FFmpeg hstack: How to Stack 3 Videos Horizontally

This article provides a quick, step-by-step guide on how to combine three separate video files side-by-side into a single row using FFmpeg’s hstack filter. You will learn the exact command-line syntax required to merge these inputs, how to handle videos of different heights, and how to manage the accompanying audio streams.

The Basic Command for Equal Height Videos

If your three input videos already have the exact same vertical resolution (height), you can stack them horizontally using a simple filter_complex command. Run the following command in your terminal:

ffmpeg -i input1.mp4 -i input2.mp4 -i input3.mp4 -filter_complex "hstack=inputs=3" output.mp4

In this command: * -i input1.mp4 -i input2.mp4 -i input3.mp4 specifies the three input video files. * -filter_complex "hstack=inputs=3" tells FFmpeg to use the horizontal stack filter and expects three video inputs. * output.mp4 is the resulting video file.

Handling Videos with Different Heights

The hstack filter requires all input videos to have the identical height. If your videos have different heights, the command will fail. To resolve this, you must scale the videos to a uniform height before stacking them.

The following command scales all three videos to a height of 720 pixels while automatically adjusting the width to maintain their original aspect ratios:

ffmpeg -i input1.mp4 -i input2.mp4 -i input3.mp4 -filter_complex "[0:v]scale=-1:720[v0];[1:v]scale=-1:720[v1];[2:v]scale=-1:720[v2];[v0][v1][v2]hstack=inputs=3" output.mp4

• [0:v]scale=-1:720[v0] takes the video stream of the first input (0:v), scales its height to 720, automatically calculates the proportional width (-1), and labels the temporary output as [v0].
• This process is repeated for the second ([v1]) and third ([v2]) videos.
• [v0][v1][v2]hstack=inputs=3 takes those three scaled video streams and stacks them side-by-side.

Combining Audio Streams

By default, the commands above may only copy the audio from the first input file or omit audio entirely depending on your system configuration. If you want to mix the audio from all three videos so they play simultaneously, use the amix filter:

ffmpeg -i input1.mp4 -i input2.mp4 -i input3.mp4 -filter_complex "[0:v]scale=-1:720[v0];[1:v]scale=-1:720[v1];[2:v]scale=-1:720[v2];[v0][v1][v2]hstack=inputs=3[outv];[0:a][1:a][2:a]amix=inputs=3[outa]" -map "[outv]" -map "[outa]" output.mp4

• [0:a][1:a][2:a]amix=inputs=3[outa] takes the audio tracks from all three inputs and mixes them into a single audio stream labeled [outa].
• -map "[outv]" and -map "[outa]" explicitly tell FFmpeg to use the newly created stacked video and mixed audio in the final output file.