How to get ffmpeg to split on keyframe?

[yetanotherlogin]

Hello

I use the following commands to split an MP4 file:

ffmpeg -ss 00:00:00 -i input.mp4 -t 1:00:00 -c copy input.part1.mp4
ffmpeg -ss 00:59:50 -i input.mp4 -c copy input.part2.mp4

Problem is, when viewing the parts in VLC, the second part doesn't seem to start at a keyframe: The picture is crappy, and the viewer has to wait a few seconds until it looks OK; I assume ffmpeg doesn't split at keyframes.

Is there a way to tell ffmpeg to split on the closest keyframe a few seconds before?

Thank you.

[poisondeathray]

The current and common problem is ffmpeg doesn't distinguish between non -IDR keyframes. It thinks all "i" frames are keyframes, but only "I" (or IDR) frames are true keyframes. So if you have open GOP's it's not possible to cut with ffmpeg without errors

[yetanotherlogin]

Thanks. What's GOP?

[poisondeathray]

Thanks. What's GOP?

Group of Pictures , used in temporal compression
https://en.wikipedia.org/wiki/Group_of_pictures

[yetanotherlogin]

Thanks. So ffmpeg isn't the right tool to split MP4 files?

I like it because it's easy to use since it has a CLI, which is why I use it instead of GUI-based alternatives such as My MP4Box.

[poisondeathray]

If that is the reason, the problem is your original video . You won't get errors from splitting with closed GOPs . ffmpeg will automatically "jump" to the nearest I-frame (or i-frame) it doesn't distinguish between them as mentioned earlier . If GOP's are closed, the all I frames are IDR frames or true keyframes

Another option is mp4box, but you will have the same problems if you split on non IDR frames

[yetanotherlogin]

I actually use ffmpeg to split 1h+ MP4 files into multiple parts, as downloaded from eg. YouTube.

I don't know what else to use to have ffmpeg split at GOPs and get clean splits.

[poisondeathray]

If these are from youtube, AFAIK, they use closed GOP settings (at least they used to, unless something has changed recently) - so that shouldn't be the problem

Then it might be some other ffmpeg bug. Maybe try mp4box instead

[yetanotherlogin]

OK, I'll go back to mp4box instead. Thanks.

[poisondeathray]

Why did you switch to ffmpeg in the first place ? What were you using in mp4box to split ?

[yetanotherlogin]

Because ffmpeg offers more options.

[Budman1]

Near as I remember, it depends on where you place the -ss parameter. I believe before your input seeks to the exact frame and after your input seeks to key frame.

https://trac.ffmpeg.org/wiki/Seeking

[yetanotherlogin]

Thanks. I'll give it a try.