atxinsky/ffmpeg

FFmpeg Expert

Expert guide for FFmpeg command-line tool for video/audio processing, transcoding, streaming, and filtering.

Quick Start

Basic format conversion:

ffmpeg -i input.mp4 output.avi

Convert video with specific codec:

ffmpeg -i input.mp4 -c:v libx264 -c:a aac output.mp4

Extract audio from video:

ffmpeg -i input.mp4 -vn -acodec copy output.aac

Instructions

Step 1: Understand the User's Goal

First, clarify what the user wants to do:

• Format conversion: Change container format (e.g., MP4 to MKV, AVI to MP4)
• Transcoding: Re-encode video/audio with different codecs
• Stream extraction: Extract video, audio, or subtitle streams
• Stream mapping: Select specific streams from input
• Filtering: Apply video/audio filters (scale, crop, overlay, etc.)
• Streaming: Process live streams or network protocols
• Screen recording: Capture desktop or application windows
• GIF creation: Convert video to GIF or create from images

Step 2: Analyze Input Files

Use ffprobe to inspect media files:

ffprobe -v error -show_format -show_streams input.mp4

Key information to gather:

• Container format (MP4, MKV, AVI, etc.)
• Video codec (H.264, H.265, VP9, etc.)
• Audio codec (AAC, MP3, Opus, etc.)
• Resolution, frame rate, bitrate
• Duration
• Number and type of streams

Step 3: Build the Command

FFmpeg follows this syntax:

ffmpeg [global_options] {[input_file_options] -i input_url} ... {[output_file_options] output_url} ...

Input Options (placed before -i)

• -ss position: Seek to position
• -t duration: Limit input duration
• -r fps: Set frame rate (input)

Output Options (placed before output url)

• -c[:stream_specifier] codec: Set codec (use copy for streamcopy)
• -b[:stream_specifier] bitrate: Set bitrate
• -s size: Set frame size (WxH)
• -r fps: Set frame rate (output)
• -vf filtergraph: Video filter
• -af filtergraph: Audio filter
• -map input_file_id[:stream_specifier]: Map streams
• -an / -vn / -sn: Disable audio/video/subtitle

Step 4: Handle Common Operations

Format Conversion (Streamcopy)

Fast, no quality loss:

ffmpeg -i input.mp4 -c copy output.mkv

Video Transcoding

# CRF-based quality (recommended)
ffmpeg -i input.mp4 -c:v libx264 -crf 23 -c:a aac output.mp4

# Bitrate-based
ffmpeg -i input.mp4 -c:v libx264 -b:v 2M -maxrate 2M -bufsize 4M -c:a aac output.mp4

Resolution Scaling

# Scale to specific size
ffmpeg -i input.mp4 -vf scale=1280:720 output.mp4

# Scale maintaining aspect ratio
ffmpeg -i input.mp4 -vf scale=-1:720 output.mp4  # Height 720, auto width
ffmpeg -i input.mp4 -vf scale=1280:-1 output.mp4  # Width 1280, auto height

Extract Streams

# Extract audio
ffmpeg -i input.mp4 -vn -acodec copy audio.aac

# Extract video
ffmpeg -i input.mp4 -an -vcodec copy video.h264

# Extract subtitle
ffmpeg -i input.mkv -map 0:s:0 subs.srt

Trim Video

# Using -ss (inputseek - fast)
ffmpeg -ss 00:01:00 -i input.mp4 -to 00:02:00 -c copy output.mp4

# Using -ss (outputseek - accurate but slow)
ffmpeg -i input.mp4 -ss 00:01:00 -to 00:02:00 -c copy output.mp4

Combine Videos

# Concatenate with filter
ffmpeg -f concat -safe 0 -i filelist.txt -c copy output.mp4
# filelist.txt contains:
# file '/path/to/video1.mp4'
# file '/path/to/video2.mp4'

Create GIF

# High quality GIF
ffmpeg -i input.mp4 -vf "fps=10,scale=320:-1:flags=lanczos,split[s0][s1];[s0]palettegen[p];[s1][p]paletteuse" output.gif

# Simple GIF
ffmpeg -i input.mp4 -vf "fps=10,scale=320:-1" output.gif

Screen Recording

# macOS
ffmpeg -f avfoundation -i "1:0" -r 30 output.mp4

# Linux (X11)
ffmpeg -f x11grab -s 1920x1080 -r 30 -i :0.0 output.mp4

# Windows
ffmpeg -f gdigrab -framerate 30 -i desktop output.mp4

Step 5: Use Stream Specifiers

Apply options to specific streams:

• :v - All video streams
• :a - All audio streams
• :s - All subtitle streams
• :v:0 - First video stream
• :a:1 - Second audio stream
• :0:v:0 - Video stream 0 from input 0

Examples:

# Different codecs for different streams
ffmpeg -i input.mkv -c:v:0 libx264 -c:v:1 libx265 output.mkv

# Copy audio, re-encode video
ffmpeg -i input.mp4 -c:v libx264 -c:a copy output.mp4

Step 6: Apply Filters

Video Filters (via -vf)

# Deinterlace
ffmpeg -i input.mp4 -vf yadif output.mp4

# Crop
ffmpeg -i input.mp4 -vf crop=1280:720:0:0 output.mp4  # W:H:X:Y

# Rotate
ffmpeg -i input.mp4 -vf "transpose=1" output.mp4

# Overlay text
ffmpeg -i input.mp4 -vf "drawtext=text='Hello':fontsize=24:fontcolor=white:x=10:y=10" output.mp4

# Overlay image on video
ffmpeg -i video.mp4 -i overlay.png -filter_complex overlay output.mp4

# Stack videos horizontally
ffmpeg -i left.mp4 -i right.mp4 -filter_complex hstack output.mp4

Audio Filters (via -af)

# Volume adjustment
ffmpeg -i input.mp4 -af "volume=1.5" output.mp4

# Normalize audio
ffmpeg -i input.mp4 -af "loudnorm" output.mp4

# Fade in/out
ffmpeg -i input.mp4 -af "afade=t=in:st=0:d=2,afade=t=out:st=50:d=2" output.mp4

Step 7: Advanced Scenarios

Two-Pass Encoding

# Pass 1
ffmpeg -i input.mp4 -c:v libx264 -pass 1 -an -f null /dev/null

# Pass 2
ffmpeg -i input.mp4 -c:v libx264 -pass 2 -b:v 2M output.mp4

Hardware Acceleration

# NVIDIA (NVENC)
ffmpeg -hwaccel cuda -i input.mp4 -c:v h264_nvenc output.mp4

# Intel (QSV)
ffmpeg -hwaccel qsv -i input.mp4 -c:v h264_qsv output.mp4

# macOS (VideoToolbox)
ffmpeg -hwaccel videotoolbox -i input.mp4 -c:v h264_videotoolbox output.mp4

HLS Streaming

ffmpeg -i input.mp4 -c:v libx264 -c:a aac -f hls -hls_time 4 -hls_playlist_type vod output.m3u8

Extract Frames

# Extract one frame per second
ffmpeg -i input.mp4 -vf "fps=1" frame_%04d.png

# Extract frame at specific time
ffmpeg -ss 00:00:05 -i input.mp4 -frames:v 1 frame.png

Best Practices

Quality Settings

• CRF (Constant Rate Factor): Recommended for quality-based encoding

  • Range: 0-51 (lower = better quality, larger file)
  • Recommended: 18-28 for H.264
  • Default: 23 for H.264

• Presets: Balance encoding speed vs compression

  • ultrafast, superfast, veryfast, faster, fast
  • medium (default)
  • slow, slower, veryslow

Performance Tips

1. Use streamcopy (-c copy) when not transcoding - much faster
2. Avoid re-encoding when possible - quality loss accumulates
3. Use hardware acceleration when available
4. Presets: veryfast for live, veryslow for final encode
5. Thread count: -threads option (usually auto is best)

Common Pitfalls

• Aspect ratio: Always consider aspect ratio when scaling
• Audio sync: Use -vsync options if audio drift occurs
• Codec compatibility: Ensure target device supports chosen codecs
• Bitrate vs quality: Don't set both CRF and bitrate
• File extensions: Use correct extension for output format

Common Codecs

Video Codecs

• libx264 - H.264 (most compatible)
• libx265 / hevc - H.265 (better compression)
• libvpx-vp9 - VP9 (WebM)
• libaom-av1 - AV1 (next-gen)
• mpeg4 - MPEG-4 Part 2 (legacy)
• copy - Streamcopy (no re-encoding)

Audio Codecs

• aac - AAC (standard)
• libmp3lame - MP3
• libopus - Opus (modern)
• libvorbis - Vorbis (WebM)
• pcm_s16le - Uncompressed PCM
• copy - Streamcopy

Container Formats

• mp4 - Most compatible
• mkv - Feature-rich
• webm - Web optimized (VP8/VP9)
• avi - Legacy
• mov - QuickTime
• flv - Flash video

Stream Selection Examples

# Map all streams from input
ffmpeg -i input.mp4 -map 0 output.mkv

# Map only video from first input
ffmpeg -i input.mp4 -map 0:v output.mp4

# Map specific streams
ffmpeg -i1.mkv -i2.mp4 -map 0:0 -map 1:1 output.mp4

# Negative mapping (exclude)
ffmpeg -i input.mp4 -map 0 -map -0:a output.mp4  # Exclude all audio

# Map by language
ffmpeg -i input.mkv -map 0:m:language:eng output.mp4

Troubleshooting

Command fails

• Check if input file exists and is readable
• Verify codec is supported: ffmpeg -codecs
• Check syntax: options go before the file they affect

Quality issues

• Increase CRF value for better quality (lower number)
• Use slower preset for better compression
• Check bitrate settings

Audio sync issues

• Try different -vsync modes
• Use -async 1 to sync audio to video
• Check frame rate settings

Performance issues

• Use hardware acceleration
• Try faster preset
• Reduce resolution first, then encode
• Check CPU usage with -threads

Advanced Reference

For complex filtergraphs, see filter documentation

For codec options, use:

ffmpeg -h encoder=libx264
ffmpeg -h decoder=h264

For format options:

ffmpeg -h muxer=mp4
ffmpeg -h demuxer=mp4