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I was asked by a number of people about how I created http://users.freedesktop.org:8080/~noelp/zoom/JumpingAnchoredShapes.ods

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 zoom.srt -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 transcode

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 http://en.flossmanuals.net/ogg-theora/subtitles/embedding-subtitles/ 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

Noel


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