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Extracting frames from a video with ffmpeg very slow if not using jpeg
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this post was submitted on 14 Jul 2024
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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.
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?
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.
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?
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
-ssand-tflags 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.