[FFmpeg-devel] [PATCH v6 0/3] ffmpeg: add video heartbeat capability to fix_sub_duration

[FFmpeg-devel] [PATCH v6 0/3] ffmpeg: add video heartbeat capability to fix_sub_duration

[Jan Ekström]

Changes from v5:
 - Rebased things on top of current master (option related code moved to
   ffmpeg_mux_init etc).
 - Added notes in the documentation regarding drawbacks of this option and
   that it should not be utilized in cases where latency of when the subtitle
   event is passed on to output is not relevant.

Jan Ekström (3):
  ffmpeg: refactor post-decoding steps for subtitles into a function
  ffmpeg: move decoded frame counter from after post-processing to
    decode
  ffmpeg: add video heartbeat capability to fix_sub_duration

 doc/ffmpeg.texi                               |  14 ++
 fftools/ffmpeg.c                              | 194 +++++++++++++++---
 fftools/ffmpeg.h                              |   8 +
 fftools/ffmpeg_mux_init.c                     |   4 +
 fftools/ffmpeg_opt.c                          |   5 +
 tests/fate/ffmpeg.mak                         |  14 ++
 .../fate/ffmpeg-fix_sub_duration_heartbeat    |  48 +++++
 7 files changed, 264 insertions(+), 23 deletions(-)
 create mode 100644 tests/ref/fate/ffmpeg-fix_sub_duration_heartbeat

-- 
2.38.1

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[FFmpeg-devel] [PATCH v6 1/3] ffmpeg: refactor post-decoding steps for subtitles into a function

[Jan Ekström]

From: Jan Ekström <jan.ekstrom@24i.com>

This enables us to later call this when generating additional
subtitles for splitting purposes.

Co-authored-by: Andrzej Nadachowski <andrzej.nadachowski@24i.com>

Signed-off-by: Jan Ekström <jan.ekstrom@24i.com>
---
 fftools/ffmpeg.c | 49 +++++++++++++++++++++++++++---------------------
 1 file changed, 28 insertions(+), 21 deletions(-)

diff --git a/fftools/ffmpeg.c b/fftools/ffmpeg.c
index 881d6f0af2..eaf0a40328 100644
--- a/fftools/ffmpeg.c
+++ b/fftools/ffmpeg.c
@@ -2187,27 +2187,15 @@ fail:
     return err < 0 ? err : ret;
 }
 
-static int transcode_subtitles(InputStream *ist, AVPacket *pkt, int *got_output,
-                               int *decode_failed)
+static int process_subtitle(InputStream *ist, AVSubtitle *subtitle, int *got_output)
 {
-    AVSubtitle subtitle;
+    int ret = 0;
     int free_sub = 1;
-    int ret = avcodec_decode_subtitle2(ist->dec_ctx,
-                                       &subtitle, got_output, pkt);
-
-    check_decode_result(NULL, got_output, ret);
-
-    if (ret < 0 || !*got_output) {
-        *decode_failed = 1;
-        if (!pkt->size)
-            sub2video_flush(ist);
-        return ret;
-    }
 
     if (ist->fix_sub_duration) {
         int end = 1;
         if (ist->prev_sub.got_output) {
-            end = av_rescale(subtitle.pts - ist->prev_sub.subtitle.pts,
+            end = av_rescale(subtitle->pts - ist->prev_sub.subtitle.pts,
                              1000, AV_TIME_BASE);
             if (end < ist->prev_sub.subtitle.end_display_time) {
                 av_log(NULL, AV_LOG_DEBUG,
@@ -2219,7 +2207,7 @@ static int transcode_subtitles(InputStream *ist, AVPacket *pkt, int *got_output,
         }
         FFSWAP(int,        *got_output, ist->prev_sub.got_output);
         FFSWAP(int,        ret,         ist->prev_sub.ret);
-        FFSWAP(AVSubtitle, subtitle,    ist->prev_sub.subtitle);
+        FFSWAP(AVSubtitle, *subtitle,   ist->prev_sub.subtitle);
         if (end <= 0)
             goto out;
     }
@@ -2228,20 +2216,20 @@ static int transcode_subtitles(InputStream *ist, AVPacket *pkt, int *got_output,
         return ret;
 
     if (ist->sub2video.frame) {
-        sub2video_update(ist, INT64_MIN, &subtitle);
+        sub2video_update(ist, INT64_MIN, subtitle);
     } else if (ist->nb_filters) {
         if (!ist->sub2video.sub_queue)
             ist->sub2video.sub_queue = av_fifo_alloc2(8, sizeof(AVSubtitle), AV_FIFO_FLAG_AUTO_GROW);
         if (!ist->sub2video.sub_queue)
             report_and_exit(AVERROR(ENOMEM));
 
-        ret = av_fifo_write(ist->sub2video.sub_queue, &subtitle, 1);
+        ret = av_fifo_write(ist->sub2video.sub_queue, subtitle, 1);
         if (ret < 0)
             exit_program(1);
         free_sub = 0;
     }
 
-    if (!subtitle.num_rects)
+    if (!subtitle->num_rects)
         goto out;
 
     ist->frames_decoded++;
@@ -2251,15 +2239,34 @@ static int transcode_subtitles(InputStream *ist, AVPacket *pkt, int *got_output,
             || ost->enc_ctx->codec_type != AVMEDIA_TYPE_SUBTITLE)
             continue;
 
-        do_subtitle_out(output_files[ost->file_index], ost, &subtitle);
+        do_subtitle_out(output_files[ost->file_index], ost, subtitle);
     }
 
 out:
     if (free_sub)
-        avsubtitle_free(&subtitle);
+        avsubtitle_free(subtitle);
     return ret;
 }
 
+static int transcode_subtitles(InputStream *ist, AVPacket *pkt, int *got_output,
+                               int *decode_failed)
+{
+    AVSubtitle subtitle;
+    int ret = avcodec_decode_subtitle2(ist->dec_ctx,
+                                       &subtitle, got_output, pkt);
+
+    check_decode_result(NULL, got_output, ret);
+
+    if (ret < 0 || !*got_output) {
+        *decode_failed = 1;
+        if (!pkt->size)
+            sub2video_flush(ist);
+        return ret;
+    }
+
+    return process_subtitle(ist, &subtitle, got_output);
+}
+
 static int send_filter_eof(InputStream *ist)
 {
     int i, ret;
-- 
2.38.1

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[FFmpeg-devel] [PATCH v6 2/3] ffmpeg: move decoded frame counter from after post-processing to decode

[Jan Ekström]

From: Jan Ekström <jan.ekstrom@24i.com>

This way we can call process_subtitles without causing the decoded
frame counter to get bumped.

Additionally, this now takes into mention all of the decoded
subtitle frames without fix_sub_duration latency/buffering, or filtering
out decoded reset/end subtitles without any rendered rectangles, which
matches the original intent in 4754345027eb85cfa51aeb88beec68d7b036c11e
.

Signed-off-by: Jan Ekström <jan.ekstrom@24i.com>
---
 fftools/ffmpeg.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fftools/ffmpeg.c b/fftools/ffmpeg.c
index eaf0a40328..3b94eaaa58 100644
--- a/fftools/ffmpeg.c
+++ b/fftools/ffmpeg.c
@@ -2232,8 +2232,6 @@ static int process_subtitle(InputStream *ist, AVSubtitle *subtitle, int *got_out
     if (!subtitle->num_rects)
         goto out;
 
-    ist->frames_decoded++;
-
     for (OutputStream *ost = ost_iter(NULL); ost; ost = ost_iter(ost)) {
         if (!check_output_constraints(ist, ost) || !ost->enc_ctx
             || ost->enc_ctx->codec_type != AVMEDIA_TYPE_SUBTITLE)
@@ -2264,6 +2262,8 @@ static int transcode_subtitles(InputStream *ist, AVPacket *pkt, int *got_output,
         return ret;
     }
 
+    ist->frames_decoded++;
+
     return process_subtitle(ist, &subtitle, got_output);
 }
 
-- 
2.38.1

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[FFmpeg-devel] [PATCH v6 3/3] ffmpeg: add video heartbeat capability to fix_sub_duration

[Jan Ekström]

From: Jan Ekström <jan.ekstrom@24i.com>

Splits the currently handled subtitle at random access point
packets that can be configured to follow a specific output stream.

This way the subtitle - which is known to be shown at this time
can be split and passed to muxer before its full duration is
yet known. This is also a drawback, as this essentially outputs
multiple subtitles from a single input subtitle that continues
over multiple random access points. Thus this feature should not
be utilized in cases where subtitle output latency does not matter.

Co-authored-by: Andrzej Nadachowski <andrzej.nadachowski@24i.com>
Co-authored-by: Bernard Boulay <bernard.boulay@24i.com>

Signed-off-by: Jan Ekström <jan.ekstrom@24i.com>
---
 doc/ffmpeg.texi                               |  14 ++
 fftools/ffmpeg.c                              | 141 ++++++++++++++++++
 fftools/ffmpeg.h                              |   8 +
 fftools/ffmpeg_mux_init.c                     |   4 +
 fftools/ffmpeg_opt.c                          |   5 +
 tests/fate/ffmpeg.mak                         |  14 ++
 .../fate/ffmpeg-fix_sub_duration_heartbeat    |  48 ++++++
 7 files changed, 234 insertions(+)
 create mode 100644 tests/ref/fate/ffmpeg-fix_sub_duration_heartbeat

diff --git a/doc/ffmpeg.texi b/doc/ffmpeg.texi
index 0367930a3b..d5f231e635 100644
--- a/doc/ffmpeg.texi
+++ b/doc/ffmpeg.texi
@@ -1339,6 +1339,20 @@ List all hardware acceleration components enabled in this build of ffmpeg.
 Actual runtime availability depends on the hardware and its suitable driver
 being installed.
 
+@item -fix_sub_duration_heartbeat[:@var{stream_specifier}]
+Set a specific output video stream as the heartbeat stream according to which
+to split and push through currently in-progress subtitle upon receipt of a
+random access packet.
+
+This lowers the latency of subtitles for which the end packet or the following
+subtitle has not yet been received. As a drawback, this will most likely lead
+to duplication of subtitle events in order to cover the full duration, so
+when dealing with use cases where latency of when the subtitle event is passed
+on to output is not relevant this option should not be utilized.
+
+Requires @option{-fix_sub_duration} to be set for the relevant input subtitle
+stream for this to have any effect.
+
 @end table
 
 @section Audio Options
diff --git a/fftools/ffmpeg.c b/fftools/ffmpeg.c
index 3b94eaaa58..1fde4bf0f5 100644
--- a/fftools/ffmpeg.c
+++ b/fftools/ffmpeg.c
@@ -119,6 +119,7 @@ typedef struct BenchmarkTimeStamps {
     int64_t sys_usec;
 } BenchmarkTimeStamps;
 
+static int trigger_fix_sub_duration_heartbeat(OutputStream *ost, const AVPacket *pkt);
 static BenchmarkTimeStamps get_benchmark_time_stamps(void);
 static int64_t getmaxrss(void);
 static int ifilter_has_all_input_formats(FilterGraph *fg);
@@ -870,6 +871,13 @@ static int encode_frame(OutputFile *of, OutputStream *ost, AVFrame *frame)
                    av_ts2str(pkt->duration), av_ts2timestr(pkt->duration, &enc->time_base));
         }
 
+        if ((ret = trigger_fix_sub_duration_heartbeat(ost, pkt)) < 0) {
+            av_log(NULL, AV_LOG_ERROR,
+                   "Subtitle heartbeat logic failed in %s! (%s)\n",
+                   __func__, av_err2str(ret));
+            exit_program(1);
+        }
+
         ost->data_size_enc += pkt->size;
 
         if (enc->codec_type == AVMEDIA_TYPE_VIDEO)
@@ -1817,6 +1825,16 @@ static void do_streamcopy(InputStream *ist, OutputStream *ost, const AVPacket *p
 
     opkt->duration = av_rescale_q(pkt->duration, ist->st->time_base, ost->mux_timebase);
 
+    {
+        int ret = trigger_fix_sub_duration_heartbeat(ost, pkt);
+        if (ret < 0) {
+            av_log(NULL, AV_LOG_ERROR,
+                   "Subtitle heartbeat logic failed in %s! (%s)\n",
+                   __func__, av_err2str(ret));
+            exit_program(1);
+        }
+    }
+
     of_output_packet(of, opkt, ost, 0);
 
     ost->streamcopy_started = 1;
@@ -2246,6 +2264,129 @@ out:
     return ret;
 }
 
+static int copy_av_subtitle(AVSubtitle *dst, AVSubtitle *src)
+{
+    int ret = AVERROR_BUG;
+    AVSubtitle tmp = {
+        .format = src->format,
+        .start_display_time = src->start_display_time,
+        .end_display_time = src->end_display_time,
+        .num_rects = 0,
+        .rects = NULL,
+        .pts = src->pts
+    };
+
+    if (!src->num_rects)
+        goto success;
+
+    if (!(tmp.rects = av_calloc(src->num_rects, sizeof(*tmp.rects))))
+        return AVERROR(ENOMEM);
+
+    for (int i = 0; i < src->num_rects; i++) {
+        AVSubtitleRect *src_rect = src->rects[i];
+        AVSubtitleRect *dst_rect;
+
+        if (!(dst_rect = tmp.rects[i] = av_mallocz(sizeof(*tmp.rects[0])))) {
+            ret = AVERROR(ENOMEM);
+            goto cleanup;
+        }
+
+        tmp.num_rects++;
+
+        dst_rect->type      = src_rect->type;
+        dst_rect->flags     = src_rect->flags;
+
+        dst_rect->x         = src_rect->x;
+        dst_rect->y         = src_rect->y;
+        dst_rect->w         = src_rect->w;
+        dst_rect->h         = src_rect->h;
+        dst_rect->nb_colors = src_rect->nb_colors;
+
+        if (src_rect->text)
+            if (!(dst_rect->text = av_strdup(src_rect->text))) {
+                ret = AVERROR(ENOMEM);
+                goto cleanup;
+            }
+
+        if (src_rect->ass)
+            if (!(dst_rect->ass = av_strdup(src_rect->ass))) {
+                ret = AVERROR(ENOMEM);
+                goto cleanup;
+            }
+
+        for (int j = 0; j < 4; j++) {
+            // SUBTITLE_BITMAP images are special in the sense that they
+            // are like PAL8 images. first pointer to data, second to
+            // palette. This makes the size calculation match this.
+            size_t buf_size = src_rect->type == SUBTITLE_BITMAP && j == 1 ?
+                              AVPALETTE_SIZE :
+                              src_rect->h * src_rect->linesize[j];
+
+            if (!src_rect->data[j])
+                continue;
+
+            if (!(dst_rect->data[j] = av_memdup(src_rect->data[j], buf_size))) {
+                ret = AVERROR(ENOMEM);
+                goto cleanup;
+            }
+            dst_rect->linesize[j] = src_rect->linesize[j];
+        }
+    }
+
+success:
+    *dst = tmp;
+
+    return 0;
+
+cleanup:
+    avsubtitle_free(&tmp);
+
+    return ret;
+}
+
+static int fix_sub_duration_heartbeat(InputStream *ist, int64_t signal_pts)
+{
+    int ret = AVERROR_BUG;
+    int got_output = 1;
+    AVSubtitle *prev_subtitle = &ist->prev_sub.subtitle;
+    AVSubtitle subtitle;
+
+    if (!ist->fix_sub_duration || !prev_subtitle->num_rects ||
+        signal_pts <= prev_subtitle->pts)
+        return 0;
+
+    if ((ret = copy_av_subtitle(&subtitle, prev_subtitle)) < 0)
+        return ret;
+
+    subtitle.pts = signal_pts;
+
+    return process_subtitle(ist, &subtitle, &got_output);
+}
+
+static int trigger_fix_sub_duration_heartbeat(OutputStream *ost, const AVPacket *pkt)
+{
+    int64_t signal_pts = av_rescale_q(pkt->pts, ost->mux_timebase,
+                                      AV_TIME_BASE_Q);
+
+    if (!ost->fix_sub_duration_heartbeat || !(pkt->flags & AV_PKT_FLAG_KEY))
+        // we are only interested in heartbeats on streams configured, and
+        // only on random access points.
+        return 0;
+
+    for (InputStream *ist = ist_iter(NULL); ist; ist = ist_iter(ist)) {
+        int ret = AVERROR_BUG;
+
+        if (!ist->decoding_needed ||
+            ist->dec_ctx->codec_type != AVMEDIA_TYPE_SUBTITLE)
+            continue;
+
+        if ((ret = fix_sub_duration_heartbeat(ist, signal_pts)) < 0)
+            return ret;
+    }
+
+    return 0;
+}
+
 static int transcode_subtitles(InputStream *ist, AVPacket *pkt, int *got_output,
                                int *decode_failed)
 {
diff --git a/fftools/ffmpeg.h b/fftools/ffmpeg.h
index 5527dbe49b..896297fef6 100644
--- a/fftools/ffmpeg.h
+++ b/fftools/ffmpeg.h
@@ -224,6 +224,8 @@ typedef struct OptionsContext {
     int        nb_reinit_filters;
     SpecifierOpt *fix_sub_duration;
     int        nb_fix_sub_duration;
+    SpecifierOpt *fix_sub_duration_heartbeat;
+    int        nb_fix_sub_duration_heartbeat;
     SpecifierOpt *canvas_sizes;
     int        nb_canvas_sizes;
     SpecifierOpt *pass;
@@ -625,6 +627,12 @@ typedef struct OutputStream {
 
     int sq_idx_encode;
     int sq_idx_mux;
+
+    /*
+     * bool on whether this stream should be utilized for splitting
+     * subtitles utilizing fix_sub_duration at random access points.
+     */
+    unsigned int fix_sub_duration_heartbeat;
 } OutputStream;
 
 typedef struct OutputFile {
diff --git a/fftools/ffmpeg_mux_init.c b/fftools/ffmpeg_mux_init.c
index 0280759b05..1e0abf1fca 100644
--- a/fftools/ffmpeg_mux_init.c
+++ b/fftools/ffmpeg_mux_init.c
@@ -57,6 +57,7 @@ static const char *const opt_name_disposition[]               = {"disposition",
 static const char *const opt_name_enc_time_bases[]            = {"enc_time_base", NULL};
 static const char *const opt_name_filters[]                   = {"filter", "af", "vf", NULL};
 static const char *const opt_name_filter_scripts[]            = {"filter_script", NULL};
+static const char *const opt_name_fix_sub_duration_heartbeat[] = {"fix_sub_duration_heartbeat", NULL};
 static const char *const opt_name_fps_mode[]                  = {"fps_mode", NULL};
 static const char *const opt_name_force_fps[]                 = {"force_fps", NULL};
 static const char *const opt_name_forced_key_frames[]         = {"forced_key_frames", NULL};
@@ -342,6 +343,9 @@ static OutputStream *new_output_stream(Muxer *mux, const OptionsContext *o,
     MATCH_PER_STREAM_OPT(bits_per_raw_sample, i, ost->bits_per_raw_sample,
                          oc, st);
 
+    MATCH_PER_STREAM_OPT(fix_sub_duration_heartbeat, i, ost->fix_sub_duration_heartbeat,
+                         oc, st);
+
     if (oc->oformat->flags & AVFMT_GLOBALHEADER && ost->enc_ctx)
         ost->enc_ctx->flags |= AV_CODEC_FLAG_GLOBAL_HEADER;
 
diff --git a/fftools/ffmpeg_opt.c b/fftools/ffmpeg_opt.c
index 3df02b7d7f..d187da88a0 100644
--- a/fftools/ffmpeg_opt.c
+++ b/fftools/ffmpeg_opt.c
@@ -1648,6 +1648,11 @@ const OptionDef options[] = {
     { "autoscale",        HAS_ARG | OPT_BOOL | OPT_SPEC |
                           OPT_EXPERT | OPT_OUTPUT,                               { .off = OFFSET(autoscale) },
         "automatically insert a scale filter at the end of the filter graph" },
+    { "fix_sub_duration_heartbeat", OPT_VIDEO | OPT_BOOL | OPT_EXPERT |
+                                    OPT_SPEC | OPT_OUTPUT,                       { .off = OFFSET(fix_sub_duration_heartbeat) },
+        "set this video output stream to be a heartbeat stream for "
+        "fix_sub_duration, according to which subtitles should be split at "
+        "random access points" },
 
     /* audio options */
     { "aframes",        OPT_AUDIO | HAS_ARG  | OPT_PERFILE | OPT_OUTPUT,           { .func_arg = opt_audio_frames },
diff --git a/tests/fate/ffmpeg.mak b/tests/fate/ffmpeg.mak
index d87639c596..a983ace3bd 100644
--- a/tests/fate/ffmpeg.mak
+++ b/tests/fate/ffmpeg.mak
@@ -117,6 +117,20 @@ fate-ffmpeg-fix_sub_duration: CMD = fmtstdout srt -fix_sub_duration \
   -real_time 1 -f lavfi \
   -i "movie=$(TARGET_SAMPLES)/sub/Closedcaption_rollup.m2v[out0+subcc]"
 
+# Basic test for fix_sub_duration_heartbeat, which causes a buffered subtitle
+# to be pushed out when a video keyframe is received from an encoder.
+FATE_SAMPLES_FFMPEG-$(call FILTERDEMDECENCMUX, MOVIE, MPEGVIDEO, \
+                           MPEG2VIDEO, SUBRIP, SRT, LAVFI_INDEV  \
+                           MPEGVIDEO_PARSER CCAPTION_DECODER \
+                           MPEG2VIDEO_ENCODER NULL_MUXER PIPE_PROTOCOL) \
+                           += fate-ffmpeg-fix_sub_duration_heartbeat
+fate-ffmpeg-fix_sub_duration_heartbeat: CMD = fmtstdout srt -fix_sub_duration \
+  -real_time 1 -f lavfi \
+  -i "movie=$(TARGET_SAMPLES)/sub/Closedcaption_rollup.m2v[out0+subcc]" \
+  -map 0:v -fix_sub_duration_heartbeat:v:0 \
+  -c mpeg2video -b:v 2M -g 30 -sc_threshold 1000000000 \
+  -f null -
+
 FATE_STREAMCOPY-$(call REMUX, MP4 MOV, EAC3_DEMUXER) += fate-copy-trac3074
 fate-copy-trac3074: CMD = transcode eac3 $(TARGET_SAMPLES)/eac3/csi_miami_stereo_128_spx.eac3\
                      mp4 "-codec copy -map 0" "-codec copy"
diff --git a/tests/ref/fate/ffmpeg-fix_sub_duration_heartbeat b/tests/ref/fate/ffmpeg-fix_sub_duration_heartbeat
new file mode 100644
index 0000000000..957a410921
--- /dev/null
+++ b/tests/ref/fate/ffmpeg-fix_sub_duration_heartbeat
@@ -0,0 +1,48 @@
+1
+00:00:00,968 --> 00:00:01,001
+<font face="Monospace">{\an7}(</font>
+
+2
+00:00:01,001 --> 00:00:01,168
+<font face="Monospace">{\an7}(</font>
+
+3
+00:00:01,168 --> 00:00:01,368
+<font face="Monospace">{\an7}(<i> inaudibl</i></font>
+
+4
+00:00:01,368 --> 00:00:01,568
+<font face="Monospace">{\an7}(<i> inaudible radio chat</i></font>
+
+5
+00:00:01,568 --> 00:00:02,002
+<font face="Monospace">{\an7}(<i> inaudible radio chatter</i> )</font>
+
+6
+00:00:02,002 --> 00:00:03,003
+<font face="Monospace">{\an7}(<i> inaudible radio chatter</i> )</font>
+
+7
+00:00:03,003 --> 00:00:03,103
+<font face="Monospace">{\an7}(<i> inaudible radio chatter</i> )</font>
+
+8
+00:00:03,103 --> 00:00:03,303
+<font face="Monospace">{\an7}(<i> inaudible radio chatter</i> )
+>></font>
+
+9
+00:00:03,303 --> 00:00:03,503
+<font face="Monospace">{\an7}(<i> inaudible radio chatter</i> )
+>> Safety rema</font>
+
+10
+00:00:03,504 --> 00:00:03,704
+<font face="Monospace">{\an7}(<i> inaudible radio chatter</i> )
+>> Safety remains our numb</font>
+
+11
+00:00:03,704 --> 00:00:04,004
+<font face="Monospace">{\an7}(<i> inaudible radio chatter</i> )
+>> Safety remains our number one</font>
+
-- 
2.38.1

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Re: [FFmpeg-devel] [PATCH v6 0/3] ffmpeg: add video heartbeat capability to fix_sub_duration

[Jan Ekström]

On Mon, Dec 19, 2022 at 9:34 AM Jan Ekström <jeebjp@gmail.com> wrote:
>
> Changes from v5:
>  - Rebased things on top of current master (option related code moved to
>    ffmpeg_mux_init etc).
>  - Added notes in the documentation regarding drawbacks of this option and
>    that it should not be utilized in cases where latency of when the subtitle
>    event is passed on to output is not relevant.
>
> Jan Ekström (3):
>   ffmpeg: refactor post-decoding steps for subtitles into a function
>   ffmpeg: move decoded frame counter from after post-processing to
>     decode
>   ffmpeg: add video heartbeat capability to fix_sub_duration

If there are no further comments, I will start pulling this patch set
in later this week.

Jan
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