How to Use ffmpeg -re with Multiple Input Files

This article explains how to use the -re (read at native frame rate) option in FFmpeg when working with multiple input files. You will learn how FFmpeg interprets this flag, how to correctly position it in your command line for multiple sources, and see practical examples for live streaming and file concatenation.

Understanding the -re Option in FFmpeg

The -re option tells FFmpeg to read the input at its native frame rate, simulating a live broadcast. By default, FFmpeg reads files as fast as possible to complete processing quickly. The -re option is essential when you are streaming a file to a live destination (like RTMP or SRT) to prevent the stream from buffering or flooding the ingest server.

Position Matters: How FFmpeg Applies -re

In FFmpeg, command-line options are position-sensitive. An input option like -re applies only to the input file that immediately follows it.

If you have multiple input files, you must specify the -re option before each individual input file that you want to read in real-time.

Scenario 1: Applying -re to All Inputs

If you are mixing or overlaying multiple video files and want both to be processed in real-time, place -re before both -i flags:

ffmpeg -re -i input1.mp4 -re -i input2.mp4 -filter_complex "[0:v][1:v]overlay=10:10[out]" -map "[out]" -f flv rtmp://live.twitch.tv/app/stream_key

Scenario 2: Applying -re to Only One Input

If you have a primary video file that must stream in real-time, but you are also importing a static image (like a watermark) or a loop that does not require real-time throttling, only place -re before the video file:

ffmpeg -re -i main_video.mp4 -i watermark.png -filter_complex "[0:v][1:v]overlay=main_w-overlay_w-10:10" -f flv rtmp://live.twitch.tv/app/stream_key

In this command, main_video.mp4 is read in real-time, while watermark.png is loaded normally without rate limiting.

Scenario 3: Using -re with the Concat Demuxer

If you are streaming a list of multiple files sequentially using the concat demuxer, you only need to apply the -re flag once before the text file input.

Create a playlist.txt file:
```
file 'video1.mp4'
file 'video2.mp4'
```

Run the FFmpeg command applying -re to the list input:

ffmpeg -re -f concat -safe 0 -i playlist.txt -c:v libx264 -c:a aac -f flv rtmp://live.twitch.tv/app/stream_key

This ensures the entire sequential playlist is read and streamed at native real-time speed.