FFMPEG command to SpeedUp/SlowDown a video to a specific time

Question

I've been trying to figure out how to write a filter for ffmpeg to take a set of videos and make them all the same time length.

For example I have a video that is 35 seconds and a video 15 seconds, but I want both the 2 videos to be 30seconds long.

I know i can use the setpts filter to speed up or slow down a video, but truthfully I don't understand filter. I know setpts is "Change the PTS (presentation timestamp) of the input frames." Though I don't think this correct filter to use as I think it applies to the time length of the frame and nothing to do with the time length of the video.

Answer 1

Say your input file is input.mp4 and output is output.mp4. You want output.mp4 to be approximately 30 seconds long.

Here's a one-liner:

ffmpeg -i input.mp4 -filter:v "setpts=PTS/$(($(ffprobe -v error -show_format -show_streams -i input.mp4 | grep 'duration=' | head -n 1 | sed 's/duration=//')/30))" output.mp4

Let's break it down. First, we find the file duration with:

ffprobe -v error -show_format -show_streams -i input.mp4 | grep 'duration=' | head -n 1 | sed 's/duration=//'

We are getting the duration with ffprobe, then extracting the information with grep, head, and sed.

Next, we divide this quantity by 30 using:

$(( $(ffprobe -v error -show_format -show_streams -i input.mp4 | grep 'duration=' | head -n 1 | sed 's/duration=//') / 30 ))

We plug this into the -filter:v "setpts=PTS/..." to give us the correct ratio for speedup or slowdown.

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Generic way

Now, assuming we define the input file as INPUT_FILE, the output file as OUTPUT_FILE, and the required duration as OUTPUT_DURATION, here's the generic way to write this:

ffmpeg -i $INPUT_FILE -filter:v "setpts=PTS/$(($(ffprobe -v error -show_format -show_streams -i $INPUT_FILE | grep 'duration=' | head -n 1 | sed 's/duration=//')/$OUTPUT_DURATION))" $OUTPUT_FILE

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Interpretable way

Lastly, here's the more interpretable way to write all of this:

INPUT_FILE=input.mp4
OUTPUT_FILE=output.mp4
OUTPUT_DURATION=30

CURRENT_DURATION=$(ffprobe -v error -show_format -show_streams -i $INPUT_FILE | grep 'duration=' | head -n 1 | sed 's/duration=//')

SPEEDUP=$(( $CURRENT_DURATION / $OUTPUT_DURATION ))

ffmpeg -i $INPUT_FILE -filter:v "setpts=PTS/$SPEEDUP" $OUTPUT_FILE