ffmpeg-pro

Overview

FFmpeg is the Swiss Army knife of media: conversion, compression, trimming, subtitles, streaming, thumbnails, GIFs — all scriptable, all free. Professionals use it for batch pipelines no GUI can match. This skill covers the essential FFmpeg patterns: the flags you actually need, quality control, and recipes for the 20 tasks that cover 95% of real work.

When to use

• Converting video/audio between formats and codecs

• Compressing files for web delivery or messaging apps

• Trimming, concatenating, or extracting clips without re-encoding

• Burning in subtitles or extracting subtitle tracks

• Batch-processing media files with scripts

Core concepts

• • The basic anatomy. ffmpeg -i input.mp4 [options] output.mp4. Input flags before -i, output flags after. Order matters; -ss before -i is fast seek, after is accurate seek.
• • Codecs: -c:v and -c:a. copy = stream copy (no re-encode, instant, lossless). libx264/libx265 for video, aac for audio. Default aac bitrate is low — always set -b:a 192k or higher.
• • Quality control: CRF. Constant Rate Factor: -crf 23 is default for x264 (lower = better quality, larger file). Sweet spots: 18-20 (high quality), 21-24 (web delivery), 28+ (small files, visible loss). One number beats bitrate guessing for most jobs.
• • Presets trade time for size. -preset slow compresses better than ultrafast at the same CRF — same quality, smaller file, longer encode. For one-off encodes, medium/slow is worth it; for live, ultrafast.
• • Filters: -vf and -af. Video/audio filter chains: scale, crop, fps, subtitles, loudnorm, volume. Chain with commas: -vf "scale=1280:-1,subtitles=subs.srt".
• • Never overwrite blindly. FFmpeg prompts before overwriting; in scripts use -n (never overwrite) or -y (always). Test commands on copies first — a wrong flag can destroy the only copy.

Practical workflow

Essential recipes (adapt paths as needed):

1. Compress for web (H.264): ffmpeg -i input.mp4 -c:v libx264 -crf 23 -preset medium -c:a aac -b:a 192k -movflags +faststart output.mp4 (faststart moves metadata to the front for web streaming.)

2. Compress smaller (H.265): ffmpeg -i input.mp4 -c:v libx265 -crf 26 -preset medium -c:a aac -b:a 160k -tag:v hvc1 output.mp4

3. Lossless trim (no re-encode): ffmpeg -ss 00:01:30 -i input.mp4 -t 00:00:45 -c copy clip.mp4

4. Extract audio: ffmpeg -i input.mp4 -vn -c:a libmp3lame -b:a 192k audio.mp3

5. Convert to GIF (high quality, two-pass palette): ffmpeg -i input.mp4 -vf "fps=15,scale=480:-1:flags=lanczos,palettegen" palette.png ffmpeg -i input.mp4 -i palette.png -lavfi "fps=15,scale=480:-1:flags=lanczos[x];[x][1:v]paletteuse" out.gif

6. Burn in subtitles: ffmpeg -i input.mp4 -vf "subtitles=subs.srt:force_style='FontSize=18'" -c:a copy out.mp4

7. Extract thumbnail strip / single frame: ffmpeg -ss 00:00:10 -i input.mp4 -frames:v 1 thumb.jpg

8. Concatenate same-codec files:

   • Write a list file (file 'a.mp4' per line), then: ffmpeg -f concat -safe 0 -i list.txt -c copy joined.mp4

9. Normalize loudness (podcast to -16 LUFS): ffmpeg -i in.wav -af loudnorm=I=-16:TP=-1.5:LRA=11 out.wav

10. Batch convert a folder: for f in *.mov; do ffmpeg -i "$f" -c:v libx264 -crf 23 -c:a aac -b:a 192k "${f%.mov}.mp4"; done

Common pitfalls

• • Forgetting -c:a bitrate. Default AAC bitrate (~128k or lower) can audibly degrade. Always set it explicitly.
• • Re-encoding when copy would do. Trimming or remuxing with default settings re-encodes unnecessarily — slower and lossy. Use -c copy when not changing the streams.
• • -ss placement confusion. Before -i: fast but keyframe-limited (cuts snap to keyframes). After -i: frame-accurate but slower. For precise cuts, put it after.
• • No faststart for web. Without -movflags +faststart, MP4s must fully download before playback starts. Always include it for web delivery.
• • Overwriting originals. Running conversions in-place or with -y in a loop over the source folder. Output to a separate directory, verify, then clean up.
• • Ignoring pixel formats. Some players choke on yuv444p from x264 defaults with certain inputs — add -pix_fmt yuv420p for maximum compatibility.