I use avconv to get a preview image of a video clip. The command is
avconv -i video.mp4 -vframes 1 -s 100x100 cover.jpeg
It works but I would like to have correct dimensions of the output jpeg. For example to be the half size of the input video. So I execute
avconv -i video.mp4 -vframes 1 -s 'iw/2:ih/2' cover.jpeg
but this just causes Invalid frame size: iw/2:ih/2
error. How should I pass advanced scale options to avconv in command line?