mencoder add hard-rendered subtitles on first 20 seconds without re-encoding the rest

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http://www.linuxquestions.org – Hello! I've tried reading through a number of threads on here about adding subtitles hard into the video stream with mencoder, but most of them are people asking how to do it or having problems. So apologies if this has already been asked somewhere, but I haven't found it yet. I don't have any problem getting mencoder to add the subtitles. But I only actually want to add a subtitle (for credits) during the first say 20 seconds of the video, and it seems a shame to have to re-encode all the rest just for that. So I was wondering about different ways to spl (HowTos)