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If you know the focal length of the camera and the depth (distance to the object from the camera), you can use pin-hole camera model to convert pixel values to real-world coordinates.

Focal length of the camera can be found be exploring the camera_info topic that will be published (The value will be px and you have to convert that to mm).

Assuming that you the pixel coordinates of the region of interest, you can get the depth from the depth image published by kinect.