FFmpeg filter_complex Complete Guide for Multiple Filters

FFmpeg's filter_complex option is a powerful tool for handling complex audio and video filters. It allows you to combine multiple filters in a single command, enabling various audio and video processing effects from simple to complex. Whether you're adding watermarks, subtitles, adjusting colors, or compositing images, filter_complex can handle it all.

Core Syntax and Concepts

Basic Syntax Structure

The basic format of a filter:

filtername=option1=value1:option2=value2:option3=value3...

Complete command format using filter_complex:

ffmpeg -i input0 -i input1 -i inputN -filter_complex "multiple filter combination expression" output.mp4

Multiple Filter Combination Rules

The core idea of multiple filter combination expressions is:

[input]filter[output];[input]filter[output];

Use ; semicolons to separate different filter chains (pipelines)
Use : colons to separate multiple parameters of the same filter
Use [] square brackets to identify input sources and output variables

Input Source Identification

[0]: First input file
[0:v]: Video track of the first input file
[0:a]: Audio track of the first input file
[0:s]: Subtitle track of the first input file
[0:a:1]: Second audio track of the first input file (index starts from 0)
Custom variables: Such as [temp], [out], etc., used to pass intermediate results between filter chains

Practical Examples

Example 1: Add Watermark and Subtitles

Add image watermark and subtitles to a video simultaneously:

ffmpeg -y -i v.mp4 -i logo.png -filter_complex "[0][1]overlay=(W-w)/2:(H-h)/2[watermarked];[watermarked]subtitles=s.srt:force_style='FontName=PingFang SC,Fontsize=16'" output.mp4

Note: Subtitle files are specified directly in the subtitles filter and do not need to be input separately with the -i parameter.

Example 2: Create Rotated Text Watermark

Create a PNG watermark image with rotated text and overlay it on a video:

Generate test video:

ffmpeg -y -f lavfi -i testsrc=size=192x108:rate=10:duration=10 -vcodec libx264 -pix_fmt yuv420p -f h264 input.264

Generate watermark image with rotated text:

ffmpeg -y -f h264 -i input.264 -filter_complex "color=black@0:100x100,format=rgba[c];[c]scale2ref[ct][mv];[ct]setsar=1,drawtext=text='WATERMARK':x=(W-tw)/2:y=(H-th)/2:fontfile=arial.ttf:fontsize=(w+h)/18:[email protected],split[txt][alpha],[txt][alpha]alphamerge,rotate=(-27*PI)/180:ow=iw:oh=ih:c=black@0,drawtext=text='Bottom Center':x=(W-tw)/2:y=H-th-3:fontfile=arial.ttf:fontsize=12:[email protected]:box=1:[email protected]:boxborderw=5,drawtext=text='Top Center':x=(W-tw)/2:y=3:fontfile=arial.ttf:fontsize=7:fontcolor=4F81BD@1:box=1:boxcolor=C0504D@1:boxborderw=5[fin];[mv]nullsink" -frames:v 1 -map "[fin]" rot_text.png

Overlay watermark on video:

ffmpeg -y -r 25 -f h264 -i input.264 -i rot_text.png -filter_complex "[0][1]overlay=x=(W-w)/2:y=(H-h)/2" -c:v libx264 output.mp4

Example 3: Separate Audio and Video Processing

Scale the video while adjusting audio volume:

ffmpeg -i input.mp4 -filter_complex "[0:v]scale=1280:720[video_out];[0:a]volume=1.5[audio_out]" -map "[video_out]" -map "[audio_out]" output.mp4

Common Questions

1. Does the order of filter chains matter?

Yes, the order in which filters are applied directly affects the final result. For example, scaling before adding a watermark produces different results than adding a watermark before scaling.

2. How to handle multiple input sources?

You can use multiple -i parameters to specify multiple input sources, then reference them in the filter expression using [0], [1], etc.

3. How to optimize performance for complex filters?

Minimize unnecessary filter operations
Use hardware-accelerated encoding/decoding (such as -hwaccel nvdec and -c:v h264_nvenc in examples)
Set intermediate variables appropriately to avoid redundant calculations