In that way playback speed is adjusted relative to current speed, instead of in absolute terms. In this case, the filter sets up the PTS variable for you which has the current PTS. ffmpeg’s video filters are a very powerful set of tools, but they’re not as well documented as they could be. Where is the multiplier you want (2x faster: speed=2, etc). Adjusting it with ffmpeg is done via the setpts video filter like so:įfmpeg -i input.mp4 -vf “setpts=(1/)*PTS” output.mp4 This adjusts frames’ metadata related to how long each is displayed-exactly what you want. Two bad methods out of the way, the right way to change video speed using ffmpeg is by adjusting the “ presentation time stamp” (PTS). Framerates outside of the common ranges are also handled differently by different video playback software, so the actual playback speed is dependent on that as well. Others, like H.264, have bit-rate limits so that the maximum framerate is dependent on the resolution of the video. Some standards, like NTSC, have fixed framerates, so changes aren’t going to work at all. In certain cases, this might have the desired effect, but there are severe limits. A faster or slower framerate should equate to faster or slower video playback, right? Yes and no.
This takes forever if you have a video of any length (you know, the sort’ve thing you might want to speed up), and can hurt the quality because you’re retranscoding.Īnother wrong, but popular and plausible, method is simply changing the framerate.
The method involves breaking out each individual frame and then putting them all back together, dropping or duplicating frames as necessary to get the new framerate. Searching for “slow motion ffmpeg” or “speed up ffmpeg” turns up links like this blog and this forum that offer a method for getting video at different speeds, but it’s a terrible way to do it. Out of respect for all its other virtues, we’ll be using ffmpeg today. Being common, you’d think it would be readily available in any video editing software, but you’d be wrong. A not-uncommon video effect is fast- and slow-motion, sped-up or slowed-down video.
Let’s assume that we’ve cleared those hurdles, and now actually want to do some video editing from the command line. In my last installment we looked at all the reasons why editing video isn’t as easy as it should be.
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