How to access specific pixel of image

edit

How can i access only the pixels of specific part of the image ? Is there any proper method of accessing those or i need to use hit and trial method for it . For example i want to access all the pixels within the circle ( this circle is just draw for understanding) will not have it in real problem.

I want to access only these pixels for changing values , one way which just come to mind is

getting the four corners of image

vector<Point> corners(4);
corners[0] = Point(0, 0);
corners[1] = Point(my_img.width, 0);
corners[2] = Point(0, my_img.height);
corners[3] = Point(my_img.width, my_img.height);

then find its center

Point center = Point(my_img.size().width/2, my_img.size().height/2);

make radius

double radius = 2.0;

and then put new values inside of radius or circle

image.at<cv::Vec3b>(row,col)[0] = newval[0];  //B
image.at<cv::Vec3b>(row,col)[1] = newval[1];  //G
image.at<cv::Vec3b>(row,col)[2] = newval[2];  //R

but then how to access pixels inside it , or any other logic or idea is appriciated