Convert raw depth data from depth image to meters (Kinect v2)

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Hello,

I am using Kinect v2 and I am trying to convert its raw values from the depth image to meters. I am subscribing to the image_depth_rect topic. Firstly, I would like to ask if I need to calibrate the depth image with the color image I get from the sensor ?

I have searched for similar issues, but all I found is info about the sensor from the previous version. I also found some stuff outside from ROS which didn't help.

Is there an equation to convert the unsigned 16bit data that I get in meters?

Currently I am using the following callback function for the depth image topic:

void kinectdepthCallback(const sensor_msgs::ImageConstPtr& msg2){

    cv_bridge::CvImagePtr cv_ptr2;
    try{
        cv_ptr2 = cv_bridge::toCvCopy(msg2, sensor_msgs::image_encodings::TYPE_16UC1); 
    }
    catch (cv_bridge::Exception& e){
        ROS_ERROR("Could not convert from '%s' to '16UC1'.", e.what());
    return;
    }
    cv::imshow("Depth Image",cv_ptr2->image);
    cv::waitKey(20);
    ROS_INFO("The object is in depth: %d",cv_ptr2->image.at<int>(x,y));


}

where (x,y) are some points of the detected object but I get wrong results.

To be more specific, I detect an orange marker but I cannot get its depth from the Kinect v2. I present the following images:

Color image with detected object:

Thresholded image:

Depth image is black:

Terminal messages from the above code:

I don't know what is wrong and I get zero depth even if the object is at approximately a 19 - 20 cm. I have tried to find the depth in bigger depths like 60 cm, but I was still getting 0 results.

Thanks for answering in advance,

Chris