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Fix Aspect Ratio of MKV Files

The steps below can be used to fix a video having the incorrect aspect ratio. I don't know what causes such issues, but here is how to fix it.

Prerequisites

This tutorial requires mkvmerge which can be installed with:

sudo apt install mkvtoolnix

Steps

Firstly, it is useful to know what aspect ratio you need. You could guess, but if the "bad" video is part of a series, then I would get the information from one of the other episodes using:

ffmpeg -i input.mkv

Then look for text similar to below in the output:

Stream #0:0(eng): Video: h264 (High), yuv420p(tv, bt709/unknown/unknown, progressive), 1920x872, SAR 1:1

In this case, my desired aspect ratio is 1920 (width) by 872 (height). We can now plug this information into mkvmerge to create a copy of our bad video file, with a corrected aspect ratio.

mkvmerge \
--aspect-ratio $VIDEO_TRACK_ID:$WIDTH/$HEIGHT \
-o output.mkv \
input.mkv

If you are unsure of the video track ID, use mkvmerge -i input.mkv and that will output the tracks and their IDs.

For my example, the following command creates the correct file with the name output.mkv from the input.mkv file.

mkvmerge \
--aspect-ratio 0:1920/872 \
-o output.mkv \
input.mkv