
Compression factor: 54.30912901783971
Displayed
++++++++++++++++++++++++=
Frame: 2
Original len: 921764
Compressed len: 551544
Compression factor: 40.16429368037806
Displayed
++++++++++++++++++++++++=
Frame: 3
Original len: 921764
Compressed len: 545085
Compression factor: 40.86501534015214
Displayed
++++++++++++++++++++++++=
Frame: 4
Original len: 921764
Compressed len: 548948
Compression factor: 40.445927591010275
Displayed
++++++++++++++++++++++++=
Frame: 5
Original len: 921764
Compressed len: 548948
Compression factor: 40.445927591010275
Displayed
++++++++++++++++++++++++=
Frame: 6
Original len: 921764
Compressed len: 561018
Compression factor: 39.136481789264934
Displayed
++++++++++++++++++++++++=