Corrupted x264 video after Youtube upload

[ZeB]

Originally Posted by FranceBB

What's important here is not just the keyint = framerate and the reference frame set to 4, but also the --aud, so the unit access delimiters which might play a role.

I did some testing and the number of ref frames seems to be the problem. Default is 16.

My command line:
ffmpeg.exe -i "input.avs" -c:v libx264 -profile:v high -bf 2 -g 25 -refs X -preset veryslow -crf 18 -pix_fmt yuv420p -movflags faststart "output.mp4"

I just changed the number of ref frames.

15 ref frames, corruption:
https://youtu.be/ul4BkEtCtps

14 ref frames, corruption:
https://youtu.be/QLBlqoV6KxY

13 ref frames, corruption:
https://youtu.be/JarNcaMm5n0

12 ref frames, no corruption:
https://youtu.be/kHEHzIIWQ2g

So it seems like the h264 decoder that Youtube uses has problems with a higher number of ref frames. Isn't that weird, I would guess that they would use I high "quality" decoder? Can it be because of performance? I lower quality decoder can be faster?

Thank you for your help!

[rwill]

Originally Posted by ZeB

I did some testing and the number of ref frames seems to be the problem.
------
13 ref frames, corruption:
------
12 ref frames, no corruption:
------
So it seems like the h264 decoder that Youtube uses has problems with a higher number of ref frames. Isn't that weird, I would guess that they would use I high "quality" decoder? Can it be because of performance? I lower quality decoder can be faster?

Well, 2560x1440 has 14400 Macroblocks.

H.264 Level 5.2 supports a decoded picture buffer size of up to 184320 Macroblocks, so at your resolution 12.8 ref pictures.

The next level (6.0) supports a DPB of 696320 ( so 48.35 ref pictures of your resolution ).

Maybe your stream does not have the correct level set ? Or youtubes H.264 decoder initializes at Level 5.2 for whatever reason ? I don't know if they are using hardware or software decode - a hardware decoder could explain such behavior.

[ZeB]

Originally Posted by rwill

Well, 2560x1440 has 14400 Macroblocks.

H.264 Level 5.2 supports a decoded picture buffer size of up to 184320 Macroblocks, so at your resolution 12.8 ref pictures.

The next level (6.0) supports a DPB of 696320 ( so 48.35 ref pictures of your resolution ).

Maybe your stream does not have the correct level set ? Or youtubes H.264 decoder initializes at Level 5.2 for whatever reason ? I don't know if they are using hardware or software decode - a hardware decoder could explain such behavior.

That makes a lot of sense. The encode reports as "High@L6" in Mediainfo. So the stream is correct, right?

I guess that level 5.2 (12 ref frames) is the maximum recommended when encoding for Youtube.

Command line:
ffmpeg.exe -i "input.avs" -c:v libx264 -profile:v high -level:v 5.2 -bf 2 -g 25 -preset veryslow -crf 18 -pix_fmt yuv420p -movflags faststart "output.mp4"

This works without any corruption.

No mentions of levels in their own guide, just profile "high":
https://support.google.com/youtube/answer/1722171

Originally Posted by benwaggoner

You don't want to exceed the number of reference frames for your profile, level, and frame size listed here: https://en.wikipedia.org/wiki/Advanc..._Coding#Levels. Level 4.2 is generally quite safe to use these days. As said, if you want to go above 1080p, I'd suggest using x265 and HEVC.

If using x265, what settings is recommended? When searching information everyone just seems to recommend h264 instead. Youtube also recommends h264 in the "official" guide. It just mentions that h265 is supported.

[rwill]

Originally Posted by ZeB

That makes a lot of sense. The encode reports as "High@L6" in Mediainfo. So the stream is correct, right?

I guess that level 5.2 (12 ref frames) is the maximum recommended when encoding for Youtube.

Command line:
ffmpeg.exe -i "input.avs" -c:v libx264 -profile:v high -level:v 5.2 -bf 2 -g 25 -preset veryslow -crf 18 -pix_fmt yuv420p -movflags faststart "output.mp4"

This works without any corruption.

No mentions of levels in their own guide, just profile "high":
https://support.google.com/youtube/answer/1722171



If using x265, what settings is recommended? When searching information everyone just seems to recommend h264 instead. Youtube also recommends h264 in the "official" guide. It just mentions that h265 is supported.

Regarding the level the stream is correct then.

However if you look at the evolution of the H.264 standard here:

https://www.itu.int/itu-t/recommenda...aspx?rec=14659

in appendix A, Table A-1, the level limits...

H.264 (V10) (02/2016) does not list a level higher than 5.2, like, there is no level 6.0 or higher.
H.264 (V11) (10/2016) does list level 6 and higher.

So higher resolutions and higher DPB sizes got first added in October 2016.

So maybe decoders done before ~10/2016 are somewhat untested with Levels limits above 5.2, who knows.

I wonder what would happen if one uploads some crafted 1440p video which uses 16 ref frames and then only references ref frames whose size exceed the level 5.2 limits to youtube. Maybe one would be able to dump some hardware reserved memory of the youtube server ? maybe the job crashes the youtube decoder and breaks something ? I am eager to find out but at the same time am somewhat scared that Youtube might get grumpy when messing with their setup.

For x265 I'd just use "-preset slow" or slower and "-tune ssim" with some nice high bitrate or low crf. Oh and I'd specify a level. Maybe Level 5 High Tier like some UHD Bluray ?