How do I avoid re-encoding when not needed?

Use -c copy to copy all streams to a new container; ensure codecs are compatible with the target container

What is the difference between -ss before and after -i?

-ss before -i offers fast seeking but may be inaccurate; -ss after -i is frame-accurate but slower because it decodes from the start

How can I fix audio desynchronization after cutting?

Use -async 1 or -af aresample=async=1 to fix audio drift; consider re-timing with setpts and ensuring timestamps are correctly reset

FFmpeg

Verified

@ivangdavila

npx machina-cli add skill @ivangdavila/ffmpeg --openclaw

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SKILL.md

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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

Source

git clone https://clawhub.ai/ivangdavila/ffmpegView on GitHub

Overview

FFmpeg is a command-line tool to process video and audio by selecting appropriate codecs, applying filters, and encoding to target formats. It helps control quality, compatibility, and file size across containers and codecs. Mastery covers input seeking, stream selection, encoding quality, filtering, and hardware acceleration.

How This Skill Works

FFmpeg uses command-line options to map streams, choose containers and codecs, and apply filter graphs. It supports fast or frame-accurate seeking, per-stream mapping, CRF-based quality control, and complex filter chains via -vf and -filter_complex. Audio processing, concatenation, subtitles, and hardware acceleration are handled through specialized flags like -c:v, -c:a, and -hwaccel.

When to Use It

Cutting video while trimming with fast seek (-ss before -i) and accurate trim (-ss after -i) for precise segments
Re-encoding to a target codec/quality using CRF and preset for size-quality trade-offs
Copying all streams to a new container without re-encoding (-c copy) to save time and avoid quality loss
Extracting or isolating audio from a video (-vn to drop video, -c:a copy to preserve audio)
Concatenating multiple video files with matching codecs/params using concat demuxer or filter_complex

Quick Start

Step 1: Install FFmpeg and run ffmpeg -version to verify
Step 2: Start with a simple command, e.g., ffmpeg -i input.mp4 -c copy output.mp4 to copy streams
Step 3: Incrementally add filters or re-encoding options (e.g., -vf or -c:v libx264 -crf 23) and test results

Best Practices

Explicitly map streams when you need a specific video/audio combination (-map 0:v:0 -map 0:a:1)
Use -c copy when re-encoding is unnecessary to keep speed and prevent quality loss
Choose CRF and -preset based on desired quality and file size (lower CRF = better quality; slower presets yield smaller files)
Place -ss before -i for fast seeks; use -avoid_negative_ts make_zero when cutting to fix timestamps
Ensure container and codec compatibility (e.g., H.264 in MP4; VP9 in WebM) and avoid mixing incompatible combinations

Example Use Cases

ffmpeg -ss 00:30:00 -i input.mp4 -ss 00:00:05 -t 00:00:10 -c copy clip.mp4
ffmpeg -i input.mov -c:v libx264 -crf 23 -preset medium -c:a aac -b:a 192k output.mp4
ffmpeg -i input.mkv -c copy output.mp4
ffmpeg -i video.mp4 -vn -c:a copy audio.m4a
ffmpeg -f concat -safe 0 -i list.txt -c copy output.mp4

Frequently Asked Questions

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