Ffmpeg Keyframe Extraction

FFmpeg Keyframe Extraction

Overview

FFmpeg can extract keyframes (I-frames) from video files. Keyframes are complete frames that don't depend on other frames for decoding, making them ideal for analysis and detection tasks.

Installation

bashCopy code
# Ubuntu/Debian
sudo apt-get install ffmpeg

# macOS
brew install ffmpeg

Usage

Extract All Keyframes from Video

bashCopy code
ffmpeg -i input.mp4 -vf "select=eq(pict_type\,I)" -vsync 0 output_%03d.png

Parameters:

• -i input.mp4: Input video file
• -vf "select=eq(pict_type\,I)": Filter to select only I-frames (keyframes)
• -vsync 0: Output one image per keyframe (important for proper numbering)
• output_%03d.png: Output pattern with zero-padded frame numbers (001, 002, etc.)

Extract Keyframes with Frame Rate Control

bashCopy code
ffmpeg -i input.mp4 -vf "fps=1/5,select=eq(pict_type\,I)" -vsync 0 output_%03d.png

This extracts at most 1 keyframe every 5 seconds.

Check Video Information

bashCopy code
ffprobe -v error -show_entries format=duration -of default=noprint_wrappers=1:nokey=1:noprint_wrappers=1 video.mp4

Output Naming Convention

When using output_%03d.png:

• Frame 1: output_001.png
• Frame 2: output_002.png
• Frame 100: output_100.png

Python Integration

pythonCopy code
import subprocess
import os

def extract_keyframes(video_path, output_dir, output_prefix="keyframes"):
    """Extract keyframes from video file"""
    os.makedirs(output_dir, exist_ok=True)
    output_pattern = os.path.join(output_dir, f"{output_prefix}_%03d.png")

    cmd = [
        'ffmpeg',
        '-i', video_path,
        '-vf', 'select=eq(pict_type\\,I)',
        '-vsync', '0',
        output_pattern
    ]

    result = subprocess.run(cmd, capture_output=True, text=True)
    if result.returncode != 0:
        raise RuntimeError(f"FFmpeg error: {result.stderr}")

    # Return list of extracted frames
    frames = sorted([
        os.path.join(output_dir, f)
        for f in os.listdir(output_dir)
        if f.startswith(output_prefix)
    ])
    return frames

Tips

• Keyframes are typically smaller in file size than regular frames
• For consistent results, always use -vsync 0 with keyframe extraction
• Output pattern with %03d ensures consistent naming for sorting
• Check that files are being created in the expected output directory