undefined | Better HN

0 pointsbrigade3y ago0 comments

A surprising number of mp4 files are missing the ctts atom that contains pts since it’s sort-of optional outside the Apple stack (like ffmpeg generated such files from raw h264, which is probably the actual motivation for finally fixing this.) This should allow you to not generate or fix such files.

It’s sort-of optional for most playback stacks because they leave frame reordering to individual decoders as a codec-specific implementation detail, but Apple’s stack actually cares about frame-accurate random access so it relies on the codec-independent container timestamps.

The inaccurate seeking you get without container pts is okay for playback but it falls apart with editing or stuff like av1an.

0 comments

2 comments · 1 top-level

scottlamb3y ago· 1 in thread

Thanks, that makes more sense! Raw (Annex B) h264 in particular isn't exactly an ideal media format but I've definitely gone through it while debugging stuff and can see how you'd need this plugin to get from it to a correctly muxed file with B frames.

I have a bit of a love-hate relationship with ffmpeg. Love because it does things nothing else does (e.g. has a main-profile software H.264 decoder) and is so comprehensive, hate because code quality varies (lots of long-unfixed bugs/quirks in trac) and documentation/API/CLI experience is poor. E.g., I wish the documentation for this new bsf explained what you just said.

j1elo3y ago

You're right, this comment should just be added to the file header and maybe the docs about H.264 processing in FFmpeg; otherwise this comment will get lost in a couple hours and the knowledge about all this will only be known by the people that made the change and not many more...

I really wonder why it costs people so much to write the whys of everything technical they do in at least a source code comment, but it is indeed an everyday's uphill battle.

j / k navigate · click thread line to collapse