Stitching videos from frames with ffmpeg (and audio/video editing tricks)

ffmpeg is awesome. In case you haven't heard, ffmpeg is a command-line tool for editing and converting audio and video files. It's taken me a while to warm up to it (the command-line syntax is pretty complicated), but since I've found myself returning to it again and again I thought I'd blog about it so you can use it too (and also for my own reference :P).

I have 2 common tasks I perform with ffmpeg:

  1. Converting audio/video files to a more efficient format
  2. Stitching PNG images into a video file

I've found that most tasks will fall into 1 of the 2 boxes. Sometimes I want to do something more complicated, but I'll usually just look that up and combine the flags I find there with the ones I usually use for one of the 2 above tasks.

horizontal rule made up of an orange rotating maze cube

Stitching PNG images into a video

I don't often render an animation, but when I do I always render to individual images - that way if it crashes half way through I can more easily restart where I left off (and also divide the workload more easily across multiple machines if I need it done quickly).

Individual image files are all very well, but they have 2 problems:

  1. They take up lots of space
  2. You can't easily watch them like a video

Solving #1 is relatively easy with optipng (sudo apt install optipng), for example:

find . -iname "*.png" -print0 | xargs -0 -P4 -n1 optipng -preserve

I cooked that one up a while ago and I've had it saved in my favourites for ages. It finds all png images in the current directory (recursively) and optimises them with optipng.

To resolve #2, we can use ffmpeg as I mentioned earlier in this post. Here's one I use:

ffmpeg -r 24 -i path/to/frame_%04d.png -c:v libvpx-vp9 -b:v 2000k -crf 31 path/to/output.webm

A few points on this one:

Read more about encoding VP9 webm videos here: Encode/VP9- ffmpeg

Converting audio / video files

I've talked about downmuxing audio before, so in this post I'm going to be focusing on video files instead. The above command for encoding to webm can be used with minimal adjustment to transcode videos from 1 format to another:

ffmpeg -hide_banner -i "path/to/input.avi" -c:v libvpx-vp9 -c:a libopus -crf 31 -b:v 0 "path/to/output/webm";

In this one, we handle the audio as well as the video all in 1 go, encoding the audio to use the opus codec, and the video as before. This is particularly useful if you've got a bunch of old video files generated by an old camera. I've found that often old cameras like to save videos as raw uncompressed AVI files, and that I can reclaim a sigificant amount of disk space if I transcode them to something more efficient.

Naturally, I cooked up a one-liner that finds all relevant video files recursively in a given directory and transcodes them to 1 standard efficient format:

find . -iname "*.AVI" -print0 | nice -n20 xargs --verbose -0 -n1 -I{} sh -c 'old="{}"; new="${old%.*}.webm"; ffmpeg -hide_banner -i "${old}" -c:v libvpx-vp9 -c:a libopus -crf 30 -b:v 0 "${new}" && rm "${old}"';

Let's break this down. First, let's look at the commands in play:

The old="{}"; new="${old%.*}.webm"; bit is a bit of gymnastics to pull in the path to the target file (the -I {} tells xargs where to substitute it in) and determine the new filepath by replacing the file extension with .webm.

I've found that this does the job quite nicely, and I can set it running and go off to do something else while it happily sits there and transcodes all my old videos, reclaiming lots of disk space in the process.

Found this useful? Got a cool command for processing audio/video/image files in bulk? Comment below!

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