[FFmpeg-devel] [PATCHv3 3/6] dvbsubdec: always return the whole buffer as consumed bytes

[Marton Balint]

On Tue, 15 Oct 2013, JULIAN GARDNER wrote:

>> ________________________________
>> From: Clément Bœsch <u at pkh.me>
>> To: FFmpeg development discussions and patches <ffmpeg-devel at ffmpeg.org> 
>> Sent: Tuesday, 15 October 2013, 7:53
>> Subject: Re: [FFmpeg-devel] [PATCHv3 3/6] dvbsubdec: always return the whole buffer as consumed bytes
>> 
>>
>> On Sun, Oct 06, 2013 at 09:14:12PM +0200, Marton Balint wrote:
>>> We don't want leftover junk bytes to be accidentally feeded to us again. Also
>>> show the number of skipped bytes in the debug output.
>>> 
>>> Signed-off-by: Marton Balint <cus at passwd.hu>
>>> ---
>>>   libavcodec/dvbsubdec.c | 5 ++++-
>>>   1 file changed, 4 insertions(+), 1 deletion(-)
>>> 
>>> diff --git a/libavcodec/dvbsubdec.c b/libavcodec/dvbsubdec.c
>>> index 4ce40aa..9754179 100644
>>> --- a/libavcodec/dvbsubdec.c
>>> +++ b/libavcodec/dvbsubdec.c
>>> @@ -1538,7 +1538,10 @@ static int dvbsub_decode(AVCodecContext *avctx,
>>>       if (got_segment == 15 && sub)
>>>           *data_size = dvbsub_display_end_segment(avctx, p, 0, sub);
>>>   
>>> -    return p - buf;
>>> +    if (p - buf < buf_size)
>>> +        av_log(avctx, AV_LOG_DEBUG, "skipping %d leftover junk bytes\n", buf_size - (int)(p - buf));
>>> +
>>> +    return buf_size;
>>>   }
>>
>> Any idea what those junk bytes are? Maybe empty packets to stop previous
>> sub presentation?
>>
>> -- 
>> Clément B.
>>
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>>
>
> Before adding this patch, which i dont think is needed, where is the sample Ts stream that has this problem so we can find our what is the real cause as the pes stream shoud NOT have any trailing data.
>
> Remember the implementation of DVB subs cheats in the removal cases as it does not use a real timer and/or the empty dvb subs packets.
>
> If you have a TS that is showing extraneous chars can i get a copy to chkeck why
>
> joolz

Unfortunately I don't have a sample transport stream. The only reference I 
could find why leftover junk bytes are even possible is here:

http://ffmpeg.org/pipermail/ffmpeg-devel/2011-February/103659.html
(mplayer ts demuxer outputs an extra byte)

The patch is needed to ensure that leftover bytes still do not cause any 
frame decode errors after extending avcodec_decode_subtitle2 to support 
multiple frames in a single packet. So it's for compatibility and keeping 
existing behaviour.

Regards,
Marton