accessing pixel image c++

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Hi there,

    cv::Mat depth_clean(cv_depthptr->image.rows, cv_depthptr->image.cols, CV_32FC1);
    cv::Mat img(cv_depthptr->image.rows, cv_depthptr->image.cols, CV_8UC1);
    for(int i = 0; i < cv_depthptr->image.rows; i++)
    {
        float* Di = cv_depthptr->image.ptr<float>(i);
        float* Ii = depth_clean.ptr<float>(i);
        char* Ivi = img.ptr<char>(i);
            for(int j = 0; j < cv_depthptr->image.cols; j++)
            {   
               if(Di[j] > 0.0f){                        
                Ii[j] = Di[j];
                Ivi[j] = (char) (255*((Di[j])/(5.5))); // some suitable values..
               }
                else{
                Ii[j] = 0.0f;
                Ivi[j] = 0;
                }
            }   
    }

Can anyone explain to me using that program how the accessing to the pixel has been done ? using pointer and the two loops and what the different between cv_depthptr->image.ptr<float>(i) and Di[j] ?

Thank you so much