Modifying a cv::Rect inside a cv::Mat in C++

Question

I'm pretty new to openCV and would like to ask what seems like a easy question.
I have an image in the form of a cv::Mat and I would like to change only a small part of the matrix. I've read that using a cv::Rect is the correct way but I can't seem to find a way to only modify that little ROI.

Here's the code:

cv::Mat img = cv::Mat::zeros(msg->height, msg->width, CV_64FC1);
cv::Rect rect(100, 100, 20, 50);

All I want to do is do linear tranformation to the rect and add asign it to the same part of the img.

Something like:

int a=0.1, b=20;
rect = rect*a + b;

Thanks in advance.

Answer 1

OpenCV's cv::Mat has a constructor that creates an ROI image referncing another image:

cv::Mat::Mat(const Mat & m, const Rect & roi)

Using this constructor will cause the new cv::Mat to share the data with the original one:

No data is copied by these constructors. Instead, the header pointing to m data or its sub-array is constructed and associated with it.
The reference counter, if any, is incremented. So, when you modify the matrix formed using such a constructor, you also modify the corresponding elements of m.

You can also use the operator() for that:

Mat cv::Mat::operator() (const Rect & roi)  const

In your case you can do something like the following:

#include <opencv2/core/core.hpp>

int main()
{
    int h = 320;
    int w = 640;
    cv::Mat img = cv::Mat::ones(h, w, CV_64FC1);
    cv::Rect rect(100, 100, 20, 50);
    cv::Mat roi(img, rect); // alternatively you can use:  cv::Mat roi = img(rect);
    double a = 0.1;
    double b = 20;
    roi = roi * a + b; // this will modify the relevant area in img
    return 0;
}