screencast instructions

Noel Power nopower at
Thu Nov 29 08:36:11 PST 2012

I was asked by a number of people about how I created

and in particular the captions/subtitles.

I will give as brief a description as I can, you have a number of 
choices to create a screen cast, first thing you need something
to capture your screen content and/or audio ( I choose to not use audio 
for various reasons, embarrassment, lack of patience trying to stick 
together a separate audio track when I inevitably would screw up the 
audio after doing the screen cast etc, )

I used recordmydesktop ( available from all good linux distros ), my 
commandline was something like

    recordmydesktop  --v_bitrate  2000000 -o zoom.ogv ( cntrl-c when 
finished and it will dump the .ogv file )

for me the high bit rate "--v_bitrate  2000000" was necessary to 
smoothly capture mouse movements, quickly changing items on the screen 
etc. Fool around with the value or maybe the default will work for you. 
On opensuse this just worked out of the box and although I didn't use 
the audio if you are happen to be happy with the sound of your own voice 
and you have described what you are doing while doing your screencast 
that's it, you should be done, just upload and away you go.

so.. onto the subtitles lots of google hits make me try 'aegisub' but 
this was a waste of time for me, just didn't work, next on the list was 
'gnome-subtitles', that's what I used and it worked reasonably well. 
Initially thought it wasn't working for a while ( blank video preview ), 
it suddenly started working ( perhaps it was to do with the restricted 
codec stuff I pulled in previously ) or maybe it just doesn't work every 
attempt to load a video, no idea really, anyway once it was working all 
was good. Only problem for me was the video preview was quite small to 
work with, was a little hard to see where I was in the video while 
trying to create the subtitles. I won't describe how to use 
gnome-subtiles, it is pretty basic but does the job, I was able to use 
it without use of help or examples or whatever so basically anyone 
should be able to use it. After you produce your subtitles you should 
end up with a .srt file ( mostly should be the same name as your video 
file for players to pick it up ) If you are happy to distribute the 2 
files then that's it your finished.

Me, I wanted just to upload a video with the comments in it so I went a 
little step further, here's where it gets a little hairy and the 
commands below worked for me but... ymmv, So, basically with subtitles 
you can either deliver them with a separate file ( the .srt file 
previously mentioned or there apparently is another popular 
caption/subtitle format '.ass' - I kid you not!!! ) or some video 
container formats support subtitle streams. The thing about delivering 
the subtitles either internally ( in the video container ) or externally 
as a separate file is that the player has to decide ( or you tell it ) 
to display them. I wanted to actually have the subtitles encoded in the 
video stream itself and the only way I found to do that which worked ( 
to actually re-encode subtitles into the video stream ) was to use 
mencoder. Unfortunately I couldn't get mencoder to actually deal with 
ogg directly so I cheated and just let mencode output to the default avi 
contain ( and of course whatever mpeg format mencoder chose, ) so if you 
wish to do this you most likely need to download those restricted codecs 
( at least on opensuse I think you will )

    mencoder zoom.ogv -o output-zoom.avi  -ovc lavc -lavcopts acodec=ac3 
-oac mp3lame -sub -lavcopts vbitrate=2400

again the bitrate here was to preserve the quality when transcoding ( 
there are probably better options to chose if you know what you are 
doing, I don't ) but.. that value worked well for me

to get back to ogv & theora video codec I used ffmpeg to once again 

ffmpeg -i ~/output-zoom.avi -b 14000k zoomcast.ogv

and again the random bitrate was something that worked well to preserve 
the video quality ( without it the result was smaller in size but quite 
blurry )

It is possible to create an ogv ( I found out later ) with the subtitles 
embedded. see for 
more details ( unfortunately firefox doesn't seem to show them ) Totem 
seems to show them nicely ( mplayer didn't )

I am pretty sure there is probably some html magic that you could do to 
embed the video whilst making it display the subtitles ( even when 
delivered as a separate file or in the .ogv ) however I don't know how 
to do it

I will probably try and put this info on the wiki ( or someone else 
might get there before me ) and then some clueful person can improve 
these instructions. Anyway hope the info was helpful to someone


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