message_filters code explained

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Hi,

I am using this code in my node but I would like to understand it more (line by line): [of course I read : http://wiki.ros.org/message_filters ]

message_filters::Subscriber<sensor_msgs::Image> raw_image_subscriber(nh_, image_topic, queue_size_);
message_filters::Subscriber<sensor_msgs::CameraInfo> camera_info_subscriber(nh_, camera_info_topic, queue_size_);
message_filters::TimeSynchronizer<sensor_msgs::Image, sensor_msgs::CameraInfo> image_info_sync(raw_image_subscriber, camera_info_subscriber, queue_size_);
image_info_sync.registerCallback(boost::bind(&Node::frameCallback,this, _1, _2));

As far as I understood:

message_filters::Subscriber<sensor_msgs::Image> raw_image_subscriber(nh_, image_topic, queue_size_);

Creates an object of type message_filters::Subscriber<sensor_msgs::Image> which is connected to the image_topic specified somewhere else in my code.

message_filters::Subscriber<sensor_msgs::CameraInfo> camera_info_subscriber(nh_, camera_info_topic, queue_size_);

Creates an object of type message_filters::Subscriber<sensor_msgs::CameraInfo> which is connected to the canera_info_topic specified somewhere else in my code.

message_filters::TimeSynchronizer<sensor_msgs::Image, sensor_msgs::CameraInfo> image_info_sync(raw_image_subscriber, camera_info_subscriber, queue_size_);

Creates an object of type message_filters::TymeSynchronizer<sensor_msgs::Image, sensor_msgs::CameraInfo> which is connected to the 2 subscribers created before. If I understood it correctly it synchronizes incoming channels by the timestamps contained in their headers, and outputs them in the form of a single callback that takes the same number of channels.

Is it all correct?

image_info_sync.registerCallback(boost::bind(&Node::frameCallback,this, _1, _2));

This is the most difficult expression for me to understand. Why do I need the boost::bind function? And how this function works?

Thanks in advance