Hello,
On N, 2011-12-08 at 05:49 -0800, TruBzzz wrote:
> Hi GStreamer team,
>
> I experienced issues while decoding DVB subtitles contained in mpegTS files:
> Here is a sample of the rendering
>
http://gstreamer-devel.966125.n4.nabble.com/file/n4172790/dvbsub1.png
> I used the 'dvbsuboverlay' element in following pipeline like
> /// gst-launch -v filesrc location=video.mpg ! mpegtsdemux name=d !
> // queue ! mp3parse ! mad ! audioconvert ! autoaudiosink d. !
> // queue ! ffdec_h264 ! ffmpegcolorspace ! r. d. !
> // queue ! "subpicture/x-dvb" ! dvbsuboverlay name=r !
> ffmpegcolorspace ! autovideosink
> /
>
> It may be related or not, but I got this logs from the parser:
> */WARNING **: Parsing 2bit color DVB sub-picture. This is not tested at all.
> If you see this message, please provide the developers with sample media
> with these subtitles, if possible./*
> If you are interested, I'll be able to provide a sample media with these
> subtitles.
Yes, that must be it. I never found any 2bit or 8bit samples to actually
test the code with. All channels in Estonia and Finland used 4bit; I
believe also some samples from UK/Spain were 4bit.
So some sample media would be most appreciated. Not sure how to get it,
but I believe if it can be public, there are some place on freedesktop
server we can store it once someone with access has it. Maybe they are
small enough as a bugzilla attachment, especially if just the subtitle
elementary stream?
> Furthermore, I notice that ffmpeg provide also a plugin 'ffdec_dvbsub' but I
> didn't succeed to decode the same file with this decoder.
The dvb-sub.c code from dvbsuboverlay plugin originates from libdvbsub I
wrote for a DVB box back in the day, this code is at
https://github.com/leio/libdvbsubFrom there you can see it in turn is based on that ffmpeg LGPL decoder,
as a port to glib/gstreamer utilities (the gstreamer bitreader, etc),
with other changes in there - commit log available on github. During
that porting work I noticed various problems in the decoder that I fixed
in the libdvbsub code. Though later some of those turned out to be my
porting errors instead. There are a few FIXME-FFMPEG comments iirc, that
show some potential problems in ffmpeg original code to look at by those
that care. Eventually I did see ffmpeg itself render some samples fine
though with its own unmodified code, iirc.
> Because this ffmpeg dvbsub decoder correctly decode these subtitles in VLC,
> I'll probably compare/merge the 2 plugins dvbsuboverlay/ffdec_dvbsub to make
> it work.
VLC has its own DVB subtitle decoding code, it doesn't use the one from
ffmpeg. It's GPL, so I didn't care for that for my needs then.
On hindsight I think I should have written it from scratch, to fit the
spec with objects better and avoid various porting errors and inherited
bugs - the spec is simple and straightforward enough. Maybe on a rainy
day when we can convert it from an overlay to something that just
provides subpicture regions for the videosink - then it may be more
useful to hang on to the dvbsub spec objects in a different way, etc.
> Here is the expected behavior:
>
http://gstreamer-devel.966125.n4.nabble.com/file/n4172790/dvbsub2.png
It's probably just some porting error in the bitreader code, or also a
bug in the original ffmpeg code. Shouldn't be too hard to fix with a
sample.
I suggest to record the problem on gstreamer bugzilla for tracking, and
somehow provide a sample. The dvbsub spec is available publicly and free
of charge if you want to take a stab at it yourself. With a sample I can
take a quick look too.
Regards,
Mart Raudsepp
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