Download FFmpeg https://www.ffmpeg.org/download.html FFmpeg is a command line tool, which means there is no GUI. You are going to have to input commands via the keyboard instead of clicking the mouse.
Current Limitations for WebM files on 4chan are:
Maximum file size is 4096KB.
Maximum duration is 300 seconds.
Maximum resolution is 2048x2048 pixels.
Audio streams allowed.
Convert a portion of a video file to WebM:
ffmpeg -i input.mkv -ss 00:00:10.000 -to 00:00:20.000 -c:v libvpx -c:a libvorbis -crf 4 -b:v 1500K -vf scale=1280:-1 -threads 4 -an output.webm
-ispecifies the input file you want to transcode to webm.-ssspecifies the start position in number of seconds, or in hh:mm:ss[.xxx] format. You can obtain the timecode using your video player (T in VLC and Ctrl-G in MPC-HC).-tospecifies the end position in number of seconds, or again in hh:mm:ss[.xxx] format.-c:vspecifies the video codec to use. webm isn't actually an encoding type unto itself, but an audio/video container, like mkv. Nevertheless, you will almost always use libvpx for the video.-crfsets CRF value. Must be from 4-63. Lower is higher quality. 10 is a nice starting point.-b:vspecifies the maximum allowed bit rate. Higher means better quality.-vf scale=1280:-1sets the output video width to 1280px. The height will be calculated automatically according to the aspect ratio of the input. Simply omit this parameter to keep the files original resolution.-threads 4tells FFmpeg to use multithreading. Enter the number of cores your processor has as a parameter, or set to 0 to use them all.-c:a libvorbisspecifies the audio codec for the output file.
Converting Animated gifs:
ffmpeg -i input_gif.gif -c:v libvpx -crf 10 -b:v 500K output.webm
FFMpeg documentation: https://www.ffmpeg.org/ffmpeg.html
Don't listen to the previous speaker, VP8 is still an option for old crappy devices, both encoding and decoding (playback), VP9 is comically slow for most machines i tried. And if you consider posting outside of 4chan, there might still be an imageboards with no VP9 support, which is not as uncommon as you might think, 4chan itself got VP9 only in 2022, three years after the original post has been written.
Also,
crf 10is NOT comically high due to usage of-b:vparameter, which is actual ceiling here — with-b:vhigher than 0, crf is used for better allocating bitrate stuff below-b:vvalue (although overshoot is still a thing, 2-pass for the mass), so you can go as high as crf 1 lol, but you better not, if you don't want to turn your encoding into basically CBR.As for Vorbis, it sucks, but if compatibility is a serious concern, try experimenting with audio track separately (i.e. outside of webm container, extract if needed) starting from 64kb/s (UPD: pardon me, i meant
-q:a 0). Downmixing to mono might help to save quality (or might not, it's Vorbis after all).