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/gif/ guide to creating animated gifs
Other helpful links:
http://www.video-gif-converter.com/index.html
http://www.wikihow.com/Make-an-Animated-GIF-from-a-Video-in-Photoshop-CS5
http://bengal.missouri.edu/~gghhgf/index.html
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.
Another encoding guide: https://trac.ffmpeg.org/wiki/vpxEncodingGuide
ffmpeg documentation: https://www.ffmpeg.org/ffmpeg.html
WebMConverter by WebMBro: https://github.com/WebMBro/WebMConverter "WebM for retards."
Current limits for WebM files on 4chan are:
Maximum file size is 4096KB.
Maximum duration is 300 seconds.
Maximum resolution is 2048x2048 pixels.
Click here to see a detailed guide on how to create webm files.