[FFmpeg-devel] DVB Subtitles

Thu Dec 5 15:54:21 CET 2013

>________________________________
> From: Reimar Döffinger <Reimar.Doeffinger at gmx.de>
>To: FFmpeg development discussions and patches <ffmpeg-devel at ffmpeg.org> 
>Sent: Wednesday, 4 December 2013, 16:56
>Subject: Re: [FFmpeg-devel] DVB Subtitles
> 
>
>On Wed, Dec 04, 2013 at 04:42:50PM +0100, Michael Niedermayer wrote:
>> 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
>
>Obviously :-) But the question is what would be a good way?
>Actually I think that just setting the default immediately
>should be ok as well, it's just that if ffmpeg doesn't support
>subtitle "resolution" changes it will break its DVB support fairly
>thoroughly.
>Do we have any test streams for this?
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>

Ok simple fix for my problem, in ffmpeg_filter.c i changed the routine sub2video_prepare to set w to 720, and h to 576

I then changed my command line to do the filter and overlaying

-filter_complex '[0:s:1] scale=1280:720 [sub],[0:v][sub] overlay'

This works but what is the correct variables to use to automagically get the video width and height, as using scale=iw:ih leaves the subs as 720:576

So how would we go about adding in a auto-scaling filter after making the subtitles, which will scale the subtitle frame to the correct size, if NO DDS is present.

joolz

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