[Matroska-devel] PGS subtitle questions

Moritz Bunkus moritz at bunkus.org
Mon Nov 5 13:53:14 CET 2012


Hey,

On Mon, Nov 5, 2012 at 1:35 PM, John Stebbins <stebbins at jetheaddev.com> wrote:

> In order to not invalidate existing streams, you could make the new
> element (or codec private data) optional and only necessary when scaling
> of PGS by the playback software is desired.

Even that might make existing files invalid. Also it's dangerous; you
cannot extract that codec private information properly. Also no other
software expects such data.

> I have no preference for whether it is a new element or codec private
> data.  But since there is already a format for vobsub that has the
> necessary information, would it be a good idea to reuse that format?

I don't think so, no. It's a text-based format. That's bad for at
least two reasons:

- it's easy to get the format wrong (is it "custom colors"? or
"custom_colors"? or "custom colours"?)
- it's difficult to parse (or at least it requires yet another parser
on the reader's side)

Another reason it's a bad idea (at least in my opinion) is that there
are tons of potential details inside those .idx files that might serve
no purpose with PGS subs: palette information, color, alignment,
fading... Note that I don't know much about PGS specs, so this might
be inaccurate.

Putting the information into Matroska elements makes them

a) official
b) easy to parse (they're simply two or more additional elements; I'm
thinking about keeping offset as well as size)
c) easier to create for existing tools, too, I guess

> The only part that wouldn't be relevant is the palette.  The rest could
> actually be useful I believe.

Maybe, maybe not. Do we require conforming players to evaluate each of
those settings then? That will never happen. So all of those options
would essentially be dead, and only serve to confuse users why any
change in those options didn't have any observable effect.

Kind regards,
mosu


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