Utilizing FFmpeg for Video Speed Adjustment: Slow Motion and Time-lapse Techniques

FFmpeg stands as a versatile tool in video editing, particularly for altering video playback speed to achieve effects such as slow motion or time-lapse. This capability enhances the visual impact of video content by offering a new dimension of storytelling.

To begin using FFmpeg for these purposes, ensure FFmpeg is installed on your system. A concise guide for FFmpeg installation is available here.

This guide outlines the steps to manipulate video speed using FFmpeg, covering both acceleration and deceleration of video playback.

Accelerating Video Playback

To increase the speed of an MP4 video by 8 times, use the following FFmpeg command:

ffmpeg -i input.mp4  -vf "setpts=0.125*PTS" -filter:a "atempo=8"  fast.mp4

Let’s break down what we’re doing here:

• i input.mp4: Specifies the input video file. Replace input.mp4 with your actual video filename.
• vf "setpts=0.125*PTS": Adjusts the video speed. The value 0.125 results from the inverse of the desired speed factor (1/8), indicating an 8x speed increase.
• filter:a "atempo=8": Increases the audio speed to match the video speed.
• fast.mp4: Names the output file as fast.mp4.

To target a specific video segment for acceleration, use this command:

ffmpeg -i input.mp4 -filter_complex "[0:v]trim=start=0:end=5,setpts=0.125*PTS[v1];[0:v]trim=start=5,setpts=PTS[v2];[0:a]atrim=start=0:end=5,atempo=8[a1];[0:a]atrim=start=5,atempo=1[a2];[v1][a1][v2][a2]concat=n=2:v=1:a=1[v3]" -map "[v3]" -c:v libx264 -c:a aac -strict experimental output_speedup.mp4

Here, we trim and accelerate a video segment from 0 to 5 seconds and then concatenates it with the rest of the video, which retains its original speed.

Let’s look at some specific parts of this command:

• -i input.mp4: Specifies the input file. (Replace input.mp4 with the name of your video file.) Note: instead of -vf, we’re using -fliter_complex
• [0:v]trim=start=0:end=5,setpts=0.125*PTS[v1]****: Trims the video from 0 to 5 seconds.
  • setpts=0.125*PTS****: Speeds up this segment of the video.
• [0:a]atrim=start=0:end=5,atempo=8[a1]****: Speeds up the audio by 8x
• [0:v]trim=start=5,setpts=PTS[v2]****: Trims video from 5 until the end of the video, have the video at their normal speed to this part using setpts=PTS
• [0:a]atrim=start=5,atempo=1[a2]****: Trims the audio from 5 seconds to the end
  • atempo=1****: Plays the video at normal speed in this segment
  • [v1][a1][v2][a2]concat=n=2:v=1:a=1[v3]: Merges two video segments with the corresponding audio using the concat function
• -map "[v3]"****: Maps the output of the complex filtergraph ([v3]) to be used as the video stream.
• -c:v libx264: Sets the video codec to libx264, which is a widely used and efficient H.264 video codec.
• -c:a aac: Sets the audio codec to AAC, a commonly used audio codec.
• -strict experimental: Allows the use of experimental codecs, necessary in some FFmpeg versions for certain codec combinations.
• output_speedup.mp4: Specifies the output file.

Slow down a video:

Here’s an FFmpeg command to slow down a mp4 video by 2x:

ffmpeg -i input.mp4 -vf "setpts=2*PTS" slow.mp4

Again, let’s get our hands dirty:

• -i input.mp4: Specifies the input file. (Replace input.mp4 with the name of your video file.)
• -vf "setpts=2*PTS": Specifies how much to slow down or speed up the video (in our example its 2x).
• -filter:a "atempo=8": Slows down the audio by 2x
• slow.mp4: Specifies the output file (in this case, an MP4 file named slow.mp4).

Finally, here’s an FFmpeg command that decelerates a specific part of a video:

ffmpeg -i input.mp4 -filter_complex "[0:v]trim=start=0:end=5,setpts=PTS*2[v1];[0:v]trim=start=5,setpts=PTS[v2];[0:a]atrim=start=0:end=5,atempo=0.5[a1];[0:a]atrim=start=5,atempo=1[a2];[v1][a1][v2][a2]concat=n=2:v=1:a=1[v3]" -map "[v3]" -c:v libx264 -c:a aac -strict experimental output_slowdown.mp4

Let’s dive in one last time:

• -i input.mp4: Specifies the input file. (Replace input.mp4 with the name of your video file.)
• -filter_complex "[0:v]trim=start=0:end=5,setpts=PTS*2[v1];[0:v]trim=start=5,setpts=PTS[v2];[0:a]atrim=start=0:end=5,atempo=8[a1];[0:a]atrim=start=5,atempo=1[a2];[v1][a1][v2][a2]concat=n=2:v=1:a=1[v3]"
  • Note: Instead of -vf, we’re using -fliter_complex
  • [0:v]trim=start=0:end=5,setpts=PTS*2[v1]****: Trims the video from 0 to 5 seconds, apples slow motion to this segment using setpts=PTS*2
  • [0:a]atrim=start=0:end=5,atempo=0.5[a1]****: Slows the audio by 2x
  • [0:v]trim=start=5,setpts=PTS[v2]: Trims the video from 5 seconds to the end, returns this segment to its normal speed using setpts=PTS
  • [0:a]atrim=start=5,atempo=1[a2]****: Trims the audio from 5 seconds to the end, returning the video to its normal speed in this segment using atempo=1
  • [v1][a1][v2][a2]concat=n=2:v=1:a=1[v3]: Merges the two parts of the video with the audio using the concat function
• -map "[v3]"****: Maps the output of the complex filtergraph ([v3]) to be used as the video stream.
• -c:v libx264: Sets the video codec to libx264 (a widely used and efficient H.264 video codec).
• -c:a aac: Sets the audio codec to AAC (a commonly used audio codec).
• -strict experimental: Allows the use of experimental codecs, necessary in some FFmpeg versions for certain codec combinations.
• output_slowdown.mp4: Specifies the output file.

That's it! You have successfully sped up or slowed down a video or a part in a video using FFmpeg. Following these instructions allows for effective speed adjustments in videos using FFmpeg, adding dynamic slow-motion or time-lapse effects to enhance your video projects.