converting only a region of image to grayscale - c++

Question

I want to get an image in which only a region has color given a color image.
Mat img = imread("lena.jpg");
Rect roi = Rect(100, 100, 300, 300);// only this should be in color in output
Mat img_yuv;
cvtColor(img, img_yuv, CV_RGB2YUV);
vector<Mat> channels(3);
split(img_yuv, channels);
Mat Y = channels[0];
Mat U = channels[1];
Mat V = channels[2];
// create mask
Mat mask = Mat::zeros(Y.size(), Y.type());
rectangle(mask, roi, Scalar(1), CV_FILLED);
// merging channels
channels[0] = Y;
channels[1] = U.mul(mask)+(Scalar::all(1)-mask).mul(Y);
channels[2] = V.mul(mask)+(Scalar::all(1)-mask).mul(Y);
Mat img_yuv_out, img_out;
merge(channels, img_yuv_out);
cvtColor(img_yuv_out, img_out, CV_YUV2RGB);
imshow("masked_color", img_out);
imshow("lena", img);
with the above opencv code here are my imput and output images respectively.
In the roi it works fine but rest of image doesn't look like a grayscale image(not exactly as we still have 3 channels.

Answer 1

You could try this:
get a copy of the image, and convert it to 3-channel grayscale (I don't know if you need to convert the grayscale explicitly back to (colored) RGB...)
get a Mat for the ROI you want to have the colors in, once for the grayscale copy and once for the original color image
assign/copy the color image ROI to the grayscale image ROI
Indeed, it's exactly as #AndreyKamaev suggests:
#include <opencv2/core/core.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include <opencv2/highgui/highgui.hpp>
int main() {
char const * const fname_in = "lena.jpg";
char const * const fname_out = "lena_out.jpg";
cv::Mat img = cv::imread(fname_in, CV_LOAD_IMAGE_COLOR);
cv::Mat tmp;
cv::cvtColor(img, tmp, CV_BGR2GRAY);
cv::cvtColor(tmp, tmp, CV_GRAY2BGR);
cv::Rect roi(100, 100, 300, 300);
img(roi).copyTo(tmp(roi));
img = tmp;
cv::imwrite(fname_out, img);
}
Output image:

Answer 2

Basically the same as #moooeeeep suggests:
Mat tmp;
cvtColor(img, tmp, COLOR_BGR2GRAY);
cvtColor(tmp, tmp, COLOR_GRAY2BGR);
img(roi).copyTo(tmp(roi));
img = tmp;

Answer 3

To convert roi into grayscale in python you can
Convert the roi to gray
Replace roi by merging the gray, such that [gray, gray, gray] inplace of BGR
Now the roi is gray.
image = cv2.imread(image.jpg')
h, w, _ = image.shape
r, c, s = h//4, w//4, min(h,w)//2
gray_portion = cv2.bitwise_not(cv2.cvtColor(image[r:r+s, c:c+s], cv2.COLOR_BGR2GRAY))
merged = cv2.merge([gray_portion, gray_portion, gray_portion]) #IMPORTANT
image[r:r+s, c:c+s] = merged
cv2.imshow(image)

Answer 4

For those who want to the other way round :-)
Mat tmp;
cvtColor(img, tmp, COLOR_BGR2GRAY);
cvtColor(tmp, tmp, COLOR_GRAY2BGR);
tmp(roi).copyTo(img(roi));
tmp = img;