How to use RVIZ fixed frame to calculate object relative to the position of the robot

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rviz is a visualizer. It draws pretty pictures. For the most part, it doesn't do any processing of your data.

The proper tool to use here is TF. If you have the position of a object in one frame, and your system knows the transformation between that frame and your robot's local frame, you can use TF to find the position of the object in your robot's local frame.

If you're unfamiliar with TF, the TF Tutorials are a good place to start.