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

[Michael Niedermayer]

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 ?

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Concerning the gods, I have no means of knowing whether they exist or not
or of what sort they may be, because of the obscurity of the subject, and
the brevity of human life -- Protagoras
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 181 bytes
Desc: Digital signature
URL: <https://ffmpeg.org/pipermail/ffmpeg-devel/attachments/20140525/6e6801b7/attachment.asc>