How can I do image processing operations only in ROI part of original image directly?

Question

Is that possible by using OpenCV to do some image processing operations only in ROI part of original image?

I search some articles on Internet. Most of codes look li

Answer 1

You can get the sub-image using one either a Rect or two Range (see OpenCV doc).

Mat3b img = imread("path_to_image");

img:

Rect r(100,100,200,200);
Mat3b roi3b(img(r));

As long as you don't change image type you can work on roi3b. All changes will be reflected in the original image img:

GaussianBlur(roi3b, roi3b, Size(), 10);

img after blur:

If you change type (e.g. from CV_8UC3 to CV_8UC1), you need to work on a deep copy, since a Mat can't have mixed types.

Mat1b roiGray;
cvtColor(roi3b, roiGray, COLOR_BGR2GRAY);
threshold(roiGray, roiGray, 200, 255, THRESH_BINARY);

You can always copy the results on the original image, taking care to correct the type:

Mat3b roiGray3b;
cvtColor(roiGray, roiGray3b, COLOR_GRAY2BGR);
roiGray3b.copyTo(roi3b);

img after threshold:

Full code for reference:

#include 
using namespace cv;

int main(void)
{
    Mat3b img = imread("path_to_image");

    imshow("Original", img);
    waitKey();

    Rect r(100,100,200,200);
    Mat3b roi3b(img(r));

    GaussianBlur(roi3b, roi3b, Size(), 10);

    imshow("After Blur", img);
    waitKey();

    Mat1b roiGray;
    cvtColor(roi3b, roiGray, COLOR_BGR2GRAY);

    threshold(roiGray, roiGray, 200, 255, THRESH_BINARY);

    Mat3b roiGray3b;
    cvtColor(roiGray, roiGray3b, COLOR_GRAY2BGR);
    roiGray3b.copyTo(roi3b);

    imshow("After Threshold", img);
    waitKey();

    return 0;
}