How to Extract B-Frames Using FFmpeg Select Filter

This article explains how to use the FFmpeg select video filter to isolate and extract B-frames (bi-directional predictive frames) from a video file. You will learn the exact command-line syntax required to save these frames as a sequence of images or compile them into a new video file.

Understanding the Select Filter for B-Frames

In video compression, B-frames rely on both past and future frames for temporal prediction. To extract only these frames, FFmpeg utilizes the select filter paired with the pict_type constant.

By evaluating the expression eq(pict_type,B), FFmpeg discards all I-frames (keyframes) and P-frames (predictive frames), processing only the B-frames.

Extracting B-Frames as Images

To extract every B-frame from a video and save them as individual PNG or JPG images, use the following command:

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

Command Breakdown:

• -i input.mp4: Specifies the input video file.
• -vf "select='eq(pict_type,B)'": Applies the video filter to select only frames where the picture type is “B”.
• -vsync vfr: Configures variable frame rate video synchronization. This prevents FFmpeg from duplicating frames to fill the gaps left by the discarded I and P frames. (Note: In newer versions of FFmpeg, you can use -fps_mode vfr instead).
• bframe_%04d.png: The output naming pattern, which generates sequentially numbered images (e.g., bframe_0001.png, bframe_0002.png).

Extracting B-Frames into a New Video

If you want to extract the B-frames and package them directly into a new video container, you must reset the presentation timestamps (PTS). Otherwise, the output video will stutter or freeze during the timestamps where the deleted I and P frames used to be.

Use this command to output a smooth video consisting solely of B-frames:

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

Command Breakdown:

• setpts=N/FRAME_RATE/TB: Recalculates the presentation timestamps for the selected frames. This forces the extracted B-frames to play back-to-back at the container’s standard frame rate.
• -an: Disables audio recording, as the audio timeline will no longer match the heavily modified video timeline.