- name
- FFmpeg
- description
- Process video and audio with correct codec selection, filtering, and encoding settings.
- metadata
- {"clawdbot":{"emoji":"🎬","requires":{"bins":["ffmpeg"]},"os":["linux","darwin","win32"]}}
Input Seeking (Major Difference)
-ssBEFORE-i: fast seek, may be inaccurate—starts from nearest keyframe-ssAFTER-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_zeroto 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
-crfand-b:vmutually 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=1280:720orscale=-1:720for 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 cudaor-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 copywhen not re-encoding—defaults to re-encode, slow and lossy -ssafter-ifor long videos—takes forever seeking- Audio desync after cutting—use
-async 1or-af aresample=async=1 - Filter on stream copy—filters require re-encoding;
-c copy+-vf= error - Output extension doesn't set codec—
output.mp4without-c:vuses default, may not be H.264