[FFmpeg-devel] ffmpeg: explicitly handle sub2video subpicture initialization

diff --git a/fftools/ffmpeg.c b/fftools/ffmpeg.c
index 544f1a1cef..efe903cc41 100644
--- a/fftools/ffmpeg.c
+++ b/fftools/ffmpeg.c
@@ -237,7 +237,7 @@  static void sub2video_push_ref(InputStream *ist, int64_t pts)
     }
 }
 
-void sub2video_update(InputStream *ist, AVSubtitle *sub)
+void sub2video_update(InputStream *ist, int64_t heartbeat_pts, AVSubtitle *sub)
 {
     AVFrame *frame = ist->sub2video.frame;
     int8_t *dst;
@@ -254,7 +254,12 @@  void sub2video_update(InputStream *ist, AVSubtitle *sub)
                                  AV_TIME_BASE_Q, ist->st->time_base);
         num_rects = sub->num_rects;
     } else {
-        pts       = ist->sub2video.end_pts;
+        /* If we are initializing the system, utilize current heartbeat
+           PTS as the start time, and show until the following subpicture
+           is received. Otherwise, utilize the previous subpicture's end time
+           as the fall-back value. */
+        pts       = ist->sub2video.initialize ?
+                    heartbeat_pts : ist->sub2video.end_pts;
         end_pts   = INT64_MAX;
         num_rects = 0;
     }
@@ -269,6 +274,7 @@  void sub2video_update(InputStream *ist, AVSubtitle *sub)
         sub2video_copy_rect(dst, dst_linesize, frame->width, frame->height, sub->rects[i]);
     sub2video_push_ref(ist, pts);
     ist->sub2video.end_pts = end_pts;
+    ist->sub2video.initialize = 0;
 }
 
 static void sub2video_heartbeat(InputStream *ist, int64_t pts)
@@ -291,9 +297,11 @@  static void sub2video_heartbeat(InputStream *ist, int64_t pts)
         /* do not send the heartbeat frame if the subtitle is already ahead */
         if (pts2 <= ist2->sub2video.last_pts)
             continue;
-        if (pts2 >= ist2->sub2video.end_pts ||
-            (!ist2->sub2video.frame->data[0] && ist2->sub2video.end_pts < INT64_MAX))
-            sub2video_update(ist2, NULL);
+        if (pts2 >= ist2->sub2video.end_pts || ist2->sub2video.initialize)
+            /* if we have hit the end of the current displayed subpicture,
+               or if we need to initialize the system, update the
+               overlayed subpicture and its start/end times */
+            sub2video_update(ist2, pts2 + 1, NULL);
         for (j = 0, nb_reqs = 0; j < ist2->nb_filters; j++)
             nb_reqs += av_buffersrc_get_nb_failed_requests(ist2->filters[j]->filter);
         if (nb_reqs)
@@ -307,7 +315,7 @@  static void sub2video_flush(InputStream *ist)
     int ret;
 
     if (ist->sub2video.end_pts < INT64_MAX)
-        sub2video_update(ist, NULL);
+        sub2video_update(ist, INT64_MAX, NULL);
     for (i = 0; i < ist->nb_filters; i++) {
         ret = av_buffersrc_add_frame(ist->filters[i]->filter, NULL);
         if (ret != AVERROR_EOF && ret < 0)
@@ -2514,7 +2522,7 @@  static int transcode_subtitles(InputStream *ist, AVPacket *pkt, int *got_output,
         return ret;
 
     if (ist->sub2video.frame) {
-        sub2video_update(ist, &subtitle);
+        sub2video_update(ist, INT64_MIN, &subtitle);
     } else if (ist->nb_filters) {
         if (!ist->sub2video.sub_queue)
             ist->sub2video.sub_queue = av_fifo_alloc(8 * sizeof(AVSubtitle));
diff --git a/fftools/ffmpeg.h b/fftools/ffmpeg.h
index eb1eaf6363..b2129a3f28 100644
--- a/fftools/ffmpeg.h
+++ b/fftools/ffmpeg.h
@@ -349,6 +349,7 @@  typedef struct InputStream {
         AVFifoBuffer *sub_queue;    ///< queue of AVSubtitle* before filter init
         AVFrame *frame;
         int w, h;
+        unsigned int initialize; ///< marks if sub2video_update should force an initialization
     } sub2video;
 
     int dr1;
@@ -646,7 +647,7 @@  int filtergraph_is_simple(FilterGraph *fg);
 int init_simple_filtergraph(InputStream *ist, OutputStream *ost);
 int init_complex_filtergraph(FilterGraph *fg);
 
-void sub2video_update(InputStream *ist, AVSubtitle *sub);
+void sub2video_update(InputStream *ist, int64_t heartbeat_pts, AVSubtitle *sub);
 
 int ifilter_parameters_from_frame(InputFilter *ifilter, const AVFrame *frame);
 
diff --git a/fftools/ffmpeg_filter.c b/fftools/ffmpeg_filter.c
index 72838de1e2..d81bb9742f 100644
--- a/fftools/ffmpeg_filter.c
+++ b/fftools/ffmpeg_filter.c
@@ -740,6 +740,12 @@  static int sub2video_prepare(InputStream *ist, InputFilter *ifilter)
         return AVERROR(ENOMEM);
     ist->sub2video.last_pts = INT64_MIN;
     ist->sub2video.end_pts  = INT64_MIN;
+
+    /* sub2video structure has been (re-)initialized.
+       Mark it as such so that the system will be
+       initialized with the first received heartbeat. */
+    ist->sub2video.initialize = 1;
+
     return 0;
 }
 
@@ -1169,7 +1175,7 @@  int configure_filtergraph(FilterGraph *fg)
             while (av_fifo_size(ist->sub2video.sub_queue)) {
                 AVSubtitle tmp;
                 av_fifo_generic_read(ist->sub2video.sub_queue, &tmp, sizeof(tmp), NULL);
-                sub2video_update(ist, &tmp);
+                sub2video_update(ist, INT64_MIN, &tmp);
                 avsubtitle_free(&tmp);
             }
         }