Ffmpeg

FFmpeg

Use this skill to produce reliable, explainable FFmpeg/FFprobe commands for media automation.

Workflow

1. Identify the user's source media, desired output container/codec, target dimensions/duration, and whether quality, speed, or file size matters most.
2. Prefer the simplest command that meets the goal.
3. Use -c copy only when no filtering, re-encoding, precise trimming, subtitle burn-in, compression, or codec change is needed.
4. Use explicit stream mapping when multiple inputs or outputs are involved.
5. For commands with filters, quote the filter graph and name streams in filter_complex when it improves readability.
6. Include -y only when the user wants non-interactive overwrite behavior.
7. Validate command intent with ffprobe or a low-duration sample when practical.

Defaults

• Web-compatible MP4: -c:v libx264 -crf 18 -preset slow -pix_fmt yuv420p -c:a aac -movflags +faststart
• High-quality archival H.264: -c:v libx264 -crf 17 or -crf 18
• Smaller modern MP4: -c:v libx265 -crf 24 -vtag hvc1 -c:a aac
• WebM/VP9: -c:v libvpx-vp9 -crf 15 -b:v 0 -c:a libopus
• Preserve audio when safe: -c:a copy
• Force square pixels after scale/pad/crop: setsar=1:1
• Playback compatibility from generated images: format=yuv420p or -pix_fmt yuv420p

Common command patterns

Inspect media

ffprobe -hide_banner -show_streams -show_format input.mp4

Remux without re-encoding

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

Resize and pad to vertical 1080x1920

ffmpeg -i input.mp4 \
  -vf "scale=1080:1920:force_original_aspect_ratio=decrease,pad=1080:1920:(ow-iw)/2:(oh-ih)/2:color=black,setsar=1:1" \
  -c:v libx264 -crf 18 -preset slow -pix_fmt yuv420p -c:a aac -movflags +faststart \
  output.mp4

Frame-accurate trim

Use output seeking and re-encode for reliable exact cuts:

ffmpeg -i input.mp4 -ss 00:00:10 -to 00:00:25 \
  -c:v libx264 -crf 18 -preset slow -c:a aac output_trimmed.mp4

Use input seeking before -i only when speed matters more than accuracy.

Replace audio

ffmpeg -i input.mp4 -i music.mp3 \
  -map 0:v -map 1:a -shortest -c:v copy -c:a aac output.mp4

Mix background music under original audio

ffmpeg -i input.mp4 -i music.mp3 \
  -filter_complex "[1:a]volume=0.2[bgm];[0:a][bgm]amix=inputs=2:duration=shortest[a]" \
  -map 0:v -map "[a]" -c:v copy -c:a aac -shortest output.mp4

Overlay logo for a time range

ffmpeg -i input.mp4 -i logo.png \
  -filter_complex "overlay=x=(main_w-overlay_w)/8:y=(main_h-overlay_h)/8:enable='gte(t,1)*lte(t,7)'" \
  -c:v libx264 -crf 18 -preset slow -c:a copy output.mp4

Burn subtitles

ffmpeg -i input.mp4 \
  -vf "subtitles=subtitles.srt:fontsdir=.:force_style='FontName=Poppins,FontSize=24,PrimaryColour=&HFFFFFF'" \
  -c:v libx264 -crf 18 -preset slow -c:a copy output_subtitled.mp4

Add soft subtitle track

ffmpeg -i input.mp4 -i subtitles.srt \
  -c copy -c:s srt -disposition:s:0 default output.mkv

Thumbnail

ffmpeg -i input.mp4 -ss 00:00:07 -frames:v 1 -q:v 2 thumbnail.jpg

GIF from video

ffmpeg -i input.mp4 \
  -vf "select='gt(trunc(t/2),trunc(prev_t/2))',setpts='PTS*0.1',scale=trunc(oh*a/2)*2:320:force_original_aspect_ratio=decrease,pad=trunc(oh*a/2)*2:320:-1:-1" \
  -loop 0 -an output.gif

Takes every second frame and accelerates playback 10x; scale=trunc(oh*a/2)*2:320 forces an even width derived from a 320px height while preserving aspect ratio. Use -loop 1 for play-once.

Gotchas

• -c copy cannot apply filters and is not frame-accurate for most cuts.
• -ss before -i is fast but approximate; -ss after -i is slower but accurate.
• MP4 playback in QuickTime and browsers often needs yuv420p for H.264 output.
• Use -movflags +faststart for web-hosted MP4/MOV/M4A so playback starts sooner.
• In filter graphs, [0:v] means first input's video stream, [1:a] means second input's audio stream, and -map chooses which streams become outputs.
• Escaping filter expressions is shell-sensitive. Prefer single quotes inside double-quoted filter graphs for time expressions, and escape commas inside nested expressions when required.
• drawtext with special characters is fragile; prefer textfile= for user-supplied text.
• Burned subtitles require re-encoding; soft subtitle tracks can often use stream copy.

When more depth is needed

Load references/command-patterns.md for additional examples and decision rules distilled from this repository's FFmpeg cheat sheet.