qtmux: Handle DTS with negative running time

As QT works with duration, simply bring back first DTS to 0 and shift
forward the PTS of the same amount.

https://bugzilla.gnome.org/show_bug.cgi?id=740575

gst/isomp4/gstqtmux.c

@@ -493,6 +493,7 @@ gst_qt_mux_pad_reset (GstQTPad * qtpad)
   qtpad->sample_size = 0;
   qtpad->sync = FALSE;
   qtpad->last_dts = 0;
+  qtpad->dts_adjustment = GST_CLOCK_TIME_NONE;
   qtpad->first_ts = GST_CLOCK_TIME_NONE;
   qtpad->prepare_buf_func = NULL;
   qtpad->create_empty_buffer = NULL;
@@ -2996,9 +2997,6 @@ gst_qt_mux_add_buffer (GstQTMux * qtmux, GstQTPad * pad, GstBuffer * buf)
         GST_BUFFER_DTS (buf)) {
       GST_BUFFER_DTS (buf) = GST_BUFFER_DTS (last_buf) + last_buf_duration;
     }
-
-    if (GST_BUFFER_PTS_IS_VALID (buf))
-      GST_BUFFER_DTS (buf) = MIN (GST_BUFFER_DTS (buf), GST_BUFFER_PTS (buf));
   }
 
   if (last_buf && !buf && !GST_BUFFER_DURATION_IS_VALID (last_buf)) {
@@ -3248,6 +3246,57 @@ not_negotiated:
   }
 }
 
+/*
+ * DTS running time can be negative. There is no way to represent that in
+ * MP4 however, thus we need to offset DTS so that it starts from 0.
+ */
+static void
+gst_qt_pad_adjust_buffer_dts (GstQTMux * qtmux, GstQTPad * pad,
+    GstCollectData * cdata, GstBuffer ** buf)
+{
+  GstClockTime pts;
+  gint64 dts;
+
+  pts = GST_BUFFER_PTS (*buf);
+  dts = GST_COLLECT_PADS_DTS (cdata);
+
+  GST_LOG_OBJECT (qtmux, "selected pad %s with PTS %" GST_TIME_FORMAT
+      " and DTS %" GST_STIME_FORMAT, GST_PAD_NAME (cdata->pad),
+      GST_TIME_ARGS (pts), GST_STIME_ARGS (dts));
+
+  if (!GST_CLOCK_TIME_IS_VALID (pad->dts_adjustment)) {
+    if (GST_CLOCK_STIME_IS_VALID (dts) && dts < 0)
+      pad->dts_adjustment = -dts;
+    else
+      pad->dts_adjustment = 0;
+  }
+
+  if (pad->dts_adjustment > 0) {
+    *buf = gst_buffer_make_writable (*buf);
+
+    dts += pad->dts_adjustment;
+
+    if (GST_CLOCK_TIME_IS_VALID (pts))
+      pts += pad->dts_adjustment;
+
+    if (GST_CLOCK_STIME_IS_VALID (dts) && dts < 0) {
+      GST_WARNING_OBJECT (pad, "Decreasing DTS.");
+      dts = 0;
+    }
+
+    if (pts < dts) {
+      GST_WARNING_OBJECT (pad, "DTS is bigger then PTS");
+      pts = dts;
+    }
+
+    GST_BUFFER_PTS (*buf) = pts;
+    GST_BUFFER_DTS (*buf) = dts;
+
+    GST_LOG_OBJECT (qtmux, "time adjusted to PTS %" GST_TIME_FORMAT
+        " and DTS %" GST_TIME_FORMAT, GST_TIME_ARGS (pts), GST_TIME_ARGS (dts));
+  }
+}
+
 static GstFlowReturn
 gst_qt_mux_handle_buffer (GstCollectPads * pads, GstCollectData * cdata,
     GstBuffer * buf, gpointer user_data)
@@ -3255,7 +3304,6 @@ gst_qt_mux_handle_buffer (GstCollectPads * pads, GstCollectData * cdata,
   GstFlowReturn ret = GST_FLOW_OK;
   GstQTMux *qtmux = GST_QT_MUX_CAST (user_data);
   GstQTPad *best_pad = NULL;
-  GstClockTime best_time = GST_CLOCK_TIME_NONE;
 
   if (G_UNLIKELY (qtmux->state == GST_QT_MUX_STATE_STARTED)) {
     if ((ret = gst_qt_mux_start_file (qtmux)) != GST_FLOW_OK)
@@ -3272,9 +3320,7 @@ gst_qt_mux_handle_buffer (GstCollectPads * pads, GstCollectData * cdata,
   /* clipping already converted to running time */
   if (best_pad != NULL) {
     g_assert (buf);
-    best_time = GST_BUFFER_PTS (buf);
+    gst_qt_pad_adjust_buffer_dts (qtmux, best_pad, cdata, &buf);
-    GST_LOG_OBJECT (qtmux, "selected pad %s with time %" GST_TIME_FORMAT,
-        GST_PAD_NAME (best_pad->collect.pad), GST_TIME_ARGS (best_time));
     ret = gst_qt_mux_add_buffer (qtmux, best_pad, buf);
   } else {
     qtmux->state = GST_QT_MUX_STATE_EOS;

gst/isomp4/gstqtmux.h

@@ -109,6 +109,9 @@ struct _GstQTPad
   /* dts of last_buf */
   GstClockTime last_dts;
 
+  /* This is compensate for CTTS */
+  GstClockTime dts_adjustment;
+
   /* store the first timestamp for comparing with other streams and
    * know if there are late streams */
   GstClockTime first_ts;