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SKILL.md
npx skills add calesthio/openmontage --skill ffmpeg
Video and audio processing with FFmpeg. Use for format conversion, resizing, compression, audio extraction, and preparing assets for Remotion. Triggers include converting GIF to MP4, resizing video, extracting audio, compressing files, or any media transformation task.
npx skills add calesthio/openmontage --skill ffmpeg
FFmpeg is the essential tool for video/audio processing. This skill covers common operations for Remotion video projects.
Why these flags:
-movflags faststart - Moves metadata to start for web streaming-pix_fmt yuv420p - Ensures compatibility with most playersscale=trunc(...) - Forces even dimensions (required by most codecs)Note: Re-encoding is recommended for trimming. Stream copy (-c copy) can silently drop video if the seek point doesn't align with a keyframe.
When to use FFmpeg vs Remotion playbackRate:
| Scenario | Use FFmpeg | Use Remotion |
|---|---|---|
| Constant speed (1.5x, 2x) | Either works | ✅ Simpler |
| Extreme speeds (>4x or <0.25x) | ✅ More reliable | May have issues |
| Variable speed (accelerate over time) | ✅ Pre-process | Complex workaround needed |
| Need perfect audio sync | ✅ Guaranteed | Usually fine |
| Demo needs to fit voiceover timing | ✅ Pre-calculate | Runtime adjustment |
Remotion limitation: playbackRate must be constant. Dynamic interpolation like playbackRate={interpolate(frame, [0, 100], [1, 5])} won't work correctly because Remotion evaluates frames independently.
Calculate speed factor:
speed = X / Y1 / speed (e.g., 3x speed = setpts=0.333*PTS)speed (e.g., 3x speed = atempo=3.0)Extreme speed (>2x audio): Chain atempo filters (each limited to 0.5-2.0 range):
Add scale filter: -vf "scale=trunc(iw/2)*2:trunc(ih/2)*2"
Use: -movflags faststart -pix_fmt yuv420p -c:v libx264
Use filter_complex with atempo: -filter_complex "[0:v]setpts=0.5*PTS[v];[0:a]atempo=2.0[a]"
Increase CRF (23→28) or reduce resolution
| Use Case | CRF | Preset | Notes |
|---|---|---|---|
| Archive/Master | 18 | slow | Best quality, large files |
| Production | 20-22 | medium | Good balance |
| Web/Preview | 23-25 | fast | Smaller files |
| Draft/Quick | 28+ | veryfast | Fast encoding |
After Remotion renders your video (typically to out/video.mp4), use FFmpeg to optimize for each distribution platform.
YouTube re-encodes everything, so upload high quality:
Twitter has strict limits: max 140s, 512MB, 1920x1200:
LinkedIn prefers MP4 with AAC audio, max 10 minutes:
| Platform | Max Resolution | Max Size | Max Duration | Audio |
|---|---|---|---|---|
| YouTube | 8K | 256GB | 12 hours | AAC 48kHz |
| Twitter/X | 1920x1200 | 512MB | 140s | AAC 44.1kHz |
| 4096x2304 | 5GB | 10 min | AAC 48kHz | |
| Instagram Feed | 1080x1350 | 4GB | 60s | AAC 48kHz |
| Instagram Reels | 1080x1920 | 4GB | 90s | AAC 48kHz |
| TikTok | 1080x1920 | 287MB | 10 min | AAC |
Common errors and fixes when processing video:
| Error | Cause | Fix |
|---|---|---|
| "No such file" | Input path wrong | Check path, use quotes for spaces |
| "Invalid data" | Corrupted input | Re-download or re-record source |
| "height not divisible by 2" | Odd dimensions | Add scale filter with trunc |
| "encoder not found" | Missing codec | Install FFmpeg with full codecs |
| Output 0 bytes | Silent failure | Check full ffmpeg output for errors |
If this skill is missing information or could be improved:
Just say "improve this skill" and I'll guide you through updating .claude/skills/ffmpeg/SKILL.md.
# To 1920x1080 (maintain aspect ratio, add black bars)
ffmpeg -i input.mp4 -vf "scale=1920:1080:force_original_aspect_ratio=decrease,pad=1920:1080:(ow-iw)/2:(oh-ih)/2" output.mp4
# To 1920x1080 (crop to fill)
ffmpeg -i input.mp4 -vf "scale=1920:1080:force_original_aspect_ratio=increase,crop=1920:1080" output.mp4
# Scale to width, auto height
ffmpeg -i input.mp4 -vf "scale=1280:-2" output.mp4# Good quality, smaller file (CRF 23 is default, lower = better quality)
ffmpeg -i input.mp4 -c:v libx264 -crf 23 -preset medium -c:a aac -b:a 128k output.mp4
# Aggressive compression for web preview
ffmpeg -i input.mp4 -c:v libx264 -crf 28 -preset fast -c:a aac -b:a 96k output.mp4
# Target file size (e.g., ~10MB for 60s video = ~1.3Mbps)
ffmpeg -i input.mp4 -c:v libx264 -b:v 1300k -c:a aac -b:a 128k output.mp4# Extract to MP3
ffmpeg -i input.mp4 -vn -acodec libmp3lame -q:a 2 output.mp3
# Extract to AAC
ffmpeg -i input.mp4 -vn -acodec aac -b:a 192k output.m4a
# Extract to WAV (uncompressed)
ffmpeg -i input.mp4 -vn output.wav# M4A to MP3 (for ElevenLabs voice samples)
ffmpeg -i input.m4a -codec:a libmp3lame -qscale:a 2 output.mp3
# WAV to MP3
ffmpeg -i input.wav -codec:a libmp3lame -b:a 192k output.mp3
# Adjust volume
ffmpeg -i input.mp3 -filter:a "volume=1.5" output.mp3# Cut from timestamp to duration (recommended - reliable)
ffmpeg -i input.mp4 -ss 00:00:30 -t 00:00:15 -c:v libx264 -c:a aac output.mp4
# Cut from timestamp to timestamp
ffmpeg -i input.mp4 -ss 00:00:30 -to 00:00:45 -c:v libx264 -c:a aac output.mp4
# Stream copy (faster but may lose frames at cut points)
# Only use when source has frequent keyframes
ffmpeg -i input.mp4 -ss 00:00:30 -t 00:00:15 -c copy output.mp4# 2x speed (video and audio)
ffmpeg -i input.mp4 -filter_complex "[0:v]setpts=0.5*PTS[v];[0:a]atempo=2.0[a]" -map "[v]" -map "[a]" output.mp4
# 0.5x speed (slow motion)
ffmpeg -i input.mp4 -filter_complex "[0:v]setpts=2.0*PTS[v];[0:a]atempo=0.5[a]" -map "[v]" -map "[a]" output.mp4
# Video only (no audio)
ffmpeg -i input.mp4 -filter:v "setpts=0.5*PTS" -an output.mp4# Create file list
echo "file 'clip1.mp4'" > list.txt
echo "file 'clip2.mp4'" >> list.txt
echo "file 'clip3.mp4'" >> list.txt
# Concatenate (same codec/resolution)
ffmpeg -f concat -safe 0 -i list.txt -c copy output.mp4
# Concatenate with re-encoding (different sources)
ffmpeg -f concat -safe 0 -i list.txt -c:v libx264 -c:a aac output.mp4# Fade in first 1 second, fade out last 1 second (30fps video)
ffmpeg -i input.mp4 -vf "fade=t=in:st=0:d=1,fade=t=out:st=9:d=1" -c:a copy output.mp4
# Audio fade
ffmpeg -i input.mp4 -af "afade=t=in:st=0:d=1,afade=t=out:st=9:d=1" -c:v copy output.mp4# Duration, resolution, codec info
ffprobe -v error -show_entries format=duration -of default=noprint_wrappers=1:nokey=1 input.mp4
# Full info
ffprobe -v quiet -print_format json -show_format -show_streams input.mp4# Speed up demo to fit a scene (e.g., 60s demo into 20s = 3x speed)
ffmpeg -i demo-raw.mp4 \
-filter_complex "[0:v]setpts=0.333*PTS[v];[0:a]atempo=3.0[a]" \
-map "[v]" -map "[a]" \
public/demos/demo-fast.mp4
# Slow motion for emphasis (0.5x speed)
ffmpeg -i action.mp4 \
-filter_complex "[0:v]setpts=2.0*PTS[v];[0:a]atempo=0.5[a]" \
-map "[v]" -map "[a]" \
public/demos/action-slow.mp4
# Speed up without audio (common for screen recordings)
ffmpeg -i demo.mp4 -filter:v "setpts=0.5*PTS" -an public/demos/demo-2x.mp4
# Timelapse effect (10x speed, drop audio)
ffmpeg -i long-demo.mp4 -filter:v "setpts=0.1*PTS" -an public/demos/timelapse.mp4# 4x speed audio
-filter_complex "[0:a]atempo=2.0,atempo=2.0[a]"
# 8x speed audio
-filter_complex "[0:a]atempo=2.0,atempo=2.0,atempo=2.0[a]"# Standard 1080p, 30fps, Remotion-ready
ffmpeg -i raw-recording.mp4 \
-vf "scale=1920:1080:force_original_aspect_ratio=decrease,pad=1920:1080:(ow-iw)/2:(oh-ih)/2,fps=30" \
-c:v libx264 -crf 18 -preset slow \
-c:a aac -b:a 192k \
-movflags faststart \
public/demos/demo.mp4# From iPhone/iPad recording (usually 60fps, variable resolution)
ffmpeg -i iphone-recording.mov \
-vf "scale=1920:-2,fps=30" \
-c:v libx264 -crf 20 \
-an \
public/demos/mobile-demo.mp4for f in assets/*.gif; do
ffmpeg -i "$f" -movflags faststart -pix_fmt yuv420p \
-vf "scale=trunc(iw/2)*2:trunc(ih/2)*2" \
"public/demos/$(basename "$f" .gif).mp4"
doneRemotion render (master) FFmpeg optimization Platform upload
↓ ↓ ↓
out/video.mp4 ────────→ out/video-youtube.mp4 ───→ YouTube
────────→ out/video-twitter.mp4 ───→ Twitter/X
────────→ out/video-linkedin.mp4 ───→ LinkedIn
────────→ out/video-web.mp4 ───→ Website embed# YouTube optimized (1080p)
ffmpeg -i out/video.mp4 \
-c:v libx264 -preset slow -crf 18 \
-profile:v high -level 4.0 \
-bf 2 -g 30 \
-c:a aac -b:a 192k -ar 48000 \
-movflags +faststart \
out/video-youtube.mp4
# YouTube Shorts (vertical 1080x1920)
ffmpeg -i out/video.mp4 \
-vf "scale=1080:1920:force_original_aspect_ratio=decrease,pad=1080:1920:(ow-iw)/2:(oh-ih)/2" \
-c:v libx264 -crf 18 -c:a aac -b:a 192k \
out/video-shorts.mp4# Twitter optimized (under 15MB target for fast upload)
ffmpeg -i out/video.mp4 \
-c:v libx264 -preset medium -crf 24 \
-profile:v main -level 3.1 \
-vf "scale='min(1280,iw)':'min(720,ih)':force_original_aspect_ratio=decrease" \
-c:a aac -b:a 128k -ar 44100 \
-movflags +faststart \
-fs 15M \
out/video-twitter.mp4
# Check file size and duration
ffprobe -v error -show_entries format=duration,size -of csv=p=0 out/video-twitter.mp4# LinkedIn optimized
ffmpeg -i out/video.mp4 \
-c:v libx264 -preset medium -crf 22 \
-profile:v main \
-vf "scale='min(1920,iw)':'min(1080,ih)':force_original_aspect_ratio=decrease" \
-c:a aac -b:a 192k -ar 48000 \
-movflags +faststart \
out/video-linkedin.mp4# Web-optimized MP4 (small file, progressive loading)
ffmpeg -i out/video.mp4 \
-c:v libx264 -preset medium -crf 26 \
-profile:v baseline -level 3.0 \
-vf "scale=1280:720" \
-c:a aac -b:a 128k \
-movflags +faststart \
out/video-web.mp4
# WebM alternative (better compression, wider browser support)
ffmpeg -i out/video.mp4 \
-c:v libvpx-vp9 -crf 30 -b:v 0 \
-vf "scale=1280:720" \
-c:a libopus -b:a 128k \
-deadline good \
out/video-web.webm# High-quality GIF (first 5 seconds)
ffmpeg -i out/video.mp4 -t 5 \
-vf "fps=15,scale=480:-1:flags=lanczos,split[s0][s1];[s0]palettegen[p];[s1][p]paletteuse" \
out/preview.gif
# Smaller file GIF
ffmpeg -i out/video.mp4 -t 3 \
-vf "fps=10,scale=320:-1:flags=lanczos,split[s0][s1];[s0]palettegen[p];[s1][p]paletteuse" \
out/preview-small.gif#!/bin/bash
# save as: export-all-platforms.sh
INPUT="out/video.mp4"
# YouTube (high quality)
ffmpeg -i "$INPUT" -c:v libx264 -preset slow -crf 18 \
-c:a aac -b:a 192k -movflags +faststart \
out/video-youtube.mp4
# Twitter (compressed)
ffmpeg -i "$INPUT" -c:v libx264 -crf 24 \
-vf "scale='min(1280,iw)':'-2'" \
-c:a aac -b:a 128k -movflags +faststart \
out/video-twitter.mp4
# LinkedIn
ffmpeg -i "$INPUT" -c:v libx264 -crf 22 \
-c:a aac -b:a 192k -movflags +faststart \
out/video-linkedin.mp4
# Web embed (small)
ffmpeg -i "$INPUT" -c:v libx264 -crf 26 \
-vf "scale=1280:720" \
-c:a aac -b:a 128k -movflags +faststart \
out/video-web.mp4
echo "Exported:"
ls -lh out/video-*.mp4# Check if FFmpeg succeeded
ffmpeg -i input.mp4 -c:v libx264 output.mp4 && echo "Success" || echo "Failed: check input file"
# Validate output file is playable
ffprobe -v error -select_streams v:0 -show_entries stream=codec_name -of csv=p=0 output.mp4
# Get detailed error info
ffmpeg -v error -i input.mp4 -f null - 2>&1 | head -20