Adding (not burning) subs using Avisynth

[pooksahib]

Hi. I know that you can burn subs with the following as part of an avisynth script:

LoadPlugin("D:\MeGUI\tools\lsmash\VSFilter.dll")
a=LWLibavVideoSource("D:\movie.mkv")
b=LWLibavaudioSource("D:\movie.mkv")
audiodub(a,b)
TextSub("D:\moviesubs.srt")

but is there a way to do it so that the subs can be demuxed at a later date? I realise I could run the script without the subs and then use mkvtoolnix to make a copy of the output file with subs added but that seems a waste of diskspace. I use MeGUI to run my scripts.
Many thanks.

[ProWo]

Try

LoadPlugin("D:\MeGUI\tools\lsmash\VSFilter.dll")
a=LWLibavVideoSource("D:\movie.mkv").TextSub("D:\moviesubs.srt")
b=LWLibavaudioSource("D:\movie.mkv")
audiodub(a,b)

[pooksahib]

Thanks, ProWo. Unsuccessful, unfortunately but I'm not overly bothered - I don't mind doing it the long way.

[ProWo]

Then try

LoadPlugin("D:\MeGUI\tools\lsmash\VSFilter.dll")
a=LWLibavVideoSource("D:\movie.mkv")
a=TextSub("D:\moviesubs.srt")
b=LWLibavaudioSource("D:\movie.mkv")
audiodub(a,b)

[zing269]

Avisynth is a frame server so what comes out of it is a raw video frame. Depending on what software you are feeding the script into you may be able to mux in the subs at the same time.

[jagabo]

zing is correct, AviSynth will always burn the subs into the video stream. If you want soft subs, include them when you mux.

[_Al_]

I could run the script without the subs and then use mkvtoolnix to make a copy of the output file with subs added but that seems a waste of diskspace. I use MeGUI to run my scripts.
Many thanks.

Is that how ffmpeg works? Using little buffer here or there as necesarry, but stitching everything on the go?
Was always thinking about that as a ffmpeg advantage, but never really investigated how much temporary memory it uses.

[jagabo]

ffmpeg can use the AviSynth script as input. So you don't need multiple steps or multiple copies of the video:

ffmpeg -i video.avs -i audio.aac -i subs.srt -c:a copy -c:s copy -c:v libx264 (any x264 args here) output.mkv

[pooksahib]

Thanks guys, esp jagabo who looks to have come up with the solution. I don't know how use that ffmpeg script yet but it doesn't matter as I've discovered a further problem: despite my careful calculations when using the 'burn' method the subs were still a little out of sync. So it seems best to make an output file first, get the subs properly timed and then mux a final output vid.