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My answer with complete code (c++) : https://answers.ros.org/question/141741/calculate-depth-from-point-cloud-xyz/?answer=381085#post-id-381085

Code funtionality:

Subscribed to /camera/depth/image_raw rostopic,
Convert ROS image to OpenCV image
Get image dimension
Get global max and min value,
Get depth value at specific point,

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