ffmpeg（ffmpeg 音频）

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)

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

# 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

# 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

# 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

# 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

# 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

# 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

# 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

# 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

# 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:

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.

# 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):

# 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]"

Prepare Demo Recording for Remotion

# 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

Screen Recording to Remotion Asset

# 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.mp4

Batch Convert GIFs

for 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"
done

Common Issues

"Height not divisible by 2"

Add scale filter: -vf "scale=trunc(iw/2)*2:trunc(ih/2)*2"

Video won't play in browser

Use: -movflags faststart -pix_fmt yuv420p -c:v libx264

Audio out of sync after speed change

Use filter_complex with atempo: -filter_complex "[0:v]setpts=0.5*PTS[v];[0:a]atempo=2.0[a]"

File too large

Increase CRF (23→28) or reduce resolution

Quality Guidelines

Platform-Specific Output Optimization

After Remotion renders your video (typically to out/video.mp4), use FFmpeg to optimize for each distribution platform.

Workflow Integration

Remotion 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 (Recommended Settings)

YouTube re-encodes everything, so upload high quality:

# 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/X

Twitter has strict limits: max 140s, 512MB, 1920x1200:

# 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

LinkedIn prefers MP4 with AAC audio, max 10 minutes:

# 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

Website/Embed (Optimized for Fast Loading)

# 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

GIF (for Previews/Thumbnails)

# 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

Platform Requirements Quick Reference

Batch Export for All Platforms

#!/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

Error Handling

Common errors and fixes when processing video:

# 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

Handling Common Failures

Feedback & Contributions

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