Cropping videos using ffmpeg / libav / avconv

Explanatory note:

Ubuntu (my distro of choice) and others are transitioning from ffmpeg to libav, libav is a fork of ffmpeg and most tools are drop in compatible, the method described in this post should work with recent versions of either, the command line tools ffmpeg and avconv are interchangeable.

Old Method

Historically ffmpeg had -croptop, -cropleft etc. parameters used for cropping videos, these have now been replaced by the -vf or video filter option which is a little more complex.

Current Method

The -vf option can be used to specify a section of the source video to use in the output by specifying the size and position of a rectangle to crop to:

The -vf option takes the argument crop=outw:outh:x:y - to create a new video file output.mpeg cropped to 720px x 600px and aligned 240px from the top:

avconv -i input.webm -vf crop=720:600:0:240 output.mpeg

In the example I'm also converting a webm video to mpeg along with cropping it, to convert webm to mpeg at the same dimensions just remove the cropping options.