4chan’s guide to converting GIF to WebM - https://boards.4chan.org/g/res/41212767

webm.md

Grab ffmpeg from https://www.ffmpeg.org/download.html

It's a command line tool which means you will have to type things with your keyboard instead of clicking on buttons.

The most trivial operation would be converting gifs:

ffmpeg -i your_gif.gif -c:v libvpx -crf 12 -b:v 500K output.webm

• -crf values can go from 4 to 63. Lower values mean better quality.
• -b:v is the maximum allowed bitrate. Higher means better quality.

To convert a part of a video file:

ffmpeg -i your_video.mkv -ss 00:00:10.000 -to 00:00:20.000 -c:v libvpx -crf 4 -b:v 1500K -vf scale=640:-1 -an output.webm

• -ss is the start position in number of seconds, or in hh:mm:ss[.xxx] format. You can get it using your video player (Ctrl-G in MPC-HC).
• -to is the end position.
• -vf scale=640:-1 sets the width to 640px. The height will be calculated automatically according to the aspect ratio of the input.
• -an disables audio. 4chan will reject your files if they contain audio streams.

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• Another encoding guide: https://trac.ffmpeg.org/wiki/vpxEncodingGuide
• ffmpeg documentation: https://www.ffmpeg.org/ffmpeg.html

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Current limits for WebM files on 4chan are:

• Maximum file size is 3072KB.
• Maximum duration is 120 seconds.
• Maximum resolution is 2048x2048 pixels.
• No audio streams.

You might wanna see this for more options to encode in VP8.

On mac os x and homebrew you need to to install ffmpeg with

brew install ffmpeg --with-libvpx

For -c:v libvpx to work.

Audio streams are accepted by 4chan, they must be libvorbis so add -c:a libvorbis, you can also use one audio stream instead of 2 to reduce file size through -ac 1. This would result in:

ffmpeg -i your_video.mkv -ss 00:00:10.000 -to 00:00:20.000 -c:v libvpx -crf 4 -b:v 1500K -vf scale=640:-1 -ac 1 -c:a libvorbis output.webm

Another tip, if you exceed the 4MB file limit you can reduce the video bitrate from 1500K to something lower.