[FFmpeg-devel] [PATCH] Add flag to drop non-forced subtitle

[Oliver Fromme]

Michael Niedermayer wrote:
 > On Thu, May 22, 2014 at 07:39:11PM +0200, Oliver Fromme wrote:
 > > Hello,
 > > 
 > > As I mentioned earlier this week, I'm trying to improve support
 > > for forced and partially forced subtitles.  The patch below is
 > > the next step.
 > > 
 > > The patch changes four files.  I'll explain in detail what the
 > > changes are good for:
 > > 
 > > libavcodec/avcodec.h -- Introduce a new codec flag for subtitle
 > > encoders, it's called CODEC_FLAG_FORCED_SUBS.  When set, this
 > > flag indicates that the encoder should write only such subtitle
 > > frames that are marked as forced.  Frames that are *not* forced
 > > will be ignored by the encoder.
 > > 
 > > By the way, is there a rule for allocating bit masks for new
 > > flags?  I just took the first unused one, that is 0x0080.
 > > 
 > > libavcodec/options_table.h -- Add an option so the new flag can
 > > be set on the command line like this:  -flags +forced_subs
 > > 
 > > Before I continue, I need to explain some background:  A single
 > > subtitle frame can consist of multiple rectangles, each one has
 > > its own forced flag.  PGS (i.e. the Blu-Ray subtitle format)
 > > supports multiple rectangles, but VOBSUB (a.k.a. dvdsub) does
 > > not.  So, if we have a subtitle frame with multiple rectangles,
 > > we have to merge them into one rectangle when encoding to dvdsub.
 > > The dvdsubenc encoder already does that, but it doesn't respect
 > > the forced flag yet.  Now comes my patch ...
 > > 
 > > libavcodec/dvdsubenc.c -- If CODEC_FLAG_FORCED_SUBS is set and
 > > there are multiple rectangles, we need to figure out which ones
 > > are forced and which ones are not, and then merge only the ones
 > > that are forced.  The new variable "first" contains the index
 > > of the first rectangle that needs to be merged (otherwise it
 > > contains 0, so the behaviour is unchanged if the codec flag is
 > > not set).
 > > 
 > > In all the following loops that iterate over the rectangles,
 > > any rectangles that are non-forced are skipped when the codec
 > > flag is set.  That's all.
 > > 
 > > Unfortunately, there's a special case that cannot be handled
 > > here in the encoder:  When *all* rectangles of this subtitle
 > > are non-forced (and the codec flag is set).  In this case we
 > > would have to skip all rectangles and produce an empty frame.
 > > This is not possible in the encoder ...  The API expects that
 > > the codec's encode() function produces a frame for every input
 > > frame.  Therefore, this case has to be handled at a higher
 > > level:
 > > 
 > > ffmpeg.c -- In the do_subtitle_out() function, we check if
 > > CODEC_FLAG_FORCED_SUBS is set.  If it is, we check if *all*
 > > of the rectangles of the current subtitle frame are non-forced.
 > > If that's the case, we return right away, not calling the
 > > encoder at all, not calling write_frame(), not increasing the
 > > frame counter etc.
 > > 
 > > You can test the new functionality with the file that I had
 > > uploaded on May 18th, called subt-forced-test.mkv.  It
 > > contains one dvdsub subtitle track that has 991 frames, 2 of
 > > which are forced.
 > > For example, the following command could be used to "split"
 > > the subtitles in two output files:  The first one contains
 > > all of them, the second one contains only the forced frames:
 > > 
 > >     $ ffmpeg -i subt-forced-test.mkv \
 > >             -codec:s dvdsub test1.mkv \
 > >             -codec:s dvdsub -flags:s +forced_subs test2.mkv
 > > 
 > >     $ mkvextract tracks test1.mkv 0:test1.idx
 > >     $ mkvextract tracks test2.mkv 0:test2.idx
 > >     $ grep -c timestamp test[12].idx
 > >     test1.idx:991
 > >     test2.idx:2
 > > 
 > > Voila.
 > > 
 > > Best regards
 > >    Oliver
 > > 
 > > 
 > > --- ffmpeg/libavcodec/avcodec.h.orig	2014-05-13 19:20:05.000000000 +0200
 > > +++ ffmpeg/libavcodec/avcodec.h	2014-05-21 18:52:28.000000000 +0200
 > > @@ -758,6 +758,7 @@
 > >   */
 > >  #define CODEC_FLAG_MV0    0x0040
 > >  #endif
 > > +#define CODEC_FLAG_FORCED_SUBS     0x0080   ///< Encode forced subtitles only.
 > >  #if FF_API_INPUT_PRESERVED
 > >  /**
 > >   * @deprecated passing reference-counted frames to the encoders replaces this
 > > --- ffmpeg/libavcodec/options_table.h.orig	2014-05-13 19:20:05.000000000 +0200
 > > +++ ffmpeg/libavcodec/options_table.h	2014-05-21 18:55:15.000000000 +0200
 > > @@ -60,6 +60,7 @@
 > >  #if FF_API_MV0
 > >  {"mv0", "always try a mb with mv=<0,0>", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_MV0 }, INT_MIN, INT_MAX, V|E, "flags"},
 > >  #endif
 > > +{"forced_subs", "encode forced subtitle captions only", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_FORCED_SUBS }, INT_MIN, INT_MAX, S|E, "flags"},
 > >  #if FF_API_INPUT_PRESERVED
 > >  {"input_preserved", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_INPUT_PRESERVED }, INT_MIN, INT_MAX, 0, "flags"},
 > >  #endif
 > 
 > > --- ffmpeg/libavcodec/dvdsubenc.c.orig	2014-05-16 23:35:29.000000000 +0200
 > > +++ ffmpeg/libavcodec/dvdsubenc.c	2014-05-21 20:03:55.000000000 +0200
 > 
 > why is this done in dvdsubenc and not utils.c for all subtitle
 > encoders ?

utils.c has no knowledge if the subtitle codec supports 
multiple rectangles per subtitle frame.  dvdsub does not,
so dvdsubenc has to merge multiple rectangles into one
frame, and also handle per-rectangle forced flags.

By the way, dvdsubenc is ffmpeg's *only* encoder for
graphical subtitles, as far as I can see.

Best regards
   Oliver

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