matroskamux: Add new property to offset all streams to start at zero

This takes the timestamp of the earliest stream and offsets it so that
it starts at 0. Some software (VLC, ffmpeg-based) does not properly
handle Matroska files that start at timestamps much bigger than zero.

Closes https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/issues/449

gst/matroska/matroska-mux.c

@@ -72,7 +72,8 @@ enum
   PROP_STREAMABLE,
   PROP_TIMECODESCALE,
   PROP_MIN_CLUSTER_DURATION,
-  PROP_MAX_CLUSTER_DURATION
+  PROP_MAX_CLUSTER_DURATION,
+  PROP_OFFSET_TO_ZERO,
 };
 
 #define  DEFAULT_DOCTYPE_VERSION         2
@@ -82,6 +83,7 @@ enum
 #define  DEFAULT_TIMECODESCALE           GST_MSECOND
 #define  DEFAULT_MIN_CLUSTER_DURATION    500 * GST_MSECOND
 #define  DEFAULT_MAX_CLUSTER_DURATION    65535 * GST_MSECOND
+#define  DEFAULT_OFFSET_TO_ZERO          FALSE
 
 /* WAVEFORMATEX is gst_riff_strf_auds + an extra guint16 extension size */
 #define WAVEFORMATEX_SIZE  (2 + sizeof (gst_riff_strf_auds))
@@ -363,6 +365,10 @@ gst_matroska_mux_class_init (GstMatroskaMuxClass * klass)
           "0 means no maximum duration.", 0,
           G_MAXINT64, DEFAULT_MAX_CLUSTER_DURATION,
           G_PARAM_READWRITE | G_PARAM_STATIC_STRINGS));
+  g_object_class_install_property (gobject_class, PROP_OFFSET_TO_ZERO,
+      g_param_spec_boolean ("offset-to-zero", "Offset To Zero",
+          "Offsets all streams so that the " "earliest stream starts at 0.",
+          DEFAULT_OFFSET_TO_ZERO, G_PARAM_READWRITE | G_PARAM_STATIC_STRINGS));
 
   gstelement_class->change_state =
       GST_DEBUG_FUNCPTR (gst_matroska_mux_change_state);
@@ -3067,6 +3073,7 @@ gst_matroska_mux_start (GstMatroskaMux * mux, GstMatroskaPad * first_pad,
   GSList *collected;
   int i;
   guint tracknum = 1;
+  GstClockTime earliest_time = GST_CLOCK_TIME_NONE;
   GstClockTime duration = 0;
   guint32 segment_uid[4];
   GTimeVal time = { 0, 0 };
@@ -3226,6 +3233,29 @@ gst_matroska_mux_start (GstMatroskaMux * mux, GstMatroskaPad * first_pad,
     if (collect_pad->track->codec_id == NULL)
       continue;
 
+    /* Find the smallest timestamp so we can offset all streams by this to
+     * start at 0 */
+    if (mux->offset_to_zero) {
+      GstClockTime ts;
+
+      if (collect_pad == first_pad)
+        buf = first_pad_buf ? gst_buffer_ref (first_pad_buf) : NULL;
+      else
+        buf = gst_collect_pads_peek (mux->collect, collected->data);
+
+      if (buf) {
+        ts = gst_matroska_track_get_buffer_timestamp (collect_pad->track, buf);
+
+        if (earliest_time == GST_CLOCK_TIME_NONE)
+          earliest_time = ts;
+        else if (ts != GST_CLOCK_TIME_NONE && ts < earliest_time)
+          earliest_time = ts;
+      }
+
+      if (buf)
+        gst_buffer_unref (buf);
+    }
+
     /* For audio tracks, use the first buffers duration as the default
      * duration if we didn't get any better idea from the caps event already
      */
@@ -3254,6 +3284,8 @@ gst_matroska_mux_start (GstMatroskaMux * mux, GstMatroskaPad * first_pad,
   }
   gst_ebml_write_master_finish (ebml, master);
 
+  mux->earliest_time = earliest_time == GST_CLOCK_TIME_NONE ? 0 : earliest_time;
+
   /* chapters */
   toc = gst_toc_setter_get_toc (GST_TOC_SETTER (mux));
   if (toc != NULL && !mux->ebml_write->streamable) {
@@ -3910,6 +3942,11 @@ gst_matroska_mux_write_data (GstMatroskaMux * mux, GstMatroskaPad * collect_pad,
 
   buffer_timestamp =
       gst_matroska_track_get_buffer_timestamp (collect_pad->track, buf);
+  if (buffer_timestamp >= mux->earliest_time) {
+    buffer_timestamp -= mux->earliest_time;
+  } else {
+    buffer_timestamp = 0;
+  }
 
   /* hm, invalid timestamp (due to --to be fixed--- element upstream);
    * this would wreak havoc with time stored in matroska file */
@@ -4188,6 +4225,14 @@ gst_matroska_mux_handle_buffer (GstCollectPads * pads, GstCollectData * data,
   g_assert (buf);
 
   buffer_timestamp = gst_matroska_track_get_buffer_timestamp (best->track, buf);
+  if (buffer_timestamp >= mux->earliest_time) {
+    buffer_timestamp -= mux->earliest_time;
+  } else {
+    GST_ERROR_OBJECT (mux,
+        "PTS before first PTS (%" GST_TIME_FORMAT " < %" GST_TIME_FORMAT ")",
+        GST_TIME_ARGS (buffer_timestamp), GST_TIME_ARGS (mux->earliest_time));
+    buffer_timestamp = 0;
+  }
 
   GST_DEBUG_OBJECT (best->collect.pad, "best pad - buffer ts %"
       GST_TIME_FORMAT " dur %" GST_TIME_FORMAT,
@@ -4304,6 +4349,9 @@ gst_matroska_mux_set_property (GObject * object,
     case PROP_MAX_CLUSTER_DURATION:
       mux->max_cluster_duration = g_value_get_int64 (value);
       break;
+    case PROP_OFFSET_TO_ZERO:
+      mux->offset_to_zero = g_value_get_boolean (value);
+      break;
     default:
       G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
       break;
@@ -4341,6 +4389,9 @@ gst_matroska_mux_get_property (GObject * object,
     case PROP_MAX_CLUSTER_DURATION:
       g_value_set_int64 (value, mux->max_cluster_duration);
       break;
+    case PROP_OFFSET_TO_ZERO:
+      g_value_set_boolean (value, mux->offset_to_zero);
+      break;
     default:
       G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
       break;

gst/matroska/matroska-mux.h

@@ -111,6 +111,9 @@ struct _GstMatroskaMux {
   guint64        max_cluster_duration;
   guint64        min_cluster_duration;
 
+  /* earliest timestamp (time, ns) if offsetting to zero */
+  gboolean       offset_to_zero;
+  guint64        earliest_time;
   /* length, position (time, ns) */
   guint64        duration;