How to find the position of a keypoint (in RGBD image) in the Pointcloud [closed]

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Hi all, suppose there is a keypoint in the RGB + depth(depth_registered/image_rect) image and the problem is to find the corresponding point in the generated pointcloud2 (placing a marker in that location) + getting the distance (in meters) of that point from the base_link of the robot. images are collected from the Kinect on the head of the robot. I'm looking for a c++ sample code for a such problem. any suggestion? thanks.