ffmpeg: explicitly handle sub2video subpicture initialization

Each time the sub2video structure is initialized, the sub2video
subpicture is initialized together with the first received heartbeat.
The heartbeat's PTS is utilized as the subpicture start time.

Additionally, add some documentation on the stages.

1 files changed, 15 insertions, 7 deletions

diff --git a/fftools/ffmpeg.c b/fftools/ffmpeg.c
index e5fbd479a8..aaaf241314 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)
@@ -2507,7 +2515,7 @@ 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));