FFmpeg

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

Security Scan

The skill is an instruction-only FFmpeg helper whose requirements and instructions align with its stated purpose and do not request unrelated credentials or risky installs.

Purpose & Capability

Name/description (FFmpeg processing) match the SKILL.md content. The only required binary is ffmpeg, which is exactly what the skill needs to perform the documented tasks.

Instruction Scope

SKILL.md contains usage guidance and example ffmpeg command lines (seeking, mapping, filters, encoding, hardware accel, etc.). It does not instruct the agent to read unrelated system files, environment variables, or to transmit data to external endpoints; examples reference user media files and local lists/subtitle files, which is expected for this functionality.

Install Mechanism

No install spec (instruction-only). Nothing is downloaded or written to disk by the skill itself, which minimizes install-time risk.

Credentials

The skill requests no environment variables, credentials, or config paths. That is proportionate for a tool that just builds/runs ffmpeg commands locally.

Persistence & Privilege

always is false and the skill does not request persistent agent-level privileges or modify other skills/configs. Autonomous invocation is allowed (platform default) but not an additional privilege here.

Assessment

This skill is an instruction-only FFmpeg guide and appears coherent. Before using it, ensure you have ffmpeg installed from a trusted source and that you trust any media files you allow the agent to operate on. The agent will construct and run ffmpeg commands — review commands before execution to avoid accidental overwrites or processing of sensitive files. If the agent runs shell commands on filenames supplied by users, watch for possible command/filename injection depending on how execution is implemented; prefer safe argument passing (no unescaped shell interpolation) or sandboxing. Finally, note hardware-acceleration flags require appropriate drivers and devices and may behave differently on different hosts.

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