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Extracting frames from a video with ffmpeg very slow if not using jpeg (discuss.tchncs.de)

submitted 1 month ago by [email protected] to c/[email protected]

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I'm trying to extract the frames of a video as individual images but it's really slow, except when I'm using jpeg. The obvious issue with jpegs is the data loss from the compression, I want the images to be lossless. Extracting them as jpegs manages about 50-70 fps but as pngs it's only 4 fps and it seems to continue getting slower, after 1 minute of the 11 minute video it's only 3.5 fps.

I suspect it's because I'm doing this on an external 5tb hard drive, connected over USB 3.0 and the write speed can't keep up. So my idea was to use a different image format. I tried lossless jpeg xl and lossless webp but both of them are even slower, only managing to extract at about 0.5 fps or something. I have no idea why that's so slow, the files are a lot smaller than png, so it can't be because of the write speed.

I would appreciate it if anyone could help me with this.

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[–] [email protected] 6 points 1 month ago (1 children)

It probably becomes CPU limited with those other compression algorithms.

You could use something like atop to find the bottleneck.

[–] [email protected] 3 points 1 month ago (1 children)

Yeah, that's the probably the case for those. I looked at CPU usage when using webp and one CPU core was always at 100%. Even tough it seems to not be able to use multiple cores, that's still really slow, no? Or is that normal?

Also, my CPU is a Ryzen 5 3600, just to get an idea of what performance would be expected.

[–] [email protected] 2 points 1 month ago (2 children)

My first thought was similar - there might be some hardware acceleration happening for the jpgs that isn't for the other formats, resulting in a CPU bottleneck. A modern harddrive over USB3.0 should be capable of hundreds of megabits to several gigabits per second. It seems unlikely that's your bottleneck (though you can feel free to share stats and correct the assumption if this is incorrect - if your pngs are in the 40 megabyte range, your 3.5 per second would be pretty taxing).

If you are seeing only 1 CPU core at 100%, perhaps you could split the video clip, and process multiple clips in parallel?

[–] [email protected] 1 points 1 month ago

At this point I'm very sure that the drive speed is actually the bottleneck. I'm not sure why it's so slow tho. Splitting it is an interesting idea, maybe it's also possible to tell ffmpeg to only extract every 6th frame and start at a different frame for each of the 6 cores.

[–] [email protected] 1 points 1 month ago (1 children)

Coming back to this, what you said at the end was really interesting. I could manually split up the file and run the frame extract script for each one at the same time but do you know if it's possible to automate this? Or even better, run each instance of ffmpeg on the same video file and just extract every nth frame, like I said in my earlier reply?

[–] [email protected] 2 points 1 month ago

If your drive is the bottleneck, this will make things worse. If you want to proceed:

You're already using ffmpeg to get the sequence of frames, correct? You can add the -ss and -t flags to give a start time and a duration. Generate a list of offsets by dividing the length of video by the number of processes you want, and feed them through gnu parallel to your ffmpeg command.