[Matroska-devel] PGS subtitle questions

John Stebbins stebbins at jetheaddev.com
Mon Nov 5 15:58:40 CET 2012

On 11/05/2012 03:41 PM, Moritz Bunkus wrote:
> 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?

It's coded pixels size.

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