FFmpeg Skill

Natural language FFmpeg operations with opinionated best practices for speed, reliability, and quality.

Core Principles

1. Stream copy by default - When converting video containers, use -c copy to avoid re-encoding (10x faster, no quality loss)
2. CPU-only encoding - No GPU encoders (more reliable, hardware-agnostic)
3. Fast presets - Default to fast preset for x264/x265 (good balance of speed/quality)
4. Context-aware audio - Ask about transcription when converting to MP3 (Whisper prefers 16kHz mono)

Common Operations

Audio Extraction

When extracting audio from video:

# Standard audio extraction (AAC/MP3)
ffmpeg -i input.mp4 -vn -acodec copy output.aac

# For transcription (Whisper-optimized)
ffmpeg -i input.mp4 -vn -ar 16000 -ac 1 -c:a libmp3lame -b:a 64k output.mp3

Always ask: "Is this for transcription?" before choosing the format.

Video Conversion

When converting video containers (e.g., MKV → MP4):

# Fast conversion (stream copy - no re-encoding)
ffmpeg -i input.mkv -c copy output.mp4

If stream copy fails (incompatible codecs), fall back to re-encoding:

# Fallback: re-encode with fast preset
ffmpeg -i input.mkv -c:v libx264 -preset fast -crf 23 -c:a aac -b:a 128k output.mp4

Trimming

When trimming video (preserving quality):

# Fast trim (stream copy)
ffmpeg -ss 00:01:30 -to 00:02:45 -i input.mp4 -c copy output.mp4

# Accurate trim (re-encode if needed)
ffmpeg -i input.mp4 -ss 00:01:30 -to 00:02:45 -c:v libx264 -preset fast -crf 23 -c:a copy output.mp4

Resizing

When resizing video:

# Resize to 720p (maintains aspect ratio)
ffmpeg -i input.mp4 -vf scale=-2:720 -c:v libx264 -preset fast -crf 23 -c:a copy output.mp4

# Resize to specific width (maintains aspect ratio)
ffmpeg -i input.mp4 -vf scale=1280:-2 -c:v libx264 -preset fast -crf 23 -c:a copy output.mp4

Compression

For WhatsApp/Telegram:

# Aggressive compression (under 50MB target)
ffmpeg -i input.mp4 -vf scale=-2:480 -c:v libx264 -preset fast -crf 28 -b:a 64k -ac 1 output.mp4

General compression:

# Balanced compression
ffmpeg -i input.mp4 -c:v libx264 -preset fast -crf 28 -c:a aac -b:a 96k output.mp4

Decision Tree

Audio Conversion

1. Ask: "What's the purpose?" (listening, transcription, archival)
2. For transcription: Use 16kHz mono MP3
3. For listening: Use AAC or high-quality MP3
4. For archival: Use FLAC or original codec with stream copy

Video Conversion

1. Check: Is this just a container change? (e.g., MKV → MP4)
   • YES → Use stream copy
   • NO → Continue
2. Check: Does the user want quality preservation or compression?
   • Preservation → Use CRF 18-23
   • Compression → Use CRF 26-30
3. Check: Is speed critical?
   • YES → Use veryfast preset
   • NO → Use fast preset

Helper Script

Use scripts/ffmpeg_helper.py for common operations:

# Extract audio (auto-detects purpose)
python3 scripts/ffmpeg_helper.py extract-audio input.mp4 --ask-purpose

# Convert video (smart defaults)
python3 scripts/ffmpeg_helper.py convert input.mkv output.mp4

# Trim video (fast mode by default)
python3 scripts/ffmpeg_helper.py trim input.mp4 output.mp4 --start 00:01:30 --end 00:02:45

# Resize video
python3 scripts/ffmpeg_helper.py resize input.mp4 output.mp4 --height 720

# Compress for messaging
python3 scripts/ffmpeg_helper.py compress input.mp4 output.mp4 --target whatsapp

Presets Reference

See references/presets.md for detailed preset explanations and use cases.

Error Handling

When stream copy fails:

1. Inform the user: "Stream copy failed (incompatible codecs). Re-encoding with fast preset..."
2. Retry with re-encoding
3. Show the command used for transparency

When output is too large:

1. Suggest compression options
2. Offer CRF adjustment (higher = smaller file)
3. Offer resolution downscaling