How to Extract I-frames with FFmpeg Select Filter

Extracting keyframes, or I-frames, from a video is a common task for video analysis, editing, and creating preview thumbnails. This article provides a quick, practical guide on how to use the FFmpeg select video filter to isolate and export only the I-frames from a video file, saving them as individual images.

Understanding the Command

An I-frame (Intra-coded frame) is a complete image that does not require data from other frames to be decoded. To extract these frames using FFmpeg, you use the select filter with the pict_type parameter set to I.

Here is the standard command to extract I-frames as PNG images:

ffmpeg -i input.mp4 -vf "select='eq(pict_type,I)'" -vsync vfr frame_%04d.png

Command Breakdown

• -i input.mp4: Specifies the input video file.
• -vf "select='eq(pict_type,I)'": Applies the video filter (-vf). The select filter evaluates each frame. The expression eq(pict_type,I) tells FFmpeg to only keep frames where the picture type equals “I”.
• -vsync vfr: Stands for Variable Frame Rate. This flag (or its modern equivalent -fps_mode vfr) is crucial because it tells FFmpeg to drop the unselected frames (P and B frames) rather than duplicating the extracted I-frames to maintain the original frame rate.
• frame_%04d.png: Specifies the output format and naming convention. %04d is a placeholder that is replaced by a sequential, four-digit number (e.g., frame_0001.png, frame_0002.png).

Extracting I-frames as a New Video

If you want to concatenate the extracted I-frames into a new, fast-forwarded video instead of individual images, you must reset the presentation timestamps (PTS) using the setpts filter:

ffmpeg -i input.mp4 -vf "select='eq(pict_type,I)',setpts=N/FRAME_RATE/TB" -an output.mp4

In this command, setpts=N/FRAME_RATE/TB recalculates the timestamps of the selected frames so they play sequentially without pauses. The -an flag removes the audio, as the original audio stream will no longer sync with the modified video stream.