File size - YCbCr 422 vs YCbCr 420

[nsesh]

Hi,
Thanks for wonderful documentation.

I'm having some doubt on YCbCr 422 vs YCbCr 420 file size. As per my understanding in case of 4:2:0, the file size reduction is more than 50% compared to 4:4:4. In case of 1280x720 the alternate lines will not take Cb or Cr. Am I right?
If yes then 'Y' pixels = 1280x720x8 bits
CbCr pixels = 640x360x8 bits
Total Bytes = 1152000
Size = 115GBytes.

For 4:2:2, the alternate lines do take chroma samples and hence
Y pixels = 1280x720x8 bits
CbCr pixels = 640x720x8
Total Bytes = 139GBytes

Kindly correct me.

[leandromoreira]

Hi @nsesh how are you?

Let me put a summary I saw on FFmpeg docs:

• 4:4:4: There is no chroma subsampling
• 4:2:2: Cb' and Cr' are sampled at half the sample rate of luma, the horizontal chroma resolution is halved
• 4:2:0: Cb' and Cr' are each subsampled at a factor of two, both horizontally and vertically

And the Wiki's drawing:

Both might help you to visualize the difference.

[nsesh]

@leandromoreira, thanks for your reply. Yes, if you see in the diagram you shared, it is clear that for 4:2:0, both horizontal and vertical resolution are half. Hence, in my calculation the resolution for CbCr was halved i.e. 640x360. In case of 4:2:2, I kept it as 640x720.

Hence, wanted to clarify if my understanding is correct and my calculation of file is correct or not.

As per the section "Chroma Subsampling", it is mentioned 12bits for 4:2:0 and file size is mentioned half of original size. We can't just sum up the ratio's and say that file size is half of original. In fact for 4:2:0, the average bits are 10 and not 12. Hence this confusion, but please correct me if I'm still wrong :)

[leandromoreira]

It's really hard because it's conceptual, and the mapping examples given are only theoretical and for illustration.

But to calculate required bandwidth factor relative to 4:4:4, one needs to sum all the factors and divide the result by 12.

• 4:2:0 = 6/12 = 0.5 = 50%
• 4:2:2 = 8/12 = 0.666 = 66.66%

Let's try to put it to test.

# cut a small chunk of video
ffmpeg -i small_bunny_1080p_60fps.mp4 -t 00:00:02 2s.mp4

# Now let's generate rawvideo format with each of the proper format

ffmpeg -i 2s.mp4 -c:v rawvideo -pix_fmt yuv444p yuv444p.yuv
ffmpeg -i 2s.mp4 -c:v rawvideo -pix_fmt yuv420p yuv420p.yuv
ffmpeg -i 2s.mp4 -c:v rawvideo -pix_fmt yuv422p yuv422p.yuv

ls -lah *yuv

-rw-r--r--  1 leandro.moreira  wheel   356M Apr 28 23:19 yuv420p.yuv
-rw-r--r--  1 leandro.moreira  wheel   475M Apr 28 23:21 yuv422p.yuv
-rw-r--r--  1 leandro.moreira  wheel   712M Apr 28 23:20 yuv444p.yuv

712M = 100%
356M = 50%
475M = 66.6%

[leandromoreira]

A python view of a single frame can be seen as:

# A single frame in KB 


# full cr and cb (4:4:4)
y_plane = 1280*720*8
cr_plane = 1280*720*8
cb_plane = 1280*720*8
print((y_plane+cr_plane+cb_plane) / 8 / 1024)


# half cr and cb (4:2:0)
y_plane = 1280*720*8
cr_plane = 640*360*8
cb_plane = 640*360*8
print((y_plane+cr_plane+cb_plane) / 8 / 1024)

[leandromoreira]

• 4:2:0 - For each 4 pixes in Y we have only 1 for Cb and 1 for Cr = 6 pixels ( where it should be 4+4+4)
• 4:2:2 - For each 4 pixes in Y we have only 2 for Cb and 2 for Cr = 8 pixels (where it should be 4+4+4)

[leandromoreira]

I hope it can help you to understand @nsesh

[nsesh]

@leandromoreira, thanks so much. The above pic and python view of single frame help me to understand the mistake I did.

For 4:2:0
'Y' pixels = 1280x720x8 bits
CbCr pixels = 640x360x8 bits + 640x360x8 bits
Total Bytes = 139GBytes (~50%)

For 4:2:2
Y pixels = 1280x720x8 bits
CbCr pixels = 640x720x8 bits + 640x720x8 bits
Total Bytes = 185GBytes (~66%)

Once again thanks very much for clearly explaining.

[nsesh]

@leandromoreira, just want to understand on 4:1:1

Y pixels = 1280x720x8 bits
CbCr pixels = 320x720x8 bits + 320x720x8 bits
Total Bytes = 139GBytes (50%).

Is my understanding correct?

If you can get the similar picture as explained above that will be helpful.

[nsesh]

got understanding from above diagram and hence closing this issue. Once again thanks very much.

[leandromoreira]

I'll leave this open as it can help other people!