ffmpeg: Do not fill gap before the first decodable frame on single stream input files unless the user explicitly requests it.

Fixes different behavior to JM and probably several if not all
reference decoders.

We cannot just do this unconditionally as it would ruin AV sync in
some use cases.

Bug-Found-by: BugMaster
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.c

@@ -823,9 +823,21 @@ static void do_video_out(AVFormatContext *s,
             format_video_sync = VSYNC_VFR;
         } else
             format_video_sync = (s->oformat->flags & AVFMT_VARIABLE_FPS) ? ((s->oformat->flags & AVFMT_NOTIMESTAMPS) ? VSYNC_PASSTHROUGH : VSYNC_VFR) : VSYNC_CFR;
+        if (   ist
+            && format_video_sync == VSYNC_CFR
+            && input_files[ist->file_index]->ctx->nb_streams == 1
+            && input_files[ist->file_index]->input_ts_offset == 0) {
+            format_video_sync = VSYNC_VSCFR;
+        }
     }
 
     switch (format_video_sync) {
+    case VSYNC_VSCFR:
+        if (ost->frame_number == 0 && delta - duration >= 0.5) {
+            av_log(NULL, AV_LOG_DEBUG, "Not duplicating %d initial frames\n", (int)lrintf(delta - duration));
+            delta = duration;
+            ost->sync_opts = lrint(sync_ipts);
+        }
     case VSYNC_CFR:
         // FIXME set to 0.5 after we fix some dts/pts bugs like in avidec.c
         if (delta < -1.1)
@@ -2365,7 +2377,7 @@ static int transcode_init(void)
                 if (ost->filter && !(codec->time_base.num && codec->time_base.den))
                     codec->time_base = ost->filter->filter->inputs[0]->time_base;
                 if (   av_q2d(codec->time_base) < 0.001 && video_sync_method != VSYNC_PASSTHROUGH
-                   && (video_sync_method == VSYNC_CFR || (video_sync_method == VSYNC_AUTO && !(oc->oformat->flags & AVFMT_VARIABLE_FPS)))){
+                   && (video_sync_method == VSYNC_CFR || video_sync_method == VSYNC_VSCFR || (video_sync_method == VSYNC_AUTO && !(oc->oformat->flags & AVFMT_VARIABLE_FPS)))){
                     av_log(oc, AV_LOG_WARNING, "Frame rate very high for a muxer not efficiently supporting it.\n"
                                                "Please consider specifying a lower framerate, a different muxer or -vsync 2\n");
                 }

ffmpeg.h

@@ -51,6 +51,7 @@
 #define VSYNC_PASSTHROUGH 0
 #define VSYNC_CFR         1
 #define VSYNC_VFR         2
+#define VSYNC_VSCFR       0xfe
 #define VSYNC_DROP        0xff
 
 #define MAX_STREAMS 1024    /* arbitrary sanity check value */
@@ -281,6 +282,7 @@ typedef struct InputFile {
     int eof_reached;      /* true if eof reached */
     int eagain;           /* true if last read attempt returned EAGAIN */
     int ist_index;        /* index of first stream in input_streams */
+    int64_t input_ts_offset;
     int64_t ts_offset;
     int64_t last_ts;
     int64_t start_time;   /* user-specified start time in AV_TIME_BASE or AV_NOPTS_VALUE */

ffmpeg_opt.c

@@ -852,6 +852,7 @@ static int open_input_file(OptionsContext *o, const char *filename)
     f->ist_index  = nb_input_streams - ic->nb_streams;
     f->start_time = o->start_time;
     f->recording_time = o->recording_time;
+    f->input_ts_offset = o->input_ts_offset;
     f->ts_offset  = o->input_ts_offset - (copy_ts ? 0 : timestamp);
     f->nb_streams = ic->nb_streams;
     f->rate_emu   = o->rate_emu;