How to Overlay Video on Video with FFmpeg

This article provides a straightforward guide on how to use the FFmpeg overlay filter to place one video on top of another. You will learn the basic command syntax, how to position the overlay video using coordinates and variables, and how to handle audio and video duration sync.

To overlay one video on top of another in FFmpeg, you use the -filter_complex option with the overlay filter. This filter takes two video inputs, overlays the second input (the foreground) onto the first input (the background), and outputs the combined stream.

The Basic Command Syntax

The fundamental command structure for overlaying a video is as follows:

ffmpeg -i background.mp4 -i foreground.mp4 -filter_complex "[0:v][1:v]overlay=x=10:y=10[out]" -map "[out]" output.mp4

In this command: * -i background.mp4 is the first input (index 0). * -i foreground.mp4 is the second input (index 1). * [0:v][1:v] tells FFmpeg to send the video stream of the first input and the video stream of the second input into the filter. * overlay=x=10:y=10 positions the top-left corner of the foreground video 10 pixels from the left (x) and 10 pixels from the top (y) of the background video. * [out] names the output stream of the filter, which is then mapped to the final output file via -map "[out]".

Positioning the Overlay Using Variables

FFmpeg provides built-in variables to make positioning easier without needing to calculate exact pixel values manually. The most common variables are: * main_w or W: Width of the background video. * main_h or H: Height of the background video. * overlay_w or w: Width of the foreground/overlay video. * overlay_h or h: Height of the foreground/overlay video.

Using these variables, you can easily place the overlay in common positions:

Top-Right Corner:
```
overlay=W-w:0
```
Bottom-Left Corner:
```
overlay=0:H-h
```
Bottom-Right Corner (with a 20-pixel padding):
```
overlay=W-w-20:H-h-20
```
Center of the Screen:
```
overlay=(W-w)/2:(H-h)/2
```

Handling Video Endings and Audio

By default, the output video will keep playing until the longer of the two input videos finishes. If the overlay video is shorter, it will freeze on its last frame while the background continues. If the background video is shorter, the video will end but the file may continue encoding.

To force the output to stop encoding as soon as the shortest input finishes, add the shortest=1 option inside the overlay filter:

ffmpeg -i background.mp4 -i foreground.mp4 -filter_complex "[0:v][1:v]overlay=10:10:shortest=1[out]" -map "[out]" output.mp4

To include the audio from the background video in the final output, map the audio channel from the first input:

ffmpeg -i background.mp4 -i foreground.mp4 -filter_complex "[0:v][1:v]overlay=10:10[out]" -map "[out]" -map 0:a output.mp4