[FFmpeg-devel] DVB Subtitles

[Michael Niedermayer]

On Wed, Dec 04, 2013 at 04:25:00PM +0100, Reimar Döffinger wrote:
> On Wed, Dec 04, 2013 at 03:57:14PM +0100, Reimar Döffinger wrote:
> > On 04.12.2013, at 14:37, JULIAN GARDNER <joolzg at btinternet.com> wrote:
> > > ----- Original Message -----
> > >> From: Reimar Döffinger <Reimar.Doeffinger at gmx.de>
> > >> To: FFmpeg development discussions and patches <ffmpeg-devel at ffmpeg.org>
> > >> Cc: 
> > >> Sent: Wednesday, 4 December 2013, 14:10
> > >> Subject: Re: [FFmpeg-devel] DVB Subtitles
> > >> 
> > >> On Wed, Dec 04, 2013 at 12:26:15PM +0000, JULIAN GARDNER wrote:
> > >>>   So as you see the canvas is being set to 1280x720 where as the subtitles 
> > >> are sized for a 720x576 screen.
> > >>>   >>2013-12-04_11:44:10.28027 [mpegts @ 0x3231f80] sub2video: using 
> > >> 1280x720 canvas
> > >>> 
> > >>>   There is no DDS so the subtitles are draw in the top left corner and when 
> > >> overlayed over the live video are in the wrong place and small compared to the 
> > >> video.
> > >>> 
> > >>>   So my 1st idea is to change the way we set the canvas size, in this case it 
> > >> would be based on 720x576 as there is no DDS.
> > >> 
> > >> Unfortunately my experience from MPlayer is that this will most likely
> > >> break many streams.
> > >> I don't know whether that is a sample with or without DDS (nor if it
> > >> maybe specifies the size correctly in the stream properties), so it might
> > >> not quite demonstrate my point, but if you create any patch make sure it
> > >> will not break http://samples.mplayerhq.hu/sub/largeres_vobsub.mkv
> > >> Remuxing that one to MPEG-TS and checking it still plays correctly might
> > >> be a good idea, too.
> > >> I'll tell you if I manage to find any other samples.
> > >> But in principle the assumption of missing DDS -> use video size as
> > >> canvas size seems more reasonable to me than assuming it to be in SD
> > >> resolution (again, who creates HD content with SD subtitles? That must
> > >> look like crap).
> > >> _______________________________________________
> > >> ffmpeg-devel mailing list
> > >> ffmpeg-devel at ffmpeg.org
> > >> http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
> > >> 
> > > 
> > > I think the assumption is wrong on your part. DDS was added later to the DVBSubtitle spec to help fix this exact problem, it was not part of the 1st spec i worked with.
> > 
> > That isn't very relevant if the DDS was added before HD streams became common IMHO.
> > 
> > > All the DVBsubtitle engines i have used base their osd drawing on a standard screen UNLESS DDS is included.
> > 
> > That is a good argument. I confused myself thinking of DVD instead of DVB subtitles, I have a hard time to keep them apart, sorry.
> > 
> > > Read Appendix B of http://www.etsi.org/deliver/etsi_en/300700_300799/300743/01.03.01_60/en_300743v010301p.pdf
> > > 
> > > This being the one that pertains to this problem
> > > 
> > > "b) DVB subtitles for an HDTV service where the on-screen graphics display is standard definition (720 by 576) 
> > > and is upconverted by the receiver before being overlaid on the HDTV video image:"
> > > 
> > > The spec talks in terms of "standard definition" which is what is taken as default if no DDS is present
> > 
> > I still find it silly to both have a HD stream _and_ not have subtitles in HD _and_ not include a DDS, it seems really crappy way to do it.
> > Still, at least the specs seems quite clear to me (though personally I think they should have strongly recommended including a DDS at least for HD streams where backwards-compatibility should not be much of an issue).
> > I think that means that our decoder should set width/height to 720x576 unless it finds a DDS.
> > Even that still leaves the d) case unhandled though (lower resolution subtitles that should be displayed on the bottom centre instead of scaled), but that would probably be more messy.
> > Or does anyone think differently?
> 
> Unfortunately the decoder is even more broken:
>     if (!avctx->width || !avctx->height) {
>         avctx->width  = display_def->width;
>         avctx->height = display_def->height;
>     }
> 
> This means if we start out with one DDS but then the DDS changes to something else,
> the context width/height will not be updated.
> The problem is: we'd really want to have probing to scan for a DDS and
> set the right values. However probing stops as soon as we set width/height.
> So how can we default to a 720x576 width/height while still having probing
> continue until a DDS is found or the limit is reached?
> Does anyone know how this should be working?

one way would be to set the default immedeatly and make the code
use a different stop condition for DVB subtites

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Dictatorship naturally arises out of democracy, and the most aggravated
form of tyranny and slavery out of the most extreme liberty. -- Plato
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