Why does Youtube player stretch MPEG1/2 video not H.264?

[parumaru]

Before uploading the video, I have this MKV file,



so I used ffmpeg to convert it to MP4 and hard code the ass subtitle into it.

Now my question is

When I simply converting the video without a sub and uploaded it, so the video retained it original vcodec,

ffmpeg -i "input.mkv" -c:v copy -c:a aac "output.mp4"

The video (vcodec MPEG-1/2) is played with 16:9 ratio on Youtube.

However, when I put a sub on it, so the video need to be re-encoded. (In this case, I chose to preserve the quality by making it lossless.)

ffmpeg -i "input.mkv" -preset veryslow -crf 0 -c:a aac -vf "ass=sub.ass" "output.mp4"

Now the subbed video has vcodec of H.264.

However, Youtube player still keep its original aspect of 720x480 and there is a black bar on the side of the video.

Therefore, I am curious why is it such a case? even though VLC told me that both has the same resolution of 720x480.

-----------------------------------------------------------------------------------------------------------------------------------------------

I just tried adjust the one with the sub with Youtube recommended scale of 854x480.

ffmpeg -i "input.mkv" -preset veryslow -crf 0 -s 854x480 -c:a aac -vf "ass=sub.ass" "output.mp4"

and confirmed that it has this resolution with VLC, but Youtube still play with the original 720x480.

-----------------------------------------------------------------------------------------------------------------------------------------------

UPDATE:

I followed the suggestion from these kind users, so I wanna update my result here ^ ^

Try -vf "ass=sub.ass,setsar=40:33"

VLC: vcodec=720x480
Youtube: 854 x 470
File Size: same the the 2nd video

-vf "ass=sub.ass,scale=848:480,setsar=1:1"

VLC: vcodec=848x480
Youtube: 848x480
BUT create the biggest file size

"-aspect 16:9"

VLC: vcodec=720x480
Youtube: 854 x 480
File Size: same the the 2nd video

[sneaker]

DVDs are anamorphic. Try -vf "ass=sub.ass,setsar=40:33"

or:
-vf "ass=sub.ass,scale=848:480,setsar=1:1"

[jagabo]

Or "-aspect 16:9"

[parumaru]

DVDs are anamorphic. Try -vf "ass=sub.ass,setsar=40:33"

or:
-vf "ass=sub.ass,scale=848:480,setsar=1:1"

Thank you for the reply!
so does this mean that Youtube recognised MPEG1/2 as anamorphic -> Instantly use 16:9 with it?
Sorry for so many questions, I really want to try understand the cause of it.

[sneaker]

I don't know. Aspect ratio information can be in MPEG stream and/or in mkv container elements. Possible that Youtube reads those.

[jagabo]

so does this mean that Youtube recognised MPEG1/2 as anamorphic -> Instantly use 16:9 with it?

Probably. Because all DVDs and most video captures are anamorphic. 720x480 (3:2) is used for both 16:9 and 4:3 NTSC DVD, or with PAL DVD 720x576 (5:4) is used for both.

[manono]

so does this mean that Youtube recognised MPEG1/2 as anamorphic -> Instantly use 16:9 with it?

Yes, these days YouTube resizes taking into account the DAR if your source is some kind of an MPEG.

I have no idea how it handles that for H.264 sources. Since they're going to resize anyway, you'd be better off resizing them yourself before the upload.

Instantly use 16:9 with it?

They're not just using the DAR to stretch it during playback. It's been resized to 854x480 or somesuch. Play the video and right-click the screen followed by hitting 'Stats For Nerds' to see the reencoded resolution of the video.

[parumaru]

They're not just using the DAR to stretch it during playback. It's been resized to 854x480 or somesuch. Play the video and right-click the screen followed by hitting 'Stats For Nerds' to see the reencoded resolution of the video.

Thank you for your reply. I did what you said and
The first video with no sub;
- VLC: MPEG-1/2 (mpgv) 720x480
- Youtube: 854x480

The 2nd video with sub
-VLC: H264-MPEG-4 AVC (part 10) (avc1)
-Youtube:720x480

Does this mean Youtube reencoded the first video to this size?
Sorry for so many questions, I really wanna try learning more about this!!

[jagabo]

The first video with no sub;
- VLC: MPEG-1/2 (mpgv) 720x480
- Youtube: 854x480

This video is encoded with the MPEG 16:9 display aspect ratio (DAR) flag. Youtube is obeying the DAR flag and resizing to 854x480. 480 * 16 / 9 = 853.333... but YV12 video must use even frame sizes. so 853.333 rounds up to 854. Personally, I would round to 852 (mod 4) or 856 (mod 8) to avoid a mod 2 frame size -- and accept the slightly larger aspect ratio error.

The 2nd video with sub
-VLC: H264-MPEG-4 AVC (part 10) (avc1)
-Youtube:720x480

When you encoded this video you lost the DAR flag. Encode with a SAR of 32:27 and youtube might resize it correctly. The x264 command line option is "--sar 32:27". I don't know how you specify it in ffmpeg.

The general equation for video aspect ratios:

DAR = FAR * SAR

DAR = display aspect ratio, the shape of the final picture that's displayed
FAR = frame aspect ratio, the ratio of the frame width:height
SAR = sampling aspect ratio, the relative distance between pixels, horizontally:vertically

Note that SAR is sometimes called pixel aspect ratio (PAR), the shape of individual pixels. But theoretically, pixels are dimensionless points, not little boxes. So the term SAR is more accurate.

For a 720x480 16:9 DVD the MPEG data specifies the DAR, not the SAR:

DAR = FAR * SAR
DAR / FAR = SAR
(16/9) / (720/480) = SAR
(16 * 480) / (9 * 720) = SAR
7680 / 6480 = SAR
(divide both by 240)
32 / 27 = SAR
32:27 = SAR

[parumaru]

Thank you so much for taking your time to explain this to me.
There are so much thing that I need to be learnt!
I finally understand the actual meaning of Anamorphic video just now OTL, sorry for being so stupid...

This video is encoded with the MPEG 16:9 display aspect ratio (DAR) flag.

Ohhh Does this mean that original video is intentionally required to be seen in 16:9, which is why there are 16:9 DAR in the first place?

When you encoded this video you lost the DAR flag.

Hence I should “unsqueezed” the 2nd video by using -aspect 16:9 to make it 854x480 on Youtube right?

--------------------------------------------------------------------------------------------------------------------------------

I also checked the original specification of the video from the website and it said

16:9LB/片面2層/リニアPCM => 16:9LB/One side 2 layers????/Linear PCM

For 16:9LB, so does this mean that when the video is played on 4:3 screen, there will be a letter box?

--------------------------------------------------------------------------------------------------------------------------------

I also uploaded both videos for you to see

A: 720x480

https://youtu.be/rWKPiyIyA5g

B: 854x480 using -> -aspect 16:9 command

https://youtu.be/NTrzElMcIaM

My apologies for so many questions guys! I'm trying hard to understand this !!

[jagabo]

I finally understand the actual meaning of Anamorphic video

It comes from the original usage in film:
https://en.wikipedia.org/wiki/Anamorphic_format

This video is encoded with the MPEG 16:9 display aspect ratio (DAR) flag.

Ohhh Does this mean that original video is intentionally required to be seen in 16:9, which is why there are 16:9 DAR in the first place?

Yes.

When you encoded this video you lost the DAR flag.

Hence I should “unsqueezed” the 2nd video by using -aspect 16:9 to make it 854x480 on Youtube right?

Yes. Or maybe flag the DAR/SAR before uploading. I don't know if youtube will respond to the flags in formats other than MPG.

--------------------------------------------------------------------------------------------------------------------------------

I also checked the original specification of the video from the website and it said

16:9LB/片面2層/リニアPCM => 16:9LB/One side 2 layers????/Linear PCM

For 16:9LB, so does this mean that when the video is played on 4:3 screen, there will be a letter box?

--------------------------------------------------------------------------------------------------------------------------------

I don't know what that's saying.

I also uploaded both videos for you to see

A: 720x480

https://youtu.be/rWKPiyIyA5g

B: 854x480 using -> -aspect 16:9 command

https://youtu.be/NTrzElMcIaM

My apologies for so many questions guys! I'm trying hard to understand this !!

That would match the earlier explanations.

[manono]

For 16:9LB, so does this mean that when the video is played on 4:3 screen, there will be a letter box?

Yes, that's what it means. It might have also been set up as 16:9 PS (Pan and Scan) meaning when viewed on 4:3 televisions it would be cropped on the sides and fill the screen.

[parumaru]

@jagabo @ @manono

Thank you so much for your help

Now I am encountering another problem T-T

so the video looks perfect on VLC !!!, but the quality was not as good on Youtube (even though I already selected 480p).

The color is faded and the subtitle was not as sharp (the stroke of the font is slightly blurr)



I guess this is because I go over the bitrate too much? (I guess from making the video lossless....)

since the recommended for 480p is 2.5 Mbps..... or do you think there are something else?

https://support.google.com/youtube/answer/1722171?hl=en-GB

[manono]

but the quality was not as good on Youtube...

It never is as YouTube reencodes everything using low bitrates. Download it and compare the bitrate YouTube used with your source video. And the more complex the video (the more movement), the worse it will look on YouTube. One way to get around that is to do some filtering to make it more compressible. It might look worse on your computer but better on YouTube.

[parumaru]

Download it and compare the bitrate YouTube used with your source video. And the more complex the video (the more movement), the worse it will look on YouTube. One way to get around that is to do some filtering to make it more compressible. It might look worse on your computer but better on YouTube.

Thank you for your suggestion. I did what you said and my 480p video was only give only 500-600 kb/s.

However, I found a way to get around this!!!! so I will share this here ^ ^ (in case someone who need help like me found this post lol)

Basically, you just encode video with a higher resolution, so I just go with 1280x720 and everything looks perfect like VLC! (I mean not as good as, but still...)

The colour no longer faded and the video was give a bitrate of 1900ish kb/s.

I think you can get a higher bitrate by making the video 1080p as well, but I think I'm satisfied with mine.

Now I can finally work on something else aaaaaaa