Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

FFMPEG from Zero to Hero

ffmpegfromzerotohero.com

Post reply on HN
Print view View original

61–70 of 140 posts

Re: FFMPEG from Zero to Hero

#61

Post by regus »

Confiks wrote: post #2

Might have come in handy while I was struggling to type out ffmpeg -i part0.mp4 -i part1.mp4 -filter_complex "[1:v]scale=1280:720:force_original_aspect_ratio=decrease,pad=1280:720:(ow-iw)/2:(oh-ih)/2[v1]; [0:v] [0:a] [v1] [1:a] concat=n=2:v=1:a=1 [v] [a]" -map "[v]" -map "[a]" out.mp4 to concatenate two videos with different sizes today. My relation with it is almost identical to my relation with any bash scripting m…

> My relation with it is almost identical to my relation with any bash scripting more than a one-liner: I have to relearn it every time I want to engage with it.

This is how I feel about jq, every time I want to parse some JSON I have to re-read their documentation. Their API is not very intuitive (at least to me).

Re: FFMPEG from Zero to Hero

#62

Post by chenxiaolong »

umaar wrote: post #57
Noctem wrote: post #53

Earlier quoted context omitted.

ffmpeg seeks accurately when transcoding. [1] Cutting on non-keyframes when stream copying results in broken video until the next keyframe. Handbrake does have a CLI. [2] I haven't used it and I'm not sure what advantage it might have over ffmpeg. I personally use mkvmerge or ffmpeg for my muxing/cutting and VapourSynth for encoding. [1] https://trac.ffmpeg.org/wiki/Seeking [2] https://handbrake.fr/docs/en/latest/cli…

I battled with this a lot with https://github.com/umaar/video-everyday and still haven't found a better solution. What I don't understand is, how can professional video editing tools trim accurately (and very quickly)? What are they doing differently to ffmpeg? If do things the "fast way" with ffmpeg, the exported video has random black frames which I think is related to the keyframe issue you mention. If I do things…

All of the proprietary tools I know of for doing frame-perfect cuts (VideoRedo, TMPGEnc, SolveigMM) work by determining (guessing?) the original encoding parameters and then only reencoding the first and last GOP. The rest of the video is just remuxed.

Re: FFMPEG from Zero to Hero

#64

Post by 91aintprime »

magicalhippo wrote: post #54
molticrystal wrote: post #30

Earlier quoted context omitted.

I never had to use it since I never had anything complicated enough, but next time you relearn things, I've heard good things about ffmpeg-python [0] , it supports multiple inputs, outputs, custom filters, etc. Their example from the readme: ffmpeg -i input.mp4 -i overlay.png -filter_complex "[0]trim=start_frame=10:end_frame=20[v0];\ [0]trim=start_frame=30:end_frame=40[v1];[v0][v1]concat=n=2[v2];[1]hflip[v3];\ [v2][v…

At this point I'd prefer it if ffmpeg included Lua and allowed you to pass Lua scripts, kinda like what ZFS did[1]. [1]: https://zfsonlinux.org/manpages/0.8.3/man8/zfs-program.8.htm...

I feel like it's better to embed the tool in a language, or many community supported languages than embed a programming language in the tool.

Re: FFMPEG from Zero to Hero

#66

Post by fireattack »

Noctem wrote: post #53
shazmosushi wrote: post #43

I love FFMPEG, but it has truly awful handling of timestamps by default. You can't easily extract a clip using an exact timestamp because it rounds to the nearest keyframe, which may be many seconds earlier. It's such a powerful command-line tool, but I find the user-interface far more difficult than it needs to be. In "git" terminology, there's so much plumbing but not enough porcelain I wish there was a scriptable…

ffmpeg seeks accurately when transcoding. [1] Cutting on non-keyframes when stream copying results in broken video until the next keyframe. Handbrake does have a CLI. [2] I haven't used it and I'm not sure what advantage it might have over ffmpeg. I personally use mkvmerge or ffmpeg for my muxing/cutting and VapourSynth for encoding. [1] https://trac.ffmpeg.org/wiki/Seeking [2] https://handbrake.fr/docs/en/latest/cli…

Yeah, it is literally not possible to not seek to keyframe if you are stream copying.

There are however, plenty of great software that can but at any frame and only re-encode the frames that are outside the whole GOP. Most of them are commercial though, I haven't find one that is free and good.

----

Also, seeking in FFMPEG in practice, is actually more complicated than the guide [1] you linked. Below is a note I keep for own reference for keyframe-copy. Hope someone will find it useful.

How to keyframe-cut video properly with FFMPEG

FFMPEG supports "input seeking" and "output seeking". The output seeking is very slow (it needs to decode the whole video until the timestamp of your -ss) so you want to avoid it if unnecessary.

However, while -ss (seek start) works fine with input seeking, "-to/-t" (seek ending) is somehow vastly inaccurate in input seeking for FFMPEG. It could be off by a few seconds, or sometimes straight up does not work (for some mepeg-ts files recorded from TV).

The best of the two worlds is to use input seeking for -ss and then output seeking for -to. However, this way, the timestamp will restart from 0 in output seeking. So instead of using -to, you should calculate -t (duration) yourself by subtracting -ss from -to, and use `-t duration` instead. Below is a quick Python script to do so.

https://gist.github.com/fireattack/9a100c5a200154937babd1823...

(You can also try to use -copyts to keep timestamp, but not recommended because it doesn't work if the video file has non-zero start time.)

Re: FFMPEG from Zero to Hero

#67

Post by magicalhippo »

91aintprime wrote: post #64
magicalhippo wrote: post #54

Earlier quoted context omitted.

At this point I'd prefer it if ffmpeg included Lua and allowed you to pass Lua scripts, kinda like what ZFS did[1]. [1]: https://zfsonlinux.org/manpages/0.8.3/man8/zfs-program.8.htm...

I feel like it's better to embed the tool in a language, or many community supported languages than embed a programming language in the tool.

My feeling is that they're already there with the command line (just look at the filter stuff). Might as well just go all the way and have something sane that's supported all over.

Re: FFMPEG from Zero to Hero

#70

Post by m463 »

shazmosushi wrote: post #43

I love FFMPEG, but it has truly awful handling of timestamps by default. You can't easily extract a clip using an exact timestamp because it rounds to the nearest keyframe, which may be many seconds earlier. It's such a powerful command-line tool, but I find the user-interface far more difficult than it needs to be. In "git" terminology, there's so much plumbing but not enough porcelain I wish there was a scriptable…

I'm not a video expert, but one thing I've noticed is that there seems to be a lot of discrepancies between video files and the way programs use them.

I think it might be differences between how the the container describes the video and the video itself, and which one is chosen as the truth during operations.

Post reply on HN