Removed throttling of image topics.

It was a good solution to a problem caused by a bad solution. This makes it by implication a bad solution. I've decided that it makes more sense to address the original issue and this shouldn't be part of the library.
If you need throttled image topics create a throttled publisher or better don't use ROS for images if network is an issue.

docs/pages/ImageTransport.rst

@@ -30,22 +30,12 @@ The :cpp:class:`ImageTransportSubscription <qml_ros2_plugin::ImageTransportSubsc
 the source of a ``VideoOutput`` to display the camera images as they are received.
 Additionally, it can be configured to show a blank image after x milliseconds using the ``timeout`` property
 which is set to 3000ms (3s) by default. This can be disabled by setting the ``timeout`` to 0.
-If you do not want the full camera rate, you can throttle the rate by setting ``throttleRate`` to a value
-greater than 0 (which is the default and disables throttling). E.g. a rate of 0.2 would show a new frame
-every 5 seconds.
-Since there is no ROS functionality for a throttled subscription, this means the ``image_transport::Subscriber``
-is shut down and newly subscribed for each frame. This comes at some overhead, hence, it should only be used
-to throttle to low rates <1.
-To avoid all throttled subscribers subscribing at the same time causing huge network spikes, the throttled rates
-are load balanced by default. This can be disabled globally using
-:cpp:func:`ImageTransportManager::setLoadBalancingEnabled <qml_ros2_plugin::ImageTransportManagerSingletonWrapper::setLoadBalancingEnabled>`
-which is available in QML using the singleton ``ImageTransportManager``.
+
+The QML ROS2 plugin internally keeps track of all image transport subscriptions and will only create one
+shared subscription per topic to avoid unnecessary computation and bandwidth overhead.
 
 API
 ---
 
 .. doxygenclass:: qml_ros2_plugin::ImageTransportSubscription
    :members:
-
-.. doxygenclass:: qml_ros2_plugin::ImageTransportManagerSingletonWrapper
-   :members:

examples/image_subscription.qml

@@ -13,11 +13,12 @@ ApplicationWindow {
   // This connection makes sure the application exits if this ROS node is requested to shutdown
   Connections {
     target: Ros2
+
     function onShutdown() {
       Qt.quit()
     }
   }
-  
+
   ImageTransportSubscription {
     id: imageSubscription
     // Enter a valid image topic here
@@ -26,12 +27,23 @@ ApplicationWindow {
     // Requires the compressed_image_transport package to be installed
     defaultTransport: "compressed"
   }
-
-  VideoOutput {
+  GridLayout {
     anchors.fill: parent
-    // Can be used in increments of 90 to rotate the video
-    orientation: 0
-    source: imageSubscription
+    anchors.margins: 12
+    columns: 1
+    Text {
+      text: "For this example,\n run the following in a separate terminal:"
+    }
+    TextInput {
+      readOnly: true
+      selectByMouse: true
+      text: "ros2 run examples_rclcpp_minimal_service service_main"
+    }
+    VideoOutput {
+      // Can be used in increments of 90 to rotate the video
+      orientation: 0
+      source: imageSubscription
+    }
   }
 
   Component.onCompleted: {

include/qml_ros2_plugin/image_transport_manager.hpp

@@ -12,14 +12,6 @@ namespace qml_ros2_plugin
 {
 class ImageTransportSubscriptionHandle;
 
-class ImageTransportManagerSingletonWrapper : public QObject
-{
-  Q_OBJECT
-public:
-  //! @copydoc ImageTransportManager::setLoadBalancingEnabled
-  Q_INVOKABLE void setLoadBalancingEnabled( bool value );
-};
-
 /*!
  * Encapsulates the image transport communication to share the subscription resources, avoiding multiple conversions of
  *  the same image and subscription overhead if multiple cameras are set to throttle.
@@ -32,15 +24,9 @@ class ImageTransportManager
 
   class Subscription;
 
-  class LoadBalancer;
-
 public:
   static ImageTransportManager &getInstance();
 
-  //! Sets whether the manager should try to balance throttled subscriptions_ to ensure they don't update at the same
-  //!   time which would result in network spikes.
-  void setLoadBalancingEnabled( bool value );
-
   /*!
    * Note: Can only be called with a ready NodeHandle!
    *
@@ -60,11 +46,10 @@ class ImageTransportManager
   subscribe( const rclcpp::Node::SharedPtr &node, const QString &qtopic, quint32 queue_size,
              const image_transport::TransportHints &transport_hints,
              const std::function<void( const QVideoFrame & )> &callback,
-             QAbstractVideoSurface *surface = nullptr, int throttle_interval = 0 );
+             QAbstractVideoSurface *surface = nullptr );
 
 private:
-  std::map<std::string, std::shared_ptr<SubscriptionManager>> subscriptions_;
-  std::unique_ptr<LoadBalancer> load_balancer_;
+  std::shared_ptr<SubscriptionManager> subscription_manager_;
 
   friend class ImageTransportSubscriptionHandle;
 };
@@ -74,13 +59,6 @@ class ImageTransportSubscriptionHandle
 public:
   ~ImageTransportSubscriptionHandle();
 
-  //! The interval in ms the subscription waits between receiving images.
-  int throttleInterval() const { return throttle_interval; }
-
-  //! Set the interval in ms the subscription may wait between images.
-  //! The images may still arrive at a higher rate if other subscriptions request it.
-  void updateThrottleInterval( int interval );
-
   //! The subscribed topic. Once subscribed this is the full topic name without the transport.
   std::string getTopic() const;
 
@@ -101,7 +79,6 @@ class ImageTransportSubscriptionHandle
   QAbstractVideoSurface *surface = nullptr;
   std::function<void( const QVideoFrame & )> callback;
   double framerate_ = 0;
-  int throttle_interval = 0;
   int network_latency = -1;
   int processing_latency = -1;
 

include/qml_ros2_plugin/image_transport_subscription.hpp

@@ -32,21 +32,19 @@ class ImageTransportSubscription : public QObjectRos2
                   defaultTransportChanged )
   //! Whether or not this ImageTransportSubscriber is subscribed to the given topic (readonly)
   Q_PROPERTY( bool subscribed READ subscribed NOTIFY subscribedChanged )
-  //! The latency from the sender to the received time in ms not including the conversion latency before displaying.
-  //! This latency is based on the ROS time of the sending and receiving machines, hence, they need to be synchronized. (readonly)
+  //! The latency from the sender to the received time in ms not including the conversion latency before displaying. (readonly)
+  //! This is computed based on the timestamp and relies on your system clock being synchronized with the camera host.
   Q_PROPERTY( int networkLatency READ networkLatency NOTIFY networkLatencyChanged )
   //! The latency (in ms) from the reception of the image until it is in a displayable format. (readonly)
   Q_PROPERTY( int processingLatency READ processingLatency NOTIFY processingLatencyChanged )
   //! The full latency (in ms) from the camera to your display excluding drawing time.  (readonly)
+  //! This is computed based on the timestamp and relies on your system clock being synchronized with the camera host.
   Q_PROPERTY( int latency READ latency NOTIFY latencyChanged )
   //! The framerate of the received camera frames in frames per second.  (readonly)
   Q_PROPERTY( double framerate READ framerate NOTIFY framerateChanged )
   //! The timeout when no image is received until a blank frame is served. Set to 0 to disable and
   //! always show last frame. Default is 3000 ms.
   Q_PROPERTY( int timeout READ timeout WRITE setTimeout NOTIFY timeoutChanged )
-  //! The update rate to throttle image receiving in images per second. Set to 0 to disable
-  //! throttling. Default is 0 (disabled).
-  Q_PROPERTY( double throttleRate READ throttleRate WRITE setThrottleRate NOTIFY throttleRateChanged )
   //! Whether the subscriber is active or not. Setting to false will shut down subscribers
   Q_PROPERTY( bool enabled READ enabled WRITE setEnabled NOTIFY enabledChanged )
 public:
@@ -72,10 +70,6 @@ class ImageTransportSubscription : public QObjectRos2
 
   void setTimeout( int value );
 
-  double throttleRate() const;
-
-  void setThrottleRate( double value );
-
   bool enabled() const;
 
   void setEnabled( bool value );