How can i get object distance using camera/depth/image_raw

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Hello all, I am using the ros topic /camera/depth/image_raw. It publish sensor_msgs::image data with depth information. I can get the image but I do not exactly how can i get depth information. I need to what is the minimum distance of any pixel infront of the camera. I actually do not want to use the topic /camera/depth/points to measure the distance, as it is said /camera/depth/image_raw provides depth infromation. Can anyone please share some suggestion? I am using ros indigo, kinect, freenect.launch, opencv. Can anyone suggest me which library function is appropriate for me here

void callback(const sensor_msgs::Image::ConstPtr& msg){

 if(check_moving_obs)
 {
     if(img_previous.data.size() == 0)
     {
         img_previous = *msg;
         img_primary = *msg;
     }
     else
         {img_previous = img_current;}
    img_current = *msg;

     cout<<"------------------------header of msg :---------:"<< msg->header.seq <<"\n";
    cout<<"------------------------current image info :---------:"<<img_current.data.size()<<"\n";
    cout<<"------------------------Height :---------:"<<img_current.height<<"\n";
    cout<<"------------------------Width :---------:" <<img_current.width<<"\n";

    cout<<"------------------------previous image info :---------:"<<img_previous.data.size()<<"\n";
    cout<<"------------------------Height :---------:"<<img_previous.height<<"\n";
    cout<<"------------------------Width :---------:" <<img_previous.width<<"\n";


        //Convert from the ROS image message to a CvImage suitable for working with OpenCV for processing
            cv_bridge::CvImagePtr cv_ptr_prev, cv_ptr_cur;
            cv::Mat image_cur, image_pre, differenceImage;
            //thresholded difference image (for use in findContours() function)
            cv::Mat thresholdImage;

            try
            {

                //Always copy, returning a mutable CvImage
                //OpenCV expects color images to use BGR channel order.
                cv_ptr_cur = cv_bridge::toCvCopy(msg, enc::TYPE_32FC1);//cv_bridge::toCvCopy(img_current, enc::BGR8);
                cv_ptr_prev = cv_bridge::toCvCopy(img_previous, enc::TYPE_32FC1);//TYPE_16UC1


                image_cur = cv_ptr_cur->image;
                image_pre = cv_ptr_prev->image;
                int count_diff=0;
                bool motion_detected = false;

                //perform frame differencing with the sequential images.
                cv::absdiff(image_cur,image_pre,differenceImage);
                //threshold intensity image at a given sensitivity value
                cv::threshold(differenceImage,thresholdImage, 20,255,cv::THRESH_BINARY);
                cout<<"~~~~~~~~~~~~IMP~~~~~~~~~~difference image info :---------:"<<thresholdImage.rows<<"\n";
                cout<<"~~~~~~~~~~~~~IMP~~~~~~~~~difference image info :---------:"<<thresholdImage.cols<<"\n";
                for(int j=0; j< thresholdImage.rows; ++j)
                {
                    //const double* Mi = differenceImage.ptr<double>(j);
                    for(int i=0; i< thresholdImage.rows; ++i)
                    {
                        //cout<<"diff img pixel val : "<<Mi[i]<<"\n";
                        //double depth = thresholdImage.at<double>(cv::Point(j,i));
                        //cout<<"diff img depth val : "<<depth<<"\n";
                        cout<<"+++++++++++++++++++++++++++++++++++++++++++++\n";
                        cout<<"+++++ i: "<<i <<"------ j : "<< j<<"++++++++++++++++++++++++++++++++++++++++\n";
                        cout<<"+++++++++++++++++++++++++++++++++++++++++++++\n";
                        cout<<"current  image    : "<< image_cur.at<cv::Vec3b>(j,i)       << "cur img depth val : "<<image_cur.at<double>(cv::Point(j,i))<<"\n";
                        cout<<"previous  image   : "<< image_pre.at<cv::Vec3b>(j,i)       << "pre img depth val : "<<image_pre.at<double>(cv::Point(j,i))<<"\n";
                        cout<<"abs-diff  image   : "<< differenceImage.at<cv::Vec3b>(j,i) << "dif img depth val : "<<differenceImage.at<double>(cv::Point(j,i))<<"\n";
                        cout<<"threshholöd image : "<< thresholdImage.at<cv::Vec3b>(j,i ) << "thr img depth val : "<<thresholdImage.at<double>(cv::Point(j,i))<<"\n";
                        cout<<"+++++++++++++++++++++++++++++++++++++++++++++\n";

                    }

                }
                /*
                for(int j=0; j<image_cur.rows; ++j)
                {
                    if(count_diff > 150)
                    {
                        break;
                    }
                        for(int i=0; i<image_cur.cols; ++i)
                        {
                            int depth = image_cur.at<short int>(cv::Point(j,i));
                            if(depth < 0.6)
                            {
                                cout<<"##############something within the range infront of the camera...########################\n";
                                if(image_cur.at<cv::Vec3b>(j,i) != image_pre.at<cv::Vec3b>(j,i))
                                {
                                    cout<<"i : "<<i<<" ... j ...