The video explain the "how to" but I will upload some screenshots to help.
TO ENCODE 2 D TO 3 D
go to this web page http://3dvision-blog.com/converting-2d-videos-to-3d-using-virtualdub-and-the-avisynth-script/
Why 3 D to 2 D?
well this has made because I have too encode one movie sterioscopic "3 D" to 2 D my system is not good (I don't have all hardware). So I add to found a solution for encode 3 D to 2 D, I goggle it but only found how to encode (convert) to 2 D to 3 D. Them came to me the idea try this on MEgui, well the result has amazing, but this as made on Windows, and I want try to on Ubuntu in Avidemux, so I use the sample of 3dvision-blog.com to make this "how to".
This is on Ubuntu 10.10 64bit this sample of the "how to" take 30min to encode on my laptop 2hz coredue 2GB memory, your miles may very.
Start, go to Applications, Sound & Video, Avidemux, (if you don't have this install it, using Software centre),
destination 16:9 and lanczos3, [on MEgui is lanczos4]
screenshot after all the settings apply,
The end result to compare here: http://dl.dropbox.com/u/16165934/avt.mkv