ffmpeg

What the source does

• FilesystemNone observedNo reads or writes in the source
• NetworkNone observedNo outbound calls in the source
• ShellNone observedNo commands run in the source
• DependenciesPinnedNothing floats
• SecretsNone foundNo keys in source

Five audit layers

Gates applied: no behavioural pass.

Audit 0.4.1 · source sha 13ec33b09272 · 29 Oct 2026Report an issue or request a re-scan

• Publishercalesthio GitHub userOwner of the repository this skill is read from
• Source foldercalesthio/openmontage/.agents/skills/ffmpegThe exact folder we audited
• LicenceAGPL-3.0As the repository states it
• Commit pinned for this verdict13ec33b09272A new audit runs when the source changes
• Repository statusNot archivedPushed today
• Sandboxed runNot runSkills are read, not executed; layer L4 is skipped

Its description: 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,…

---
name: ffmpeg
description: 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.
---

# FFmpeg for Video Production

FFmpeg is the essential tool for video/audio processing. This skill covers common operations for Remotion video projects.

## Quick Reference

### GIF to MP4 (Remotion-compatible)

```bash
ffmpeg -i input.gif -movflags faststart -pix_fmt yuv420p \
  -vf "scale=trunc(iw/2)*2:trunc(ih/2)*2" output.mp4
```

**Why these flags:**
- `-movflags faststart` - Moves metadata to start for web streaming
- `-pix_fmt yuv420p` - Ensures compatibility with most players
- `scale=trunc(...)` - Forces even dimensions (required by most codecs)

### Resize Video

```bash
# 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
```

### Compress Video

```bash
# 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 Audio

```bash
# 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
```

### Convert Audio Formats

```bash
# 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
```

### Trim/Cut Video

```bash
# 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
```

**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.

### Speed Up / Slow Down

```bash
# 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
```

### Concatenate Videos

```bash
# 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
```

### Add Fade In/Out

```bash
# 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
```

### Get Video Info

```bash
# 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
```

## Remotion-Specific Patterns

### Video Speed Adjustment for Remotion

**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.

```bash
# 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
```

**Calculate speed factor:**
- To fit X seconds of video into Y seconds of scene: `speed = X / Y`
- setpts multiplier = `1 / speed` (e.g., 3x speed = setpts=0.333*PTS)
- atempo value = `speed` (e.g., 3x speed = atempo=3.0)

**Extreme speed (>2x audio):** Chain atempo filters (each limited to 0.5-2.0 range):
```bash
# 4x speed audio
-filter_complex

Excerpt, first 6,000 characters of SKILL.md, verbatim from the audited commit. It is text from someone else’s repository, quoted so you can judge it before your agent reads it.

git clone https://github.com/calesthio/openmontage
git -C openmontage checkout 13ec33b09272

Works in

1. 2026-10-29
   ✓SAFE B 89 / 100

   first audit · source 13ec33b09272

Questions about ffmpeg

• github.com/calesthio/openmontage/tree/main/.agents/skills/ffmpeg — the audited source at commit 13ec33b09272 and every finding aboveObserved · 29 Oct 2026

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