Multiple Kinects in Rviz, image fusion?

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The camera_pose_calibration package essentially provides exactly what you´re looking for. Haven´t used it myself, so not sure how mature and easy to adapt it is. The calibration package also is able to estimate transforms, but is originally meant for robot kinematic calibration. Based on it, a approach using the Google Ceres solver is also available. There are of course also other options, like registration using some PCL tools (if you have significant point cloud overlap), using AR markers visible in multiple views and others, which likely require some more work on your part.