Create lowLatencyMode setting

src/Settings/Video.SettingsGroup.json

@@ -117,5 +117,12 @@
     "longDescription":  "Disable Video Stream when disarmed.",
     "type":             "bool",
     "defaultValue":     false
+},
+{
+    "name":             "lowLatencyMode",
+    "shortDescription": "Tweaks video for lower latency",
+    "longDescription":  "If this option is enabled, the rtpjitterbuffer is removed and the video sink is set to assynchronous mode, reducing the latency by about 200 ms.",
+    "type":             "bool",
+    "defaultValue":     false
 }
 ]

src/Settings/VideoSettings.cc

@@ -82,6 +82,7 @@ DECLARE_SETTINGSFACT(VideoSettings, enableStorageLimit)
 DECLARE_SETTINGSFACT(VideoSettings, rtspTimeout)
 DECLARE_SETTINGSFACT(VideoSettings, streamEnabled)
 DECLARE_SETTINGSFACT(VideoSettings, disableWhenDisarmed)
+DECLARE_SETTINGSFACT(VideoSettings, lowLatencyMode)
 
 DECLARE_SETTINGSFACT_NO_FUNC(VideoSettings, videoSource)
 {

src/Settings/VideoSettings.h

@@ -34,6 +34,7 @@ public:
     DEFINE_SETTINGFACT(rtspTimeout)
     DEFINE_SETTINGFACT(streamEnabled)
     DEFINE_SETTINGFACT(disableWhenDisarmed)
+    DEFINE_SETTINGFACT(lowLatencyMode)
 
     Q_PROPERTY(bool     streamConfigured        READ streamConfigured       NOTIFY streamConfiguredChanged)
     Q_PROPERTY(QString  rtspVideoSource         READ rtspVideoSource        CONSTANT)

src/VideoStreaming/VideoManager.cc

@@ -60,6 +60,7 @@ VideoManager::setToolbox(QGCToolbox *toolbox)
    connect(_videoSettings->rtspUrl(),       &Fact::rawValueChanged, this, &VideoManager::_rtspUrlChanged);
    connect(_videoSettings->tcpUrl(),        &Fact::rawValueChanged, this, &VideoManager::_tcpUrlChanged);
    connect(_videoSettings->aspectRatio(),   &Fact::rawValueChanged, this, &VideoManager::_aspectRatioChanged);
+   connect(_videoSettings->lowLatencyMode(),&Fact::rawValueChanged, this, &VideoManager::_lowLatencyModeChanged);
    MultiVehicleManager *pVehicleMgr = qgcApp()->toolbox()->multiVehicleManager();
    connect(pVehicleMgr, &MultiVehicleManager::activeVehicleChanged, this, &VideoManager::_setActiveVehicle);
 
@@ -228,6 +229,13 @@ VideoManager::_tcpUrlChanged()
     restartVideo();
 }
 
+//-----------------------------------------------------------------------------
+void
+VideoManager::_lowLatencyModeChanged()
+{
+    restartVideo();
+}
+
 //-----------------------------------------------------------------------------
 bool
 VideoManager::hasVideo()

src/VideoStreaming/VideoManager.h

@@ -97,6 +97,7 @@ protected slots:
     void _udpPortChanged            ();
     void _rtspUrlChanged            ();
     void _tcpUrlChanged             ();
+    void _lowLatencyModeChanged     ();
     void _updateUVC                 ();
     void _setActiveVehicle          (Vehicle* vehicle);
     void _aspectRatioChanged        ();

src/VideoStreaming/VideoReceiver.cc

@@ -395,7 +395,7 @@ VideoReceiver::_makeSource(const QString& uri)
         gst_element_foreach_src_pad(source, _padProbe, &probeRes);
 
         if (probeRes & 1) {
-            if (probeRes & 2) {
+            if (probeRes & 2 && !_videoSettings->lowLatencyMode()->rawValue().toBool()) {
                 if ((buffer = gst_element_factory_make("rtpjitterbuffer", nullptr)) == nullptr) {
                     qCCritical(VideoReceiverLog) << "gst_element_factory_make('rtpjitterbuffer') failed";
                     break;
@@ -414,7 +414,11 @@ VideoReceiver::_makeSource(const QString& uri)
                 }
             }
         } else {
-            g_signal_connect(source, "pad-added", G_CALLBACK(_linkPadWithOptionalBuffer), parser);
+            if (_videoSettings->lowLatencyMode()->rawValue().toBool()) {
+                g_signal_connect(source, "pad-added", G_CALLBACK(newPadCB), parser);
+            } else {
+                g_signal_connect(source, "pad-added", G_CALLBACK(_linkPadWithOptionalBuffer), parser);
+            }
         }
 
         g_signal_connect(parser, "pad-added", G_CALLBACK(_wrapWithGhostPad), nullptr);
@@ -568,6 +572,9 @@ VideoReceiver::start()
         qCWarning(VideoReceiverLog) << "Failed because video sink is not set";
         return;
     }
+
+    g_object_set(_videoSink, "sync", !_videoSettings->lowLatencyMode()->rawValue().toBool(), NULL);
+
     if(_running) {
         qCDebug(VideoReceiverLog) << "Already running!";
         return;

src/ui/preferences/GeneralSettings.qml

@@ -1028,6 +1028,16 @@ Rectangle {
                                 fact:                   QGroundControl.settingsManager.videoSettings.disableWhenDisarmed
                                 visible:                _isGst && QGroundControl.settingsManager.videoSettings.disableWhenDisarmed.visible
                             }
+
+                            QGCLabel {
+                                text:                   qsTr("Low Latency Mode")
+                                visible:                _isGst && QGroundControl.settingsManager.videoSettings.lowLatencyMode.visible
+                            }
+                            FactCheckBox {
+                                text:                   ""
+                                fact:                   QGroundControl.settingsManager.videoSettings.lowLatencyMode
+                                visible:                _isGst && QGroundControl.settingsManager.videoSettings.lowLatencyMode.visible
+                            }
                         }
                     }