How to Extract Every Nth Frame with FFmpeg Select

Extracting specific frames from a video is a fundamental task in video processing, useful for creating contact sheets, generating thumbnails, or preparing datasets for machine learning. This article provides a straightforward guide on how to use FFmpeg’s select video filter to extract every \(N\)-th frame (for example, every 10th or 100th frame) from a video file and save them as sequential image files.

The Basic Command

To extract every \(N\)-th frame, you use the select filter with the mathematical expression not(mod(n,N)). Here is the standard command to extract every 10th frame:

ffmpeg -i input.mp4 -vf "select='not(mod(n,10))'" -fps_mode vfr output_%04d.png

Command Breakdown

• -i input.mp4: Specifies the path to your input video file.
• -vf "select='not(mod(n,10))'": Applies the video filter (-vf).
  • n represents the sequential frame number starting at 0.
  • mod(n,10) calculates the modulo of the frame number divided by 10. This returns 0 for frames 0, 10, 20, 30, etc.
  • not(...) negates the result. Since not(0) is 1 (true) and not(any other number) is 0 (false), FFmpeg only selects the frames where the frame number is a multiple of 10.
• -fps_mode vfr: Sets the frame rate mode to Variable Frame Rate (VFR). This is a critical parameter. Without it, FFmpeg may attempt to duplicate the selected frames to match the original video’s frame rate, resulting in a massive sequence of duplicate images. (Note: In older versions of FFmpeg, use -vsync vfr instead).
• output_%04d.png: Specifies the naming convention for the output images. %04d is a placeholder that outputs padded four-digit sequential integers (e.g., output_0001.png, output_0002.png).

Examples for Different Intervals

To change the interval, simply replace the number 10 in the modulo function with your desired interval \(N\).

• Extract every 24th frame:

  ffmpeg -i input.mp4 -vf "select='not(mod(n,24))'" -fps_mode vfr img_%04d.png

• Extract every 100th frame:

  ffmpeg -i input.mp4 -vf "select='not(mod(n,100))'" -fps_mode vfr img_%04d.png

Alternative: Extracting Frames by Time Interval

If you prefer to extract frames based on time (seconds) rather than frame counts, you can use the t (time) variable instead of n (frame number).

To extract one frame every 5 seconds:

ffmpeg -i input.mp4 -vf "select='not(mod(t,5))'" -fps_mode vfr output_%04d.png