ffmpeg-toolkit（ffmpeg 工具包）

FFmpeg Toolkit

Comprehensive guide for video and audio processing with ffmpeg.

Overview

This skill provides guidance on using ffmpeg for common video and audio operations. ffmpeg is a powerful command-line tool for processing multimedia files - converting formats, trimming, resizing, extracting audio, creating GIFs, and much more.

Core Principles

Always Use These Flags

• -hide_banner - Suppress ffmpeg version info for cleaner output
• -y - Overwrite output file without asking (for automation)

When to Use -c copy (Stream Copy)

Use -c copy when you want to:

• Preserve original quality (no re-encoding)
• Process quickly (10-100x faster than re-encoding)
• Keep file size similar to source

Limitation: Stream copy cannot change codec, resolution, or apply filters.

When to Re-encode

Re-encode when you need to:

• Change resolution or aspect ratio
• Apply filters (crop, rotate, overlay)
• Change codec (e.g., HEVC to H.264)
• Adjust quality/bitrate
• Frame-accurate trimming

Common Operations

1. Format Conversion

Basic conversion (re-encodes):

ffmpeg -hide_banner -i input.mkv output.mp4

Lossless remux (no re-encoding):

ffmpeg -hide_banner -i input.mkv -c copy output.mp4

Convert to web-optimized MP4:

ffmpeg -hide_banner -i input.mov -c:v libx264 -crf 23 -c:a aac -movflags faststart output.mp4

2. Trimming/Cutting

Fast trim (no re-encoding, may have keyframe issues):

ffmpeg -hide_banner -ss 00:01:30 -i input.mp4 -t 00:00:30 -c copy output.mp4

• -ss 00:01:30 - Start at 1 minute 30 seconds
• -t 00:00:30 - Duration of 30 seconds

Frame-accurate trim (requires re-encoding):

ffmpeg -hide_banner -i input.mp4 -ss 00:01:30 -t 00:00:30 -c:v libx264 -c:a aac output.mp4

Trim to end of file:

ffmpeg -hide_banner -ss 00:05:00 -i input.mp4 -c copy output.mp4

3. Audio Operations

Extract audio to MP3:

ffmpeg -hide_banner -i video.mp4 -vn -c:a libmp3lame -q:a 2 audio.mp3

Extract audio to WAV (lossless):

ffmpeg -hide_banner -i video.mp4 -vn audio.wav

Remove audio from video:

ffmpeg -hide_banner -i input.mp4 -an -c:v copy output.mp4

Add audio to video:

ffmpeg -hide_banner -i video.mp4 -i audio.mp3 -c:v copy -c:a aac -map 0:v -map 1:a output.mp4

Replace audio in video:

ffmpeg -hide_banner -i video.mp4 -i new_audio.mp3 -c:v copy -c:a aac -map 0:v -map 1:a -shortest output.mp4

4. Resizing/Scaling

Scale to specific resolution:

ffmpeg -hide_banner -i input.mp4 -vf "scale=1920:1080" -c:a copy output.mp4

Scale maintaining aspect ratio (width 1280, auto height):

ffmpeg -hide_banner -i input.mp4 -vf "scale=1280:-1" -c:a copy output.mp4

Scale to fit within bounds (won't exceed 1920x1080):

ffmpeg -hide_banner -i input.mp4 -vf "scale='min(1920,iw)':'min(1080,ih)'" -c:a copy output.mp4

5. Compression

Constant Rate Factor (CRF) - Recommended:

ffmpeg -hide_banner -i input.mp4 -c:v libx264 -crf 23 -c:a aac output.mp4

• CRF 18: Visually lossless
• CRF 23: Default, good balance
• CRF 28: Smaller file, lower quality

Target bitrate:

ffmpeg -hide_banner -i input.mp4 -c:v libx264 -b:v 2M -c:a aac -b:a 128k output.mp4

Two-pass encoding (best quality for target size):

ffmpeg -hide_banner -i input.mp4 -c:v libx264 -b:v 2M -pass 1 -f null /dev/null
ffmpeg -hide_banner -i input.mp4 -c:v libx264 -b:v 2M -pass 2 output.mp4

6. Creating GIFs

Basic GIF:

ffmpeg -hide_banner -i input.mp4 -vf "fps=10,scale=480:-1" output.gif

High-quality GIF with palette:

ffmpeg -hide_banner -i input.mp4 -vf "fps=10,scale=480:-1:flags=lanczos,split[s0][s1];[s0]palettegen[p];[s1][p]paletteuse" output.gif

GIF from specific section:

ffmpeg -hide_banner -ss 00:00:05 -t 3 -i input.mp4 -vf "fps=15,scale=320:-1:flags=lanczos" output.gif

7. Combining Videos

Concatenate videos (same codec):

First create a file list (videos.txt):

file 'video1.mp4'
file 'video2.mp4'
file 'video3.mp4'

Then concatenate:

ffmpeg -hide_banner -f concat -safe 0 -i videos.txt -c copy output.mp4

Concatenate with re-encoding (different codecs/resolutions):

ffmpeg -hide_banner -f concat -safe 0 -i videos.txt -c:v libx264 -c:a aac output.mp4

8. Rotation

Rotate 90° clockwise:

ffmpeg -hide_banner -i input.mp4 -vf "transpose=1" output.mp4

Transpose values:

• 0 - 90° counterclockwise + vertical flip
• 1 - 90° clockwise
• 2 - 90° counterclockwise
• 3 - 90° clockwise + vertical flip

Rotate 180°:

ffmpeg -hide_banner -i input.mp4 -vf "transpose=1,transpose=1" output.mp4

9. Cropping

Crop to specific dimensions:

ffmpeg -hide_banner -i input.mp4 -vf "crop=640:480:100:50" output.mp4

Format: crop=width:height:x:y

Crop center square:

ffmpeg -hide_banner -i input.mp4 -vf "crop=min(iw\,ih):min(iw\,ih)" output.mp4

10. Speed Adjustment

Speed up video 2x:

ffmpeg -hide_banner -i input.mp4 -vf "setpts=0.5*PTS" -af "atempo=2.0" output.mp4

Slow down video 0.5x:

ffmpeg -hide_banner -i input.mp4 -vf "setpts=2*PTS" -af "atempo=0.5" output.mp4

Speed up without audio:

ffmpeg -hide_banner -i input.mp4 -vf "setpts=0.5*PTS" -an output.mp4

Advanced Operations

Add Subtitles

Burn subtitles into video:

ffmpeg -hide_banner -i input.mp4 -vf "subtitles=subs.srt" output.mp4

Burn ASS subtitles (styled):

ffmpeg -hide_banner -i input.mp4 -vf "ass=subs.ass" output.mp4

Add Watermark/Overlay

Image overlay (top-right corner):

ffmpeg -hide_banner -i input.mp4 -i logo.png -filter_complex "overlay=W-w-10:10" output.mp4

Extract Frames

Extract all frames:

ffmpeg -hide_banner -i input.mp4 frame_%04d.png

Extract 1 frame per second:

ffmpeg -hide_banner -i input.mp4 -vf "fps=1" frame_%04d.png

Extract single frame at timestamp:

ffmpeg -hide_banner -ss 00:00:10 -i input.mp4 -frames:v 1 thumbnail.png

Create Video from Images

Images to video:

ffmpeg -hide_banner -framerate 30 -i frame_%04d.png -c:v libx264 -pix_fmt yuv420p output.mp4

Change Metadata

Set title:

ffmpeg -hide_banner -i input.mp4 -c copy -metadata title="My Video" output.mp4

Remove all metadata:

ffmpeg -hide_banner -i input.mp4 -c copy -map_metadata -1 output.mp4

Hardware Acceleration (NVIDIA)

Encode with NVENC:

ffmpeg -hide_banner -i input.mp4 -c:v h264_nvenc -preset fast -b:v 5M output.mp4

Decode and encode with GPU:

ffmpeg -hide_banner -hwaccel cuda -i input.mp4 -c:v h264_nvenc output.mp4

Quick Reference

Best Practices

1. Always test on a short clip first before processing large files
2. Use -c copy when possible for speed and quality preservation
3. Add -movflags faststart for web-optimized MP4s
4. Use CRF for quality-based encoding instead of fixed bitrate
5. Check input info first with ffmpeg -i input.mp4 to understand source
6. Use two-pass encoding when targeting specific file size

Common Pitfalls

• Audio/video sync issues after trim: Use re-encoding instead of -c copy
• Black frames at start: Add -avoid_negative_ts make_zero
• Incompatible codec in container: Check container format supports codec
• GIF too large: Reduce FPS, scale down, or shorten duration

Additional Resources

For detailed command reference, see references/ffmpeg-commands.md.