Tree frames tf

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Hi,

This is my scenario:

In a node I publish the webcam image and retrieve the coordinates x,y when the user clicks on the screen. After I publish the coordinates.

void my_mouse_callback( int event, int x, int y, int flags, void* param ){

switch( event ){
    case CV_EVENT_MOUSEMOVE: 
        break;
    case CV_EVENT_LBUTTONDOWN:
        break;
    case CV_EVENT_LBUTTONUP:
        //publicamos x e y
        geometry_msgs::PointStamped pixel;
        pixel.header.frame_id = "webcam";
        pixel.header.stamp = ros::Time();       
        pixel.point.x = x;
        pixel.point.y = y;
        pixel.point.z = 0.0;        
        pub.publish(pixel); 
        break;
}

}

In the other node I subscribe to the coordinates and I calculate the ray of pixel with projectPixelTo3dRay().

void point_callback(const geometry_msgs::PointStampedConstPtr& p){
     tf::TransformListener listener;
     cv::Point2d pixel(p->point.x,p->point.y);
     ROS_INFO("Pixel (%.f,%.f)",pixel.x,pixel.y);
     cv::Point3d ray=model.projectPixelTo3dRay(pixel);
     ROS_INFO("ray.x: %.5f ray.y: %.5f ray.z: %.5f\n",ray.x,ray.y,ray.z);

}

As the camera is on the table, I want to calculate 3d point in the world with Z=0. I want to use tf, but do not know how to create the tree to make the transformations.

What tree should I create?

World-->base_camera-->optic_centre

World-->camera

...

Thanks