mpegtbase: Improve last PCR detection

When dealing with random-access content (such as files), we initially
search for the last PCR in order to figure out duration and to handle
other position estimation such as those used in seeking.

Previously, the code looking for that last PCR would search in the last
640kB of the file going forward, and stop at the first PCR encountered.
The problem with that was two-fold:
* It wouldn't really be the last PCR (it would be the first one within
  those last 640kB. In case of VBR files, this would put off duration
  and seek code slightly.
* It would fail on files with bitrates higher than 52Mbit/s (not common)

Instead this patch modifies that code by:
* Scanning over the last 2048kB (allows to cope with streams up to 160Mbit/s)
* Starts by the end of the file, going over chunks of 300 MPEG-TS packets
* Doesn't stop at the first PCR detected in a chunk, but instead records all
  of them, and only stop searching if there was "at least" one PCR within
  that chunk

This should improve duration reporting and seeking operations on VBR files

https://bugzilla.gnome.org/show_bug.cgi?id=708532

diff --git a/gst/mpegtsdemux/mpegtsbase.c b/gst/mpegtsdemux/mpegtsbase.c
index cb35c83..06a5980 100644 (file)
--- a/gst/mpegtsdemux/mpegtsbase.c
+++ b/gst/mpegtsdemux/mpegtsbase.c
@@ -1205,7 +1205,7 @@ mpegts_base_scan (MpegTSBase * base)
   gboolean done = FALSE;
   MpegTSPacketizerPacketReturn pret;
   gint64 tmpval;
-  gint64 upstream_size, seek_pos;
+  gint64 upstream_size, seek_pos, reverse_limit;
   GstFormat format;
   guint initial_pcr_seen;
 
@@ -1213,7 +1213,7 @@ mpegts_base_scan (MpegTSBase * base)
 
   /* Find initial sync point and at least 5 PCR values */
   for (i = 0; i < 10 && !done; i++) {
-    GST_DEBUG ("Grabbing %d => %d", i * 65536, 65536);
+    GST_DEBUG ("Grabbing %d => %d", i * 65536, (i + 1) * 65536);
 
     ret = gst_pad_pull_range (base->sinkpad, i * 65536, 65536, &buf);
     if (G_UNLIKELY (ret == GST_FLOW_EOS))
@@ -1252,7 +1252,6 @@ mpegts_base_scan (MpegTSBase * base)
   GST_DEBUG ("Seen %d initial PCR", initial_pcr_seen);
 
   /* Now send data from the end */
-  mpegts_packetizer_clear (base->packetizer);
 
   /* Get the size of upstream */
   format = GST_FORMAT_BYTES;
@@ -1261,12 +1260,19 @@ mpegts_base_scan (MpegTSBase * base)
   upstream_size = tmpval;
   done = FALSE;
 
-  /* Find last PCR value */
-  for (seek_pos = MAX (0, upstream_size - 655360);
-      seek_pos < upstream_size && !done; seek_pos += 65536) {
-    GST_DEBUG ("Grabbing %" G_GUINT64_FORMAT " => %d", seek_pos, 65536);
+  /* The scanning takes place on the last 2048kB. Considering PCR should
+   * be present at least every 100ms, this should cope with streams
+   * up to 160Mbit/s */
+  reverse_limit = MAX (0, upstream_size - 2097152);
+
+  /* Find last PCR value, searching backwards by chunks of 300 MPEG-ts packets */
+  for (seek_pos = MAX (0, upstream_size - 56400);
+      seek_pos >= reverse_limit && !done; seek_pos -= 56400) {
+    mpegts_packetizer_clear (base->packetizer);
+    GST_DEBUG ("Grabbing %" G_GUINT64_FORMAT " => %" G_GUINT64_FORMAT, seek_pos,
+        seek_pos + 56400);
 
-    ret = gst_pad_pull_range (base->sinkpad, seek_pos, 65536, &buf);
+    ret = gst_pad_pull_range (base->sinkpad, seek_pos, 56400, &buf);
     if (G_UNLIKELY (ret == GST_FLOW_EOS))
       break;
     if (G_UNLIKELY (ret != GST_FLOW_OK))
@@ -1277,17 +1283,16 @@ mpegts_base_scan (MpegTSBase * base)
     buf = NULL;
 
     if (mpegts_packetizer_has_packets (base->packetizer)) {
-      while (1) {
-        /* Eat up all packets */
+      pret = PACKET_OK;
+      /* Eat up all packets, really try to get last PCR(s) */
+      while (pret != PACKET_NEED_MORE)
         pret = mpegts_packetizer_process_next_packet (base->packetizer);
-        if (pret == PACKET_NEED_MORE)
-          break;
-        if (pret != PACKET_BAD &&
-            base->packetizer->nb_seen_offsets > initial_pcr_seen) {
-          GST_DEBUG ("Got last PCR");
-          done = TRUE;
-          break;
-        }
+
+      if (base->packetizer->nb_seen_offsets > initial_pcr_seen) {
+        GST_DEBUG ("Got last PCR(s) (total seen:%d)",
+            base->packetizer->nb_seen_offsets);
+        done = TRUE;
+        break;
       }
     }
   }