Obs-studio: B-Frame reference mode flag in ffmpeg nvenc is bugged and doesn't work.

Created on 4 Feb 2020 · 17Comments · Source: obsproject/obs-studio

Expected Behavior

This flag enables B-frames as reference and it should improve picture quality. It's basically b-pyramid from x264.

Current Behavior

When -b_ref_mode 2 is enabled my recording starts and after 2 seconds drop's bitrate to zero. If I try to stop recording, ui button just freezes and I have to kill obs process.

Steps to Reproduce

I'm using Xaymar's plugin for FFmpeg Nvenc that includes all the flags with easy to use UI.
https://obsproject.com/forum/resources/ffmpeg-encoders-for-obs-studio.826/
After installing just select the encoder in OBS Output
You can leave everything at default and just enable this option

Additional information

Enabling this flag -b_ref_mode 2 in Custom Output (FFmpeg)
should result in the same bug.

Dependency Issue Fix available

Source

maginhart

Most helpful comment

The aforementioned fix for "b_ref_mode=middle" was integrated into Zeranoe's latest Windows nightly. The change does appear to fix the issue and ffmpeg no longer produces errors.

For you OBS Studio devs, I thought I'd be cheeky and copy the .dll files from "/ffmpeg/bin/" (from a "shared" version of the ffmpeg nightly), into the /obs-studio/bin/64bit/ folder, just to see what happens. Interestingly, I can record h264/h265 using "b_ref_mode=middle" with ffmpeg custom output and Xaymar's StudioFX gui, with no problems.

*Hopefully that means little-to-no work will need to be done by yall, once the next full ffmpeg version is released and yall integrate that into a future OBS Studio version.

Thank you all again for your help!

*edited for clarity, Fenrirthviti. I didn't mean to imply that yall should do anything until an official release comes out, if I did.

cs9kc on 4 May 2020

👍2

All 17 comments

Please do check with ffmpeg custom output to confirm the issue is in obs or in the plugin , even if you expect the issue to be there. For issues with Xaymar's plugins, you should post to his repo. Not here.

pkviet on 4 Feb 2020

Xaymar answered that question. But I will check Custom Output (FFmpeg) later today.

maginhart on 4 Feb 2020

If Xaymar's answer is correct, it's an ffmpeg issue.
You should then test with ffmpeg cli, with version 4 and then master to see if the bug is fixed.
It doesn't seem to be an issue with obs at any rate.

pkviet on 4 Feb 2020

For further reference: https://trac.ffmpeg.org/ticket/7303

pkviet on 4 Feb 2020

👍1

I think we should reopen this. It is important. Technically this issue is simple to solve as it is off by one (usually 3600) error. And is only present when you stream not when you output to file.

ValZapod on 22 Apr 2020

? what's your simple fix ? @ValZapod
If there is one, feel free to PR. But it seemed to be on FFmpeg side rather than OBS.

pkviet on 22 Apr 2020

Was fixed in https://github.com/FFmpeg/FFmpeg/commit/aaadf0dce8fa7b3b5073089498a84e758ceb975a Please reopen ASAP!

ValZapod on 1 May 2020

the fix is upstream so it'll come to obs when we update the ffmpeg dll. Let's keep this closed.

pkviet on 2 May 2020

But it is very important, really. Why you think it is not?

ValZapod on 2 May 2020

There's not really a point to re-opening this if the issue is already fixed. All that needs to happen is to release a new OBS package with an updated FFmpeg, which we can do with the next release if we need. Personally I don't think this is urgent enough to warrant an immediate hotfix.

dodgepong on 2 May 2020

@dodgepong I strongly disagree with you but okay. Just remember Twitch already supported it before this patch and now it may be broken because they applied some fix, you know.

ValZapod on 2 May 2020

ValZapod, ffmpeg -just now- put in a fix that won't make it into an official release for another month or two. OBS Studio -then- needs to take that and integrate it into their program, which will take more time.

The use-case for b-frames as reference for recording is so hilariously microscopic, that we can live without it until OBS Studio developers can get an official ffmpeg release and get to work themselves.

Please don't clutter up this ticket any more.

cs9kc on 2 May 2020

The aforementioned fix for "b_ref_mode=middle" was integrated into Zeranoe's latest Windows nightly. The change does appear to fix the issue and ffmpeg no longer produces errors.

*Hopefully that means little-to-no work will need to be done by yall, once the next full ffmpeg version is released and yall integrate that into a future OBS Studio version.

Thank you all again for your help!

*edited for clarity, Fenrirthviti. I didn't mean to imply that yall should do anything until an official release comes out, if I did.

cs9kc on 4 May 2020

👍2

Not exactly. You're correct there's not much work that needs to be done to link, but you've only tested a single aspect: custom ffmpeg output. We use ffmpeg for things like media source, image source, etc. as well, so we'd need to fully test against those. Also, I am not comfortable shipping a nightly unstable build.

Once the fix makes it to a stable release, we will look in to updating, but you're more than welcome to replace the shared library files with the nightly for your own use cases!

Fenrirthviti on 4 May 2020

Any updates? BTW, what is the status of this commit? https://github.com/obsproject/obs-studio/commit/f27a00d266a380312025fa09878023592d9bb612 Should we reland it??? I think we should.

What about this, BTW? https://github.com/FFmpeg/FFmpeg/commit/9ce7de903882793749abc68eeb29813a19f50de5 Can you comment @BtbN?

This leaves open an issue with VFR content, for which some more complex
logic might be needed.

ValZapod on 7 Jun 2020

With VFR content, it's virtually impossible to properly do timestamp calculation when re-ordering is required, given that you can't possibly know the largest possible gap.
x264 also leaves that unsolved, so I didn't bother implementing it, since it's a non-issue in practice.