How to Use the FFmpeg Concat Filter

This article provides a straightforward guide on how to use the concat filter in FFmpeg to merge multiple video and audio files. You will learn the basic syntax of the filter, how to combine clips with different properties, and see practical command-line examples for seamless video concatenation.

The FFmpeg concat filter is used to join multiple video and audio streams back-to-back. Unlike the concat demuxer, which simply appends files without re-encoding, the concat filter re-encodes the files. This makes it ideal for joining videos that have different formats, codecs, resolutions, or frame rates.

The Basic Syntax

To use the concat filter, you must use a complex filtergraph (-filter_complex). The basic parameters for the filter are:

• n: The number of input files to concatenate. (Default is 2).
• v: The number of output video streams. (Default is 1).
• a: The number of output audio streams. (Default is 0).

Step-by-Step Example

Here is a standard command to concatenate two video files that both contain video and audio streams:

ffmpeg -i input1.mp4 -i input2.mp4 -filter_complex "[0:v][0:a][1:v][1:a]concat=n=2:v=1:a=1[outv][outa]" -map "[outv]" -map "[outa]" output.mp4

Command Breakdown:

1. -i input1.mp4 -i input2.mp4: Specifies the two input files.
2. [0:v][0:a]: Selects the video (v) and audio (a) streams of the first input file (index 0).
3. [1:v][1:a]: Selects the video and audio streams of the second input file (index 1).
4. concat=n=2:v=1:a=1: Tells FFmpeg to concatenate 2 segments, resulting in 1 video output and 1 audio output.
5. [outv][outa]: Names the resulting output streams.
6. -map "[outv]" -map "[outa]": Maps the named output streams to the final output file, output.mp4.

Handling Different Resolutions and Aspect Ratios

The concat filter requires all input streams to have the same width, height, and pixel format. If your input videos have different resolutions, you must scale them to matching dimensions before concatenating them.

You can chain the scale and setsar filters inside the filtergraph before applying the concat filter:

ffmpeg -i input1.mp4 -i input2.mp4 -filter_complex "[0:v]scale=1920:1080,setsar=1[v0]; [1:v]scale=1920:1080,setsar=1[v1]; [v0][0:a][v1][1:a]concat=n=2:v=1:a=1[outv][outa]" -map "[outv]" -map "[outa]" output.mp4

In this command, scale=1920:1080,setsar=1 resizes both video streams to a standard 1080p resolution and sets a 1:1 sample aspect ratio before feeding them into the concat filter as [v0] and [v1].