Better ways to create a rectangular mask by openCV - c++

Question

Creating a mask in openCV
/** result I want
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 1 1 1 1 0 0
0 0 1 1 1 1 0 0
0 0 1 1 1 1 0 0
0 0 1 1 1 1 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
*/
cv::Mat mask = cv::Mat::zeros(8, 8, CV_8U);
std::cout<<"before : \n"<<mask<<std::endl;
for(int i = 2; i != 6; ++i)
{
auto ptr = mask.ptr<uchar>(i) + 2;
for(int j = 0; j != 4; ++j)
{
*ptr++ = 1;
}
}
std::cout<<"after : \n"<<mask<<std::endl;
Do openCV provide us any build in function to create a mask like this?
It is trivial to create a function fot this task, but the function of openCV
always faster than naive handcrafted codes

Answer 1

sure, there's an easier way, use the roi operator:
cv::Mat mask = cv::Mat::zeros(8, 8, CV_8U); // all 0
mask(Rect(2,2,4,4)) = 1;
done!

Answer 2

If some one is looking for creating a non rectangular mask and then to apply it on the image then have a look here :
Mat& obtainIregularROI(Mat& origImag, Point2f topLeft, Point2f topRight, Point2f botLeft, Point2f botRight){
static Mat black(origImag.rows, origImag.cols, origImag.type(), cv::Scalar::all(0));
Mat mask(origImag.rows, origImag.cols, CV_8UC1, cv::Scalar(0));
vector< vector<Point> > co_ordinates;
co_ordinates.push_back(vector<Point>());
co_ordinates[0].push_back(topLeft);
co_ordinates[0].push_back(botLeft);
co_ordinates[0].push_back(botRight);
co_ordinates[0].push_back(topRight);
drawContours( mask,co_ordinates,0, Scalar(255),CV_FILLED, 8 );
origImag.copyTo(black,mask);
return black;
}
"black" is the image where we will finally obtain the result by cropping out the irregular ROI from the original image.
static Mat black(origImag.rows, origImag.cols, origImag.type(), cv::Scalar::all(0));
The "mask" is a Mat, initialized as the same size of original image and filled with 0.
Mat mask(origImag.rows, origImag.cols, CV_8UC1, cv::Scalar(0));
Putting the coordinates in ANTICLOCKWISE direction
vector< vector<Point> > co_ordinates;
co_ordinates.push_back(vector<Point>());
co_ordinates[0].push_back(topLeft);
co_ordinates[0].push_back(botLeft);
co_ordinates[0].push_back(botRight);
co_ordinates[0].push_back(topRight);
Now generating the mask actually
drawContours( mask,co_ordinates,0, Scalar(255),CV_FILLED, 8 );
At the end copy the masked portion/ROI from original image (origImag) and paste on the portion of ROI from the original image (using mask) into image named as "black"
origImag.copyTo(black,mask);