Slow Motion or Time Lapse with ffmpeg

Sometimes you want to slow down or speed up your H264 codec video (mind that this works smoothly only for videos without audio-track!). Here is a UNIX shell-script to do this losslessly when ffmpeg is installed. If you pass e.g. myVideo.mp4 as first parameter, the script will generate a new video-file named myVideo_timed.mp4 in the directory where the original video resides.

fileMarker=timed

[ -z "$1" -o ! -f "$1" ] && {
	echo "SYNTAX: $0 videoFile [framesPerSecond]" >&2
	echo "	Slows down or speeds up given video, creating an xxx_$fileMarker.mp4 file." >&2
	echo "	framesPerSecond default is 15, which is half speed of normal 30 fps." >&2
	exit 1
}

sourceVideo=$1
sourceDir=`dirname \$sourceVideo`
sourceFile=`basename \$sourceVideo`
filename=${sourceFile%.*}	# filename without extension
extension=${sourceFile#*.}	# extension without filename
targetVideo=$sourceDir/${filename}_$fileMarker.$extension

framesPerSecond=15
[ -n "$2" ] && framesPerSecond=$2

tempRawFile=raw.h264

cleanup()	{
	rm -rf $tempRawFile
}
error()	{
	cleanup
	exit $1
}

# copy the video to a raw bitstream format
ffmpeg -v error -y -i $sourceVideo -map 0:v -c:v copy -bsf:v h264_mp4toannexb $tempRawFile || error 2

# generate new timestamps while muxing to a container
ffmpeg -v error -y -fflags +genpts -r $framesPerSecond -i $tempRawFile -c:v copy $targetVideo || error 3

cleanup

echo "Created $targetVideo"

Normally the frames-per-second value is something like 30. By default the script will slow down the video by 50% when you do not pass an explicit FPS-value as second argument to the script call (framesPerSecond=15). If you pass a value of 60, the video will play in double speed.