[Matroska-devel] PGS subtitle questions

Moritz Bunkus moritz at bunkus.org
Mon Nov 5 15:41:37 CET 2012


Hey,

On Mon, Nov 5, 2012 at 3:06 PM, John Stebbins <stebbins at jetheaddev.com> wrote:

> I'm surprised that the matroska specification doesn't say something to
> the effect that unrecognized EBML id's should be ignored.

Oh it does. It it is implied.

>  You would
> think this would be required for an extensible format. I would hope the
> majority of players are smart enough to skip unrecognized data since the
> only alternative is to choke and die. But I've seen plenty of hardware
> players that do exactly that when they see something unexpected.  So I
> understand your point.

Unfortunately that's my experience as well (not directly by owning
said hardware, but judging from the multitude of support requests I
receive because players don't recognize tracks, don't recognize the
whole file, only show black pictures or freeze completely).

> I just did some checking to see if all this is really needed.  I figured
> someone *must* have given this at least a little thought previously.  It
> turns out there is a "video descriptor" required in the PGS subtitle
> data that defines the width and height "of the associated HDMV stream".
> So my request for original width and height is unnecessary.

Nice catch.

> Now I'm rethinking this a little.  Cropping information would still be
> helpful as a hint to a player about how to position the subtitles.
> Also, original pixel aspect ratio would be helpful in cases where the
> video has be anamorphically scaled during transcoding.  I looked and
> found nothing about pixel aspects in the PGS data.

Is the "size of associated HDMV video track" stored inside the PGS the
coded pixels size or the display size?

Kind regards,
mosu


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