videorate: Operate in a zero-latency mode if drop-only is set to TRUE

There's no reason why we would have to wait for the next buffer to decide
whether to output the current one or not. We just have to check if the
current one is earlier than our expected next time, which is the previous
frame timestamp plus the expected frame duration.

https://bugzilla.gnome.org/show_bug.cgi?id=740018

gst/videorate/gstvideorate.c

@@ -805,10 +805,12 @@ gst_video_rate_query (GstBaseTransform * trans, GstPadDirection direction,
       gboolean live;
       guint64 latency;
       guint64 avg_period;
+      gboolean drop_only;
       GstPad *peer;
 
       GST_OBJECT_LOCK (videorate);
       avg_period = videorate->average_period_set;
+      drop_only = videorate->drop_only;
       GST_OBJECT_UNLOCK (videorate);
 
       if (avg_period == 0 && (peer = gst_pad_get_peer (otherpad))) {
@@ -819,7 +821,8 @@ gst_video_rate_query (GstBaseTransform * trans, GstPadDirection direction,
               GST_TIME_FORMAT " max %" GST_TIME_FORMAT,
               GST_TIME_ARGS (min), GST_TIME_ARGS (max));
 
-          if (videorate->from_rate_numerator != 0) {
+          /* Drop only has no latency, other modes have one frame latency */
+          if (!drop_only && videorate->from_rate_numerator != 0) {
             /* add latency. We don't really know since we hold on to the frames
              * until we get a next frame, which can be anything. We assume
              * however that this will take from_rate time. */
@@ -983,7 +986,7 @@ gst_video_rate_transform_ip (GstBaseTransform * trans, GstBuffer * buffer)
   intime = in_ts + videorate->segment.base;
 
   /* we need to have two buffers to compare */
-  if (videorate->prevbuf == NULL) {
+  if (videorate->prevbuf == NULL || videorate->drop_only) {
     gst_video_rate_swap_prev (videorate, buffer, intime);
     videorate->in++;
     if (!GST_CLOCK_TIME_IS_VALID (videorate->next_ts)) {
@@ -997,6 +1000,24 @@ gst_video_rate_transform_ip (GstBaseTransform * trans, GstBuffer * buffer)
         videorate->next_ts = videorate->segment.start + videorate->segment.base;
       }
     }
+
+    /* In drop-only mode we can already decide here if we should output the
+     * current frame or drop it because it's coming earlier than our minimum
+     * allowed frame period. This also keeps latency down to 0 frames
+     */
+    if (videorate->drop_only) {
+      if (intime >= videorate->next_ts) {
+        GstFlowReturn r;
+
+        /* on error the _flush function posted a warning already */
+        if ((r = gst_video_rate_flush_prev (videorate, FALSE)) != GST_FLOW_OK) {
+          res = r;
+          goto done;
+        }
+      }
+      /* No need to keep the buffer around for longer */
+      gst_buffer_replace (&videorate->prevbuf, NULL);
+    }
   } else {
     GstClockTime prevtime;
     gint count = 0;
@@ -1130,6 +1151,7 @@ gst_video_rate_set_property (GObject * object,
     guint prop_id, const GValue * value, GParamSpec * pspec)
 {
   GstVideoRate *videorate = GST_VIDEO_RATE (object);
+  gboolean latency_changed = FALSE;
 
   GST_OBJECT_LOCK (videorate);
   switch (prop_id) {
@@ -1142,10 +1164,15 @@ gst_video_rate_set_property (GObject * object,
     case PROP_SKIP_TO_FIRST:
       videorate->skip_to_first = g_value_get_boolean (value);
       break;
-    case PROP_DROP_ONLY:
+    case PROP_DROP_ONLY:{
+      gboolean new_value = g_value_get_boolean (value);
+
+      /* Latency changes if we switch drop-only mode */
+      latency_changed = new_value != videorate->drop_only;
       videorate->drop_only = g_value_get_boolean (value);
       goto reconfigure;
       break;
+    }
     case PROP_AVERAGE_PERIOD:
       videorate->average_period_set = g_value_get_uint64 (value);
       break;
@@ -1163,6 +1190,11 @@ gst_video_rate_set_property (GObject * object,
 reconfigure:
   GST_OBJECT_UNLOCK (videorate);
   gst_base_transform_reconfigure_src (GST_BASE_TRANSFORM (videorate));
+
+  if (latency_changed) {
+    gst_element_post_message (GST_ELEMENT (videorate),
+        gst_message_new_latency (GST_OBJECT (videorate)));
+  }
 }
 
 static void