ffmpeg: How to trim a video with every nth frame

2 min read 22-10-2024

the ifix

ffmpeg: How to trim a video with every nth frame

If you're looking to trim a video and keep every nth frame using FFmpeg, you're in the right place! FFmpeg is a powerful multimedia framework that allows you to manipulate audio and video files with ease. This article will guide you on how to achieve this while explaining the code and offering practical examples for better understanding.

The Problem Scenario

Suppose you have a video file and you only want to retain every nth frame from it. For example, if you want to keep every 5th frame, this could be useful in creating a time-lapse effect or reducing the size of your video by cutting down on the frame count.

Here's an example of the original command that might confuse users trying to achieve this:

ffmpeg -i input.mp4 -vf "select='not(mod(n,5))'" -vsync vfr output.mp4

Analyzing the Code

Input File: -i input.mp4 specifies the input video file. Replace input.mp4 with the path to your actual video file.
Video Filter: -vf "select='not(mod(n,5))'" is where the magic happens. This filter uses the select command to choose which frames to keep:
- n represents the frame number.
- mod(n,5) calculates the modulus of the current frame number divided by 5. It returns 0 for every 5th frame.
- not(mod(n,5)) selects frames for which the condition is true (i.e., every 5th frame).
Synchronization: -vsync vfr ensures that the output has variable frame rate (VFR) synchronization, making it suitable for the edited video.
Output File: output.mp4 is the name of your output file. You can name it anything you prefer.

Practical Example

Let’s consider a practical example to demonstrate how to apply this command effectively:

Suppose you have a video titled nature_video.mp4, and you want to extract every 10th frame. You would run the following command in your terminal:

ffmpeg -i nature_video.mp4 -vf "select='not(mod(n,10))'" -vsync vfr output_nature.mp4

After executing this command, you would have a new video file called output_nature.mp4, containing only the frames you specified, which effectively reduces the video size and can provide a fast-paced viewing experience.

Additional Considerations

Performance: Depending on the length of the video and your specified n value, this process can be resource-intensive. Make sure to run this on a capable machine, especially with long videos.
Frame Rates: Keep in mind the original frame rate of your video when selecting the nth frame. A lower frame rate may make it more noticeable when you skip frames.
Quality Settings: You may want to include options like -crf (Constant Rate Factor) for quality control. A value of 23 is the default, with lower values yielding higher quality:
```
ffmpeg -i input.mp4 -vf "select='not(mod(n,5))'" -vsync vfr -crf 18 output.mp4
```

Useful Resources

FFmpeg Documentation: For detailed explanations of all FFmpeg filters and options.
Online FFmpeg Tutorials: Additional guidance on using FFmpeg effectively.

Conclusion

Trimming a video to keep every nth frame with FFmpeg is straightforward once you understand the command structure. By using the select video filter effectively, you can customize your video editing tasks to fit your specific needs, whether for a time-lapse effect or simply to reduce file size. Don’t hesitate to experiment with different values of n and additional FFmpeg options to achieve the best results for your projects!