[FFmpeg-devel] [PATCH] Add flag to drop non-forced subtitle frames
Oliver Fromme
oliver at fromme.com
Thu May 22 19:39:11 CEST 2014
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
@@ -260,6 +260,8 @@
uint8_t *vrect_data = NULL;
int x2, y2;
int forced = 0;
+ int first = 0;
+ int want_forced_subs = (avctx->flags & CODEC_FLAG_FORCED_SUBS) != 0;
if (rects == 0 || h->rects == NULL)
return AVERROR(EINVAL);
@@ -271,9 +273,12 @@
/* Mark this subtitle forced if any of the rectangles is forced. */
for (i = 0; i < rects; i++)
if ((h->rects[i]->flags & AV_SUBTITLE_FLAG_FORCED) != 0) {
+ first = i;
forced = 1;
break;
}
+ if (!want_forced_subs)
+ first = 0;
vrect = *h->rects[0];
if (rects > 1) {
@@ -281,9 +286,11 @@
rectangle containing all actual rectangles.
The data of the rectangles will be copied later, when the palette
is decided, because the rectangles may have different palettes. */
- int xmin = h->rects[0]->x, xmax = xmin + h->rects[0]->w;
- int ymin = h->rects[0]->y, ymax = ymin + h->rects[0]->h;
+ int xmin = h->rects[first]->x, xmax = xmin + h->rects[first]->w;
+ int ymin = h->rects[first]->y, ymax = ymin + h->rects[first]->h;
for (i = 1; i < rects; i++) {
+ if (want_forced_subs && !(h->rects[i]->flags & AV_SUBTITLE_FLAG_FORCED))
+ continue;
xmin = FFMIN(xmin, h->rects[i]->x);
ymin = FFMIN(ymin, h->rects[i]->y);
xmax = FFMAX(xmax, h->rects[i]->x + h->rects[i]->w);
@@ -298,12 +305,18 @@
/* Count pixels outside the virtual rectangle as transparent */
global_palette_hits[0] = vrect.w * vrect.h;
- for (i = 0; i < rects; i++)
+ for (i = first; i < rects; i++) {
+ if (want_forced_subs && !(h->rects[i]->flags & AV_SUBTITLE_FLAG_FORCED))
+ continue;
global_palette_hits[0] -= h->rects[i]->w * h->rects[i]->h;
+ }
}
- for (i = 0; i < rects; i++)
+ for (i = first; i < rects; i++) {
+ if (want_forced_subs && !(h->rects[i]->flags & AV_SUBTITLE_FLAG_FORCED))
+ continue;
count_colors(avctx, global_palette_hits, h->rects[i]);
+ }
select_palette(avctx, out_palette, out_alpha, global_palette_hits);
if (rects > 1) {
@@ -311,7 +324,9 @@
return AVERROR(ENOMEM);
vrect.pict.data [0] = vrect_data;
vrect.pict.linesize[0] = vrect.w;
- for (i = 0; i < rects; i++) {
+ for (i = first; i < rects; i++) {
+ if (want_forced_subs && !(h->rects[i]->flags & AV_SUBTITLE_FLAG_FORCED))
+ continue;
build_color_map(avctx, cmap, (uint32_t *)h->rects[i]->pict.data[1],
out_palette, out_alpha);
copy_rectangle(&vrect, h->rects[i], cmap);
--- ffmpeg/ffmpeg.c.orig 2014-05-08 19:20:05.000000000 +0200
+++ ffmpeg/ffmpeg.c 2014-05-21 19:05:33.000000000 +0200
@@ -769,6 +769,7 @@
AVCodecContext *enc;
AVPacket pkt;
int64_t pts;
+ int forced = 0;
if (sub->pts == AV_NOPTS_VALUE) {
av_log(NULL, AV_LOG_ERROR, "Subtitle packets must have a pts\n");
@@ -779,6 +780,16 @@
enc = ost->st->codec;
+ if (enc->flags & CODEC_FLAG_FORCED_SUBS) {
+ for (i = 0; i < sub->num_rects; i++)
+ if (sub->rects[i]->flags & AV_SUBTITLE_FLAG_FORCED) {
+ forced = 1;
+ break;
+ }
+ if (!forced)
+ return;
+ }
+
if (!subtitle_out) {
subtitle_out = av_malloc(subtitle_out_max_size);
}
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