FFmpeg "ass" filter not working

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http://superuser.com – I'm haveing a bit of a problem when trying to use FFmpeg. I've mostly been using it to extract clips from videos and such. However, I've been trying to get some subtitles embedded (hardsubs) into the video. This page here told my I should use the "ass" video filter, and I tried it as shown but to no avail. There's no change. The video doesn't have the subtitles on it; they don't even show up as a stream in the file. I may as well not have included the filter. This is the line I'm using, which isn't working. The file has no audio stream attached. ffmpeg -i input.webm -vf ass=subs.ass -b:v 12 (HowTos)