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Re: Video subtitles

From: Ricardo Wurmus
Subject: Re: Video subtitles
Date: Thu, 20 Dec 2018 09:11:00 +0100
User-agent: mu4e 1.0; emacs 26.1

Hi Laura,

> The first question I need to answer is if subtitles are going to be
> present all the time as part of the video, or they should be able to
> be disabled. We had that discussion somewhere, but if they are going
> to be present as part of the video (hardsubs) only .srt and .ass - the
> ones created by default by aegisub - are supported by ffmpeg:

The subtitles should be provided as a separate file, so that they can
easily be swapped out or disabled.  Some container formats allow
embedding the subtitles, but we would need to embed all subtitles in all
language variants of all video files, and this seems like more work.
Having the subtitles as separate files is more convenient for

> I remembered how much time each audio file lasted, so I added one
> subtitle for each svg file that appears in the video - they were just
> three- with the full content of what is said in the audio.

It is more common to have a single subtitle file per video.

> The drawback of this is that each svg file in the video has the full
> subtitle text before the speaker mentions those words - I will ask
> Björn tomorrow to upload it to their IPFS so that you can see what I
> mean.

You can time the appearance of text.  I’ve only ever written srt
subtitle files by hand before, but they are all similar in that they
associate a piece of text with a timestamp.  The text is then displayed
at the specified time only.

You can figure out the appropriate timestamps by playing back the video
and recording the times when something is said that should be titled.

> When choosing the format of the subtitles, Ricardo mentioned that it
> was better using usf because it had support for styling. The good news
> is that aegisub allows us to do so in their own interface, but  in
> their .ass format, one of the two that supports ffmpeg,  which is not
> compatible,  I guess, with the formats mentioned in po4a for
> translation. However the .ass subtitle can be exported as .ttxt -
> which is an XML, and that can be converted back to .ass.

“.ass” files are very popular and very rich in styling features.  “.usf”
files are less commonly seen in the wild.

I have no strong opinions about using “.usf”.  “.ass” may just be fine.
Apparently, it can be converted to po files for translation with
“sub2po”, which is part of the “translate” toolkit.  I have no
experience with this, and I think it would be better to use a format
that didn’t require even more tools to work with.

This shouldn’t block you, though.  It is fine to use Aegisub manually to
create the initial subtitle and then convert it to another format.
Aegisub does not need to be part of the automated process to compile the


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