[FFmpeg-devel] DVB Subtitles

JULIAN GARDNER joolzg at btinternet.com
Wed Dec 4 14:37:24 CET 2013

----- 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).
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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.

All the DVBsubtitle engines i have used base their osd drawing on a standard screen UNLESS DDS is included.

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


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