tf: Adding a frame to transforms from the past

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Hi everyone,

I have a question regarding the usage of tf and hope someone can help me answer it.

The problem setup is the following: Imaging I have a robot with defined base_frame and I send the robots pose transform (in world frame) to tf with a constant rate. Now the robot has a camera rigidly mounted on it. So I define a camera frame with the robots base frame as parent frame and I also send this frame to tf. This way, I can know the cameras pose in the world frame if the robot moves. Assume at time t_0, the robot is at location s_0 and I get an image from the camera, then I do some processing to calculate the pose of a marker in the camera frame. Assume I finish processing the image at time t_K, where the robot moved to s_k. And now I define a marker frame with its parent frame being the camera frame. For this, I first define for the marker its pose transform in the camera frame, and then I send the transform to tf using the API call:

tf::TransformBroadcaster br;
br.sentTransform(tf::StampedTransform(markerTransform, t_0, "camera_frame_tf",  "marker_frame_tf"));

I stamp this marker frame tranform with time t_0 instead of the current time t_k, because the image is taken at time t_0 and the calculated distance from marker to camera should be superposed with the robots/camera position at t_0. So using the above br.sendTransform callback, will the marker frame be superposed correctly to tf from time t_0?

Also at current time t_k, how do I get the markers pose w.r.t. the world frame back at time t_0?

Any clue will be appreciated!

Many thanks in advance.