[FFmpeg-devel] [PATCH] dvbsub fix transcoding

[Michael Niedermayer]

On Fri, May 23, 2014 at 03:50:05PM +0530, anshul wrote:
> On 05/22/2014 06:43 PM, Wim Vander Schelden wrote:
> >On Thu, May 22, 2014 at 3:01 PM, Wim Vander Schelden <lists at fixnum.org>wrote:
> >
> >>These subtitles not disappearing is exactly the issue that's introduced by
> >>sending the second (empty, "clearing") packet (by removing the nb = 2).
> you mean not sending second packet here.
> >>Note that you are now relying on the DVB subtitle timeout value, which is
> >>only for "safety" purposes (ie, the encoder crashed), and is only required
> >>by the specification to be followed with a 5 second accuracy (so it is
> >>completely useless for functioning subtitles)!
> >>
> agree its for safety purpose, but as you see -0/+5 accuracy means
> that it might be possible that subtitle
> are on screen +5 second ie more then required time. so using timeout
> does not have any harm.
> though relying only on it, is harmful.
> following is copy pasted from ETSI EN 300 743 V1.3.1 (2006-11)
> page_time_out: The period, expressed in seconds, after which a page
> instance is no longer valid and consequently shall
> be erased from the screen, should it not have been redefined before
> that. The time-out period starts when the page
> instance is first displayed. The page_time_out value applies to each
> page instance until its value is redefined. The
> purpose of the time-out period is to avoid a page instance remaining
> on the screen "for ever" if the IRD happens to have
> missed the redefinition or deletion of the page instance. The
> time-out period does not need to be counted very
> accurately by the IRD: a reaction accuracy of -0/+5 s is accurate enough.
> 
> >I seem to have misformulated my ordering issue in my earlier mail: the
> >problem occurs when subtitle X only disappears after subtitle X+1 should
> >start being displayed, as the "clear" packet for X is emitted immediately
> >after the "show" packet for X, so before the "show" packet for X+1.
> >
> yes agree :)
> 
> 
> So problem is
> end_display_time is not reliable in case of dvbsub decoder,
> since end_display_time in dvbsub decoder is calculated using page_time_out.
> which is only for safety purpose.
> 
> I am trying to fix end_display_time in dvbsub decoder.
> I want to know should i cache the subtitle till i get clear_display
> region/overlap region
> or should i try to overwrite end_display_time when i have recieved
> some new region.
> 
> just now i have attached patch which stop second loop only when
> stream was decoded by
> dvbsubtile.
> 
> -Anshul

>  ffmpeg.c            |    4 +++-
>  libavcodec/dvbsub.c |    2 +-
>  2 files changed, 4 insertions(+), 2 deletions(-)
> 5d9736343922cc0aad0fe3124943e5cf7406b2c6  0001-dvbsub-fix-transcoding.patch
> From 946dc41a84a6b17d64445acb2424e18774e7f1ac Mon Sep 17 00:00:00 2001
> From: Anshul Maheshwari <er.anshul.maheshwari at gmail.com>
> Date: Fri, 23 May 2014 15:48:12 +0530
> Subject: [PATCH] dvbsub fix transcoding
> 
> Fix Ticket #2024
> ---
>  ffmpeg.c            | 4 +++-
>  libavcodec/dvbsub.c | 2 +-
>  2 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/ffmpeg.c b/ffmpeg.c
> index 5299f0e..d9539ee 100644
> --- a/ffmpeg.c
> +++ b/ffmpeg.c
> @@ -770,6 +770,7 @@ static void do_subtitle_out(AVFormatContext *s,
>      int subtitle_out_max_size = 1024 * 1024;
>      int subtitle_out_size, nb, i;
>      AVCodecContext *enc;
> +    AVCodecContext *dec;
>      AVPacket pkt;
>      int64_t pts;
>  
> @@ -781,6 +782,7 @@ static void do_subtitle_out(AVFormatContext *s,
>      }
>  
>      enc = ost->st->codec;
> +    dec = ist->st->codec;
>  
>      if (!subtitle_out) {
>          subtitle_out = av_malloc(subtitle_out_max_size);
> @@ -789,7 +791,7 @@ static void do_subtitle_out(AVFormatContext *s,
>      /* Note: DVB subtitle need one packet to draw them and one other
>         packet to clear them */
>      /* XXX: signal it in the codec context ? */
> -    if (enc->codec_id == AV_CODEC_ID_DVB_SUBTITLE)
> +    if (enc->codec_id == AV_CODEC_ID_DVB_SUBTITLE && dec->codec_id != AV_CODEC_ID_DVB_SUBTITLE)


>          nb = 2;

this case (independant of this patch) needs some kind of que
(either inside the encoder or elsewhere) to ensure the clear packet
is correctly ordered in relation to the next "add" packet
there was a patch about this but i think no new version was posted
after reviews


>      else
>          nb = 1;
> diff --git a/libavcodec/dvbsub.c b/libavcodec/dvbsub.c
> index f6b46e6..f4b4efa 100644
> --- a/libavcodec/dvbsub.c
> +++ b/libavcodec/dvbsub.c
> @@ -268,7 +268,7 @@ static int encode_dvb_subtitles(DVBSubtitleContext *s,
>      bytestream_put_be16(&q, page_id);
>      pseg_len = q;
>      q += 2; /* segment length */
> -    *q++ = 30; /* page_timeout (seconds) */
> +    *q++ = ceil(h->end_display_time /1000); /* page_timeout (seconds) */

this would need a check for the value fitting in a byte (0..255)

also this only sort of fixes/hacks around the dvb->dvb case, there
still wont be correctly set durations for dvb->something else
I think, unless iam missing something

[...]

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Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

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