FFmpeg

Name: FFmpeg
Author: bighardperson

bighardperson/computer-science-skills-collection

8 installs
33 repo stars
Updated April 26, 2026
bighardperson/computer-science-skills-collection

ffmpeg is a skill that guides video and audio processing with FFmpeg through correct codec selection, filtering, and encoding settings.

About

ffmpeg is a reference skill for processing video and audio with FFmpeg using correct codec selection, filtering, and encoding settings. A developer uses it to pick CRF and preset values, map streams, apply filters, concatenate clips, burn or mux subtitles, and choose hardware acceleration. It also lists common FFmpeg mistakes to avoid.

Reference for FFmpeg codec, container, and encoding choices
Covers seeking, stream mapping, filters, concat, subtitles, and hardware acceleration
Lists common FFmpeg mistakes like re-encoding when -c copy was intended

FFmpeg by the numbers

8 all-time installs (skills.sh)
Ranked #1,074 of 1,337 Generative Media skills by installs in the Skillselion catalog
Data as of Jul 30, 2026 (Skillselion catalog sync)

At a glance

FFmpeg capabilities & compatibility

Free; only needs the ffmpeg binary installed locally.

Capabilities: video transcode · audio process · subtitle mux
Use cases: video generation
Platforms: Linux · macOS · Windows
Pricing: Free

From the docs

What FFmpeg says it does

Process video and audio with correct codec selection, filtering, and encoding settings.

SKILL.md

CRF (Constant Rate Factor): lower = better quality, larger file—18-23 typical for H.264

SKILL.md

npx skills add https://github.com/bighardperson/computer-science-skills-collection --skill ffmpeg

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Installs	8
repo stars	★ 33
Last updated	April 26, 2026
Repository	bighardperson/computer-science-skills-collection ↗

What it does

Process video and audio with FFmpeg using correct codec, container, filter, and encoding settings.

Who is it for?

Getting FFmpeg codec, filter, and encoding flags right for video and audio jobs.

Skip if: Generating new media from prompts or non-FFmpeg pipelines.

When should I use this skill?

The user needs to transcode, trim, filter, concatenate, or subtitle video or audio with FFmpeg.

What you get

Correct FFmpeg command lines for transcoding, trimming, filtering, and muxing

By the numbers

11 topic sections from input seeking to common mistakes
CRF range 18-23 typical for H.264

Files

SKILL.mdMarkdownGitHub ↗

Input Seeking (Major Difference)

-ss BEFORE -i: fast seek, may be inaccurate—starts from nearest keyframe
-ss AFTER -i: frame-accurate but slow—decodes from start
Combine both: -ss 00:30:00 -i input.mp4 -ss 00:00:05—fast seek then accurate trim
For cutting, add -avoid_negative_ts make_zero to fix timestamp issues

Stream Selection

Default: first video + first audio—may not be what you want
Explicit selection: -map 0:v:0 -map 0:a:1—first video, second audio
All streams of type: -map 0:a—all audio streams
Copy specific: -map 0 -c copy—all streams, no re-encoding
Exclude: -map 0 -map -0:s—all except subtitles

Encoding Quality

CRF (Constant Rate Factor): lower = better quality, larger file—18-23 typical for H.264
-preset: ultrafast to veryslow—slower = smaller file at same quality
Two-pass for target bitrate: first pass analyzes, second pass encodes
-crf and -b:v mutually exclusive—use one or the other

Container vs Codec

Container (MP4, MKV, WebM): wrapper format holding streams
Codec (H.264, VP9, AAC): compression algorithm for stream
Not all codecs fit all containers—H.264 in MP4/MKV, not WebM; VP9 in WebM/MKV, not MP4
Copy codec to new container: -c copy—fast, no quality loss

Filter Syntax

Simple: -vf "scale=1280:720"—single filter chain
Complex: -filter_complex "[0:v]scale=1280:720[scaled]"—named outputs for routing
Chain filters: -vf "scale=1280:720,fps=30"—comma-separated
Filter order matters—scale before crop gives different result than crop before scale

Common Filters

Scale: scale=1280:720 or scale=-1:720 for auto-width maintaining aspect
Crop: crop=640:480:100:50—width:height:x:y from top-left
FPS: fps=30—change framerate
Trim: trim=start=10:end=20,setpts=PTS-STARTPTS—setpts resets timestamps
Overlay: overlay=10:10—position from top-left

Audio Processing

Sample rate: -ar 48000—standard for video
Channels: -ac 2—stereo
Audio codec: -c:a aac -b:a 192k—AAC at 192kbps
Normalize: -filter:a loudnorm—EBU R128 loudness normalization
Extract audio: -vn -c:a copy output.m4a—no video, copy audio

Concatenation

Same codec/params: concat demuxer—-f concat -safe 0 -i list.txt -c copy
Different formats: concat filter—-filter_complex "[0:v][0:a][1:v][1:a]concat=n=2:v=1:a=1"
list.txt format: file 'video1.mp4' per line—escape special characters
Different resolutions: scale/pad to match before concat filter

Subtitles

Burn-in (hardcode): -vf "subtitles=subs.srt"—cannot be turned off
Mux as stream: -c:s mov_text (MP4) or -c:s srt (MKV)—user toggleable
From input: -map 0:s—include subtitle streams
Extract: -map 0:s:0 subs.srt—first subtitle to file

Hardware Acceleration

Decode: -hwaccel cuda or -hwaccel videotoolbox (macOS)
Encode: -c:v h264_nvenc (NVIDIA), -c:v h264_videotoolbox (macOS)
Not always faster—setup overhead; benefits show on long videos
Quality may differ—software encoding often produces better quality

Common Mistakes

Forgetting -c copy when not re-encoding—defaults to re-encode, slow and lossy
-ss after -i for long videos—takes forever seeking
Audio desync after cutting—use -async 1 or -af aresample=async=1
Filter on stream copy—filters require re-encoding; -c copy + -vf = error
Output extension doesn't set codec—output.mp4 without -c:v uses default, may not be H.264