[FFmpeg-devel] [PATCH] DVB Subtitles Fix

[JULIAN GARDNER]

----- Original Message -----
> From: Michael Niedermayer <michaelni at gmx.at>
> To: FFmpeg development discussions and patches <ffmpeg-devel at ffmpeg.org>
> Cc: 
> Sent: Wednesday, 4 May 2011, 20:05
> Subject: Re: [FFmpeg-devel] [PATCH] DVB Subtitles Fix
> 
> On Wed, May 04, 2011 at 07:18:12PM +0100, JULIAN GARDNER wrote:
>> 
>> 
>> 
>> 
>>  ----- Original Message -----
>>  > From: Michael Niedermayer <michaelni at gmx.at>
>>  > To: FFmpeg development discussions and patches 
> <ffmpeg-devel at ffmpeg.org>
>>  > Cc: 
>>  > Sent: Wednesday, 4 May 2011, 18:25
>>  > Subject: Re: [FFmpeg-devel] [PATCH] DVB Subtitles Fix
>>  > 
>>  > On Wed, May 04, 2011 at 04:11:33PM +0100, JULIAN GARDNER wrote:
>>  >> 
>>  >> 
>>  >> 
>>  >> 
>>  >>  ----- Original Message -----
>>  >>  > From: Michael Niedermayer <michaelni at gmx.at>
>>  >>  > To: FFmpeg development discussions and patches 
>>  > <ffmpeg-devel at ffmpeg.org>
>>  >>  > Cc: 
>>  >>  > Sent: Wednesday, 4 May 2011, 15:14
>>  >>  > Subject: Re: [FFmpeg-devel] [PATCH] DVB Subtitles Fix
>>  >>  > 
>>  >>  > On Tue, Apr 26, 2011 at 11:34:10PM +0100, JULIAN GARDNER 
> wrote:
>>  >>  >>  Ok first patch to fix the decoding and re-encoding of 
> DVB 
>>  > Subtitles.
>>  >>  >> 
>>  >>  >>  I went through and validated against the spec, my own 
> engine and 
>>  > fixed the 
>>  >>  > bits that were failing or wrong.
>>  >>  > 
>>  >>  > please explain how to reproduce the bugs that your patch 
> fixes
>>  >>  > 
>>  >>  > Also please explain more elaborately what and why each 
> change is made.
>>  >>  > 
>>  >> 
>>  >>  I use LIVE and RECORDED streams from various sources which i have 
> collected 
>>  > over the past years, these are put through ffmpeg and then the output 
> stream is 
>>  > played out in VLC and a couple of my own receivers which are used in 
>>  > professional cable networks.
>>  > 
>>  > understood, but does this patch fix any problem with these ffmpeg
>>  > generated streams?
>>  > Iam asking because id like to reproduce the problem and the fix.
>>  > 
>>  > 
>>  it fixes all the problems with the encoded streams, also it allows FFMPEG 
> to process a file correctly without bailing out with an error.
>> 
>>  I can upload a couple of streams if you want for you to see the problems.
> 
> thanks, yes this would help me.
> 
> if they are small you can upload to our issue tracker 
> http://ffmpeg.org/trac/ffmpeg
> 
> 
>> 
>>  >> 
>>  >>  > 
>>  >>  >> 
>>  >>  >>  I have tested this on recorded streams and live streams 
> and found 
>>  > no 
>>  >>  > problems except for one which i will try and explain about.
>>  >>  >> 
>>  >>  >>  Here in the UK some of our channels use a live speech 
> convertor 
>>  > where you 
>>  >>  > see the words appear as they are spoken, now this works fine 
> but the 
>>  > way the 
>>  >>  > decode/encode works a lot more space is taken by this 
> encoder for the 
>>  > subsequent 
>>  >>  > encoded stream. In one test the incoming packets made up 
> around 3k of 
>>  > data, but 
>>  >>  > the amount of data in the output stream was around 15k.
>>  >>  >> 
>>  >>  >>  joolz
>>  >>  > 
>>  >>  >>   ffmpeg.c               |   11 -
>>  >>  >>   libavcodec/dvbsub.c    |  103 ++++++++---------
>>  >>  >>   libavcodec/dvbsubdec.c |  296 
>>  >>  > +++++++++++++++++++++++++++++--------------------
>>  >>  >>   libavcodec/utils.c     |    3 
>>  >>  >>   4 files changed, 233 insertions(+), 180 deletions(-)
>>  >>  >>  d22a3a04cf764a0b25138652ad82fe484311405f  dvbsubs.diff
>>  >>  >>  diff --git a/ffmpeg.c b/ffmpeg.c
>>  >>  >>  index 2c69608..e88bcfc 100644
>>  >>  >>  --- a/ffmpeg.c
>>  >>  >>  +++ b/ffmpeg.c
>>  >>  >>  @@ -1092,15 +1092,8 @@ static void 
>>  > do_subtitle_out(AVFormatContext *s,
>>  >>  >>           subtitle_out = 
> av_malloc(subtitle_out_max_size);
>>  >>  >>       }
>>  >>  >>   
>>  >>  >>  -    /* 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 == CODEC_ID_DVB_SUBTITLE)
>>  >>  >>  -        nb = 2;
>>  >>  >>  -    else
>>  >>  >>  -        nb = 1;
>>  >>  >>  -
>>  >>  >>  -    for(i = 0; i < nb; i++) {
>>  >>  >>  +    i = 0;
>>  >>  >>  +    {
>>  >>  >>           sub->pts = av_rescale_q(pts, 
>>  > ist->st->time_base, 
>>  >>  > AV_TIME_BASE_Q);
>>  >>  >>           // start_display_time is required to be 0
>>  >>  >>           sub->pts              += 
>>  >>  > av_rescale_q(sub->start_display_time, (AVRational){1, 
> 1000}, 
>>  > AV_TIME_BASE_Q);
>>  >>  > 
>>  >>  > why is this changed?
>>  >>  > 
>>  >> 
>>  >>  Because only one stream is needed to process DVB Subtitles, i 
> could not 
>>  > work out why you needed this. The NOTE is wrong regarding needed one 
> to draw and 
>>  > one to remove
>>  >> 
>>  >>  What you will get from the DVB Stream is a packet with NO 
> regions, this is 
>>  > the one that removes the subtitles from the picture, as per the spec, 
> there is a 
>>  > timeout value, which in the ffmpeg engine is always set to 30 seconds, 
> this 
>>  > would cause the receiving unit to switch off the display after 30 
> seconds of no 
>>  > valid dvb stream.
>>  > 
>>  > Lets assume something generates subtitles, lets say it generates one
>>  > displaying "hello world" at start time 0:10 and end time 
> 0:50
>>  > If i understand you correctly this needs a packet muxed with time 0:10
>>  > and a empty clear packet muxed at time 0:50
>>  > 
>>  > And this loop generates these 2 packets from 1 subtitle with start and
>>  > end times.
>>  > 
>>  > You remove it and i dont see how this could still work afterwards
>>  > maybe iam missing something ?
>> 
>>  The subtitles dont have an end time per se, what you have is a display time 
> and a timeout value, this is incase of a stream problem or corruption for a 
> missing packet.
>> 
>>  now what happens is in you example you would get a display packet with 
> regions/objects/cluts etc and a timeout of 60, then around 50 seconds into the 
> stream you would get a packet which has no objects/regions/cluts. This packet 
> would turn the subs off, now if this packet was corrupted then the timeout would 
> take care of the removal.
> 
> I understand that, but this is only so when the source is dvb subs too
> it can be something else.a different subtitle format for example.
> 

But it specifically says "/* Note: DVB subtitle need one packet to draw them and one other packet to clear them */

So i took it from the comment that it was only dvb subs that the code was written for.

Ive uploaded a stream for you and a link to some other test streams.

Try running the BBC_small.ts through ffmpeg with the old engine and then with the modified one.

joolz