Fixed problem with frame rate reduction when capturing from a source

whose PTS does not start at zero. New code will fix that if the PTS
is more than 100 seconds from zero. This may well not be the right
number, but it is a big improvement over the current state.

Originally committed as revision 1297 to svn://svn.ffmpeg.org/ffmpeg/trunk

ffmpeg.c

@@ -163,6 +163,7 @@ typedef struct AVOutputStream {
     /* input pts and corresponding output pts
        for A/V sync */
     double sync_ipts;
+    double sync_ipts_offset;
     INT64 sync_opts;
     /* video only */
     AVPicture pict_tmp;         /* temporary image for resizing */
@@ -541,12 +542,24 @@ static void do_video_out(AVFormatContext *s,
         double vdelta;
 
         if (ost->sync_ipts != AV_NOPTS_VALUE) {
-            vdelta = (double)(ost->st->pts.val) * s->pts_num / s->pts_den - ost->sync_ipts;
-            if (vdelta < -AV_DELAY_MAX)
-                nb_frames = 2;
-            else if (vdelta > AV_DELAY_MAX)
-                nb_frames = 0;
-            //printf("vdelta=%f\n", vdelta); 
+            vdelta = (double)(ost->st->pts.val) * s->pts_num / s->pts_den - (ost->sync_ipts - ost->sync_ipts_offset);
+            if (vdelta < 100 && vdelta > -100) {
+                if (vdelta < -AV_DELAY_MAX)
+                    nb_frames = 2;
+                else if (vdelta > AV_DELAY_MAX)
+                    nb_frames = 0;
+            } else {
+                ost->sync_ipts_offset -= vdelta;
+            }
+
+#if 0
+            {
+                static char *action[] = { "drop frame", "copy frame", "dup frame" };
+                printf("Input PTS %12.6f, output PTS %12.6f: %s\n",
+                    (double) ost->sync_ipts, (double) ost->st->pts.val * s->pts_num / s->pts_den,
+                    action[nb_frames]);
+            }
+#endif
         }
     }
     if (nb_frames <= 0)