Extract Frames at Regular Intervals with FFmpeg Select

Extracting specific images from a video file is a common task in video processing and analysis. This article provides a quick and practical guide on how to use the FFmpeg select video filter to extract frames at precise, regular intervals—either by time (e.g., every few seconds) or by frame count (e.g., every Nth frame)—and save them as image files.

Extracting a Frame Every N Seconds

To extract a single frame at a regular time interval (for example, every 10 seconds), use the select filter with the mod (modulo) function on the timestamp variable t.

Run the following command in your terminal:

ffmpeg -i input.mp4 -vf "select='not(mod(t\,10))',setpts=N_TB" -fps_mode vfr frame_%04d.png

How it works:

  • -vf "select='...'": Applies the video filtergraph.
  • not(mod(t\,10)): The select filter evaluates this expression for every frame. The expression returns true when the presentation time t divided by 10 has a remainder of 0 (i.e., at 10s, 20s, 30s, etc.).
  • setpts=N_TB: Resets the presentation timestamps of the selected frames to be contiguous. This prevents gaps in the output sequence.
  • -fps_mode vfr: Sets the video sync method to Variable Frame Rate (VFR), which ensures FFmpeg only outputs the selected frames instead of duplicating frames to match the original frame rate. (Note: For older versions of FFmpeg, use -vsync vfr instead).
  • frame_%04d.png: Saves the extracted frames as sequential images (e.g., frame_0001.png, frame_0002.png).

Extracting Every Nth Frame

If you prefer to extract frames based on frame count rather than time (for example, extracting every 100th frame), you can use the frame number variable n instead of t.

Run this command:

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

How it works:

  • not(mod(n\,100)): The filter selects a frame whenever the frame index n is a multiple of 100.
  • The remaining parameters function exactly the same as in the time-based extraction command, ensuring clean, sequential image output without duplicates.