How to use FFmpeg Filter? What are the common video and audio filters? - 面试题

FFmpeg Filter is one of the most powerful features in FFmpeg, allowing for various complex processing operations on audio and video.

Filter Basic Syntax

Filter Chain Syntax

bash
# Basic format
-filter "filter1=param1=value1:param2=value2,filter2=param1=value1"

# Complex filter chain
-filter_complex "[0:v]scale=1280:720[v];[0:a]volume=0.5[a];[v][a]concat=n=1:v=1:a=1"

Filter Label Explanation

[0:v]: Video stream from input 0
[0:a]: Audio stream from input 0
[v]: Named output stream
;: Separator for different filter chains
,: Separator for filters in the same chain

Common Video Filters

Scale Filter

bash
# Fixed resolution
ffmpeg -i input.mp4 -vf "scale=1280:720" output.mp4

# Maintain aspect ratio
ffmpeg -i input.mp4 -vf "scale=1280:-1" output.mp4

# Scale by ratio
ffmpeg -i input.mp4 -vf "scale=iw/2:ih/2" output.mp4

# Limit maximum size
ffmpeg -i input.mp4 -vf "scale='min(1280,iw)':'min(720,ih)'" output.mp4

Crop Filter

bash
# Crop center area
ffmpeg -i input.mp4 -vf "crop=640:480" output.mp4

# Specify crop position
ffmpeg -i input.mp4 -vf "crop=640:480:10:10" output.mp4

# Crop bottom-right corner
ffmpeg -i input.mp4 -vf "crop=640:480:in_w-640:in_h-480" output.mp4

Overlay Filter

bash
# Overlay image at specified position
ffmpeg -i video.mp4 -i logo.png -filter_complex "overlay=10:10" output.mp4

# Overlay at bottom-right
ffmpeg -i video.mp4 -i logo.png -filter_complex "overlay=W-w-10:H-h-10" output.mp4

# Overlay at center
ffmpeg -i video.mp4 -i logo.png -filter_complex "overlay=(W-w)/2:(H-h)/2" output.mp4

Transpose Filter

bash
# Rotate 90 degrees clockwise
ffmpeg -i input.mp4 -vf "transpose=1" output.mp4

# Rotate 90 degrees counter-clockwise
ffmpeg -i input.mp4 -vf "transpose=2" output.mp4

# Rotate 180 degrees
ffmpeg -i input.mp4 -vf "transpose=1,transpose=1" output.mp4

Drawtext Filter

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

# Add text with shadow
ffmpeg -i input.mp4 -vf "drawtext=text='Hello':fontcolor=white:fontsize=32:x=10:y=10:shadowcolor=black:shadowx=2:shadowy=2" output.mp4

# Add dynamic text (timestamp)
ffmpeg -i input.mp4 -vf "drawtext=text='%{pts\:hms}':fontcolor=white:fontsize=24:x=10:y=10" output.mp4

Gblur Filter

bash
# Gaussian blur
ffmpeg -i input.mp4 -vf "gblur=sigma=2" output.mp4

# Strong blur
ffmpeg -i input.mp4 -vf "gblur=sigma=5" output.mp4

Unsharp Filter

bash
# Sharpen
ffmpeg -i input.mp4 -vf "unsharp=5:5:1.0:5:5:0.0" output.mp4

# Slight sharpen
ffmpeg -i input.mp4 -vf "unsharp=3:3:0.5:3:3:0.0" output.mp4

Common Audio Filters

Volume Filter

bash
# Adjust volume (1.0 is original volume)
ffmpeg -i input.mp4 -af "volume=0.5" output.mp4

# Boost volume
ffmpeg -i input.mp4 -af "volume=2.0" output.mp4

# Use decibels
ffmpeg -i input.mp4 -af "volume=3dB" output.mp4

Afade Filter

bash
# Fade in for 3 seconds
ffmpeg -i input.mp4 -af "afade=t=in:st=0:d=3" output.mp4

# Fade out for 3 seconds
ffmpeg -i input.mp4 -af "afade=t=out:st=5:d=3" output.mp4

Amix Filter

bash
# Mix two audio tracks
ffmpeg -i audio1.mp3 -i audio2.mp3 -filter_complex "amix=inputs=2:duration=first" output.mp3

# Mix three audio tracks
ffmpeg -i audio1.mp3 -i audio2.mp3 -i audio3.mp3 -filter_complex "amix=inputs=3:duration=longest" output.mp3

Adelay Filter

bash
# Delay audio by 500 milliseconds
ffmpeg -i input.mp4 -af "adelay=500|500" output.mp4

# Different delay for left and right channels
ffmpeg -i input.mp4 -af "adelay=500|1000" output.mp4

Complex Filter Examples

Picture-in-Picture Effect

bash
ffmpeg -i main.mp4 -i pip.mp4 -filter_complex \
  "[1:v]scale=320:240[pip];[0:v][pip]overlay=10:10" \
  output.mp4

Multi-Screen Splicing

bash
ffmpeg -i video1.mp4 -i video2.mp4 -i video3.mp4 -i video4.mp4 \
  -filter_complex \
  "[0:v]scale=640:360[v0];[1:v]scale=640:360[v1];[2:v]scale=640:360[v2];[3:v]scale=640:360[v3]; \
   [v0][v1]hstack[top];[v2][v3]hstack[bottom];[top][bottom]vstack" \
  output.mp4

Video Speed Up/Down

bash
# Speed up 2x
ffmpeg -i input.mp4 -filter_complex "[0:v]setpts=0.5*PTS[v];[0:a]atempo=2.0[a]" -map "[v]" -map "[a]" output.mp4

# Slow down 0.5x
ffmpeg -i input.mp4 -filter_complex "[0:v]setpts=2.0*PTS[v];[0:a]atempo=0.5[a]" -map "[v]" -map "[a]" output.mp4

Filter Performance Optimization

Use Hardware Acceleration

bash
# Use GPU accelerated filter
ffmpeg -hwaccel cuda -i input.mp4 -vf "scale_npp=1280:720" output.mp4

Preprocessing Optimization

bash
# Crop first then scale (reduce processing pixels)
ffmpeg -i input.mp4 -vf "crop=640:480,scale=320:240" output.mp4

Filter is one of the most powerful features of FFmpeg. Mastering the use of filters allows for fine-grained control over audio and video.