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There exists a static transformation between the camera and the world frame. I would like to know the position to be reported in world frame, yes.

Dear community members,

I have a legged robot that walks around a floor and is being tracked by an overhead camera. This tracking is done with the ar_tag_alvar package which supplies me with a pose of the tag in camera frame. Please note that in a pose message no child frame is broadcasted. I would like to fuse this information with IMU data and odometry and to do this I use the robot_localization package. My understanding is that this package accepts all poses with covariance but assumes that the pose observed is the pose of the base_link in a certain frame. However, the tag I am using is statically attached on top of the robot so that a transform exists between the base link and a link that I will call "tag link". The full set up is drawn in the following figure.

In which: C = Camera frame, T = Tag frame, B = Base frame. It is clear that knowing the pose of the tag in camera frame implies knowing the pose of the base in camera frame. Mathematically this is not a hard problem to solve when using, for example, homogeneous transformation matrices. This, however, looks like a problem that is so common in ROS that I am assuming tf is able to help me solve it instead of me coding out equations for a homogeneous transformation matrix.

How would I proceed solving this problem with tf? Or am I not seeing a functionality in robot_localization that will handle this, just like it does with the IMU?