FFmpeg

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